Qass QB4,4i Book-^.„D7 ASTRONOMY OF TO-DAY ASTRONOMY OF TO-DAY A T OVULAR INTRODUCTION IN NON-TECHNICAL LANGUAGE By CECIL G. DOLMAGE, M.A., LL.D., D.C.L. Fellow of the Royal Astronomical Society ; Member of the British Astronomical Association ; Member of the Astronomical Society of the Pacific ; Membre de la Societe Astronomique de France; Membre de la Societe Beige d'Astronomie With a Frontispiece in Colour and 45 Illustrations ^ Diagrams PHILADELPHIA B. LIPPINCOTT COMPANY LONDON : SEELEY & CO. Limited 1909 »>^ <^.^^ /f 3/3^1 7 PREFACE The object of this book is to give an account of the science of Astronomy, as it is known at the present day, in a manner acceptable to \hQ general reader. It is too often supposed that it is impossible to acquire any useful knowledge of Astronomy without much laborious study, and without adventuring into quite a new world of thought. The reasoning apphed to the study of the celestial orbs is, however, of no different order from that which is employed in the affairs of everyday life. The science of mathematics is perhaps responsible for the idea that some kind of difference does exist ; but mathematical processes are, in effect, no more than ordinary logic in con- centrated form, the shorthand of reasoning ^ so to speak. I have attempted in the following pages to take the main facts and theories of Astronomy out of those mathematical forms which repel the general reader, and to present them in the ordinary language of our workaday world. The few diagrams introduced are altogether supple- mentary, and are not connected with the text by any wearying cross-references. Each diagram is com- plete in itself, being intended to serve as a pictorial aid, in case the wording of the text should not have perfectly conveyed the desired meaning. The full V Preface page illustrations are also described as adequately as possible at the foot of each. As to the coloured frontispiece, this must be placed in a category by itself. It is the work of the artist as distinct from the scientist. The book itself contains incidentally a good deal of matter concerned with the Astronomy of the past, the introduction of which has been found necessary in order to make clearer the Astronomy of our time. It would be quite impossible for me to enumerate here the many sources from which information has been drawn. But I acknowledge my especial in- debtedness to Professor F, R. Moulton's Introduction to Astronomy (Macmillan, 1906), to the works on EcHpses of the late Rev. S. ]. Johnson and of Mr. W. T. Lynn, and to the excellent Journals of the British Astronomical Association, Further, for those grand questions concerned with the Stellar Universe at large, I owe a very deep debt to the writings of the famous American astronomer, Professor Simon Newcomb, and of our own countryman, Mr. John Ellard Gore ; to the latter of whom I am under an additional obligation for much valuable information privately rendered. In my search for suitable illustrations, I have been greatly aided by the kindly advice of Mr. W. H. Wesley, the Assistant Secretary of the Royal Astronomical Society. To those who have been so good as to permit me to reproduce pictures and photographs, I desire to record my best thanks as follows : — To the French Artist, Mdlle. Andree Moch ; to the Astronomer Royal ; to Sir David Gill, K.GB., LL.D., F.R.S. ; to the Council of the Royal Astrono- vi Preface mical Society ; to Professor E. B. Frost, Director of the Yerkes Observatory ; to M. P. Puiseux, of the Paris Observatory ; to Dr. Max Wolf, of Heidelberg ; to Professor Percival Lowell ; to the Rev. Theodore E. R. PhilUps, M.A., F.R.A.S. ; to Mr. W. H. Wesley ; to the Warner and Swasey Co., of Cleveland, Ohio, U.S.A. ; to the publishers of Knowledge, and to Messrs. Sampson, Low & Co. For permission to reproduce the beautiful photograph of the Spiral Nebula in Canes Venatici (Plate XXIL), I am indebted to the distinguished astronomer, the late Dr. W. E. Wilson, D.Sc, F.R.S., whose untimely death, I regret to state, occurred in the early part of this year. Finally, my best thanks are due to Mr. John Ellard Gore, F.R.A.S., M.R.LA., to Mr. W. H. Wesley, and to Mr. John Butler Burke, M.A., of Cambridge, for their kindness in reading the proof-sheets. CECIL G. DOLMAGE. London, S.W., August 4, 1908. vu CONTENTS CHAPTER I PAGE The Ancient View 17 CHAPTER II The Modern View 20 CHAPTER III The Solar System 29 CHAPTER IV Celestial Mechanism 38 CHAPTER V Celestial Distances . . . . . 46 CHAPTER VI Celestial Measurement 55 ix Contents CHAPTER VII PAGE Eclipses and Kindred Phenomena . . 6i CHAPTER VIII Famous Eclipses of the Sun ... 83 CHAPTER IX Famous Eclipses of the Moon . . .101 CHAPTER X The Growth of Observation . . . 105 CHAPTER XI Spectrum Analysis 121 CHAPTER XII The Sun . . . . . . . .127 CHAPTER XIII The Sun — continued 134 CHAPTER XIV The Inferior Planets 146 X Contents CHAPTER XV PAGE The Earth 158 CHAPTER XVI The Moon 183 CHAPTER XVII The Superior Planets . . . . . 209 CHAPTER XVIII The Superior Planets — continued . . . 229 CHAPTER XIX Comets . . . . . . . . 247 CHAPTER XX Remarkable Comets 259 CHAPTER XXI Meteors or Shooting Stars .... 266 CHAPTER XXII The Stars . 278 xi Contents CHAPTER XXIII PAGE The Stars — continued ..... 287 CHAPTER XXIV Systems of Stars ...... 300 CHAPTER XXV The Stellar Universe 319 CHAPTER XXVI The Stellar Universe — continued . . 329 CHAPTER XXVII The Beginning of Things .... 333 CHAPTER XXVIII The End of Things 342 Index 351 Xll LIST OF ILLUSTRATIONS LIST OF PLATES The Total Eclipse of the Sun OF August 30, 1905 . I. The Total Eclipse of the Sun OF May 17, 1882 II. Great Telescope of Hevelius III. A TUBELESS, OR " AeRIAL" TELE- SCOPE .... IV. The Great Yerkes Telescope V. The Sun, showing several GROUPS OF Spots VI. Photograph of a Sunspot VII. Forms of the Solar Corona AT THE epochs OF SUNSPOT Maximum and Sunspot Minimum respectively. (A) The Total Eclipse of THE Sun of December 22, 1870. (B) The Total Eclipse of THE Sun of May 28, • Frontispiece NT . To face page 96 J3 11 IIO J? 3) 112 5) J3 118 L • 55 5) 134 )) )J 136 1900 142 Xlll List of Illustrations VIII. The Moon .... To face page 196 PLATE IX. Map of the Moon, showing THE PRINCIPAL " CRATERS," Mountain Ranges and "Seas" ....„„ 198 X. One of the most interesting Regions on the Moon . „ „ 200 XI. The Moon (showing Systems OF " Rays ") . . . „ „ 204 XII. A Map of the Planet Mars „ „ 216 XIII. Minor Planet Trails . . „ „ 226 XIV. The Planet Jupiter . . „ „ 230 XV. The Planet Saturn . . „ „ 236 XVI. Early Representations of Saturn . . . • » » 242 XVII. DoNATi's Comet . . . „ „ 256 XVIII. Daniel's Comet of 1907 . „ „ 258 XIX. The Sky around the North Pole . . . • jj » 292 XX. Orion and his Neighbours . ,, „ 296 XXI. The Great Globular Cluster in the Southern Constel- lation OF Centaurus . „ „ 306 XXII. Spiral Nebula in the Con- stellation OF Canes Vena- tici xiv 314 List of Illustrations PLATE XXIII. The Great Nebula in the Constellation of Andro- meda . . . . .To face page 316 XXIV. The Great Nebula in the Constellation of Orion . ,, „ 318 LIST OF DIAGRAMS FIG. PAGE 1. The Ptolemaic Idea of the Universe . 19 2. The Copernican Theory of the Solar System .21 3. Total and Partial Eclipses of the Moon . 64 4. Total and Partial Eclipses of the Sun . 67 5. " Baily's Beads" 70 6. Map of the World on Mercator's Projec- tion, SHOWING A portion OF THE PRO- GRESS OF THE Total Solar Eclipse of August 30, 1905, across the surface of the Earth . . . . .81 7. The " Ring with Wings " . . . .87 8. The Various Types of Telescope . . 113 9. The Solar. Spectrum 123 10. A Section through the Sun, showing how the Prominences rise from the Chro- mosphere . . . . . • 131 11. Orbit and Phases of an Inferior Planet . 148 XV List of Illustrations FIG. PAGE 12. The "Black Drop" 153 13. Summer and Winter .... 14. Orbit and Phases of the Moon . 15. The Rotation of the Moon on her Axis 16. Laplace's "Perennial Full Moon" 176 184 187 191 17. Illustrating the Author's explanation of the apparent enlargement of celestial Objects ....... 195 18. Showing how the Tail of a Comet is directed away FROM THE SUN . . 248 19. The Comet of 1066, as represented in the Bayeux Tapestry , . . . .263 20. Passage of the Earth through the thickest portion of a Meteor Swarm . .269 XVI ASTRONOMY OF TO-DAY CHAPTER I THE ANCIENT VIEW It is never safe, as we know, to judge by appearances, and this is perhaps more true of astronomy than of anything else. For instance, the idea which one would most naturally form of the earth and heaven is that the solid earth on which we live and move extends to a great distance in every direction, and that the heaven is an immense dome upon the inner surface of which the stars are fixed. Such must needs have been the idea of the universe held by men in the earliest times. In their view the earth was of paramount importance. The sun and moon were mere lamps for the day and for the night ; and these, if not gods themselves, were at any rate under the charge of special deities, whose task it was to guide their motions across the vaulted sky. Little by little, however, this simple estimate of nature began to be overturned. Difficult problems agitated the human mind. On what, for instance, did the solid earth rest, and what prevented the vaulted heaven from falling in upon men and crushing them out of exist- 17 B Astronomy of To-day ence ? Fantastic myths sprang from the vain attempts to solve these riddles. The Hindoos, for example, ima- gined the earth as supported by four elephants which stood upon the back of a gigantic tortoise, which, in its turn, floated on the surface of an elemental ocean. The early Western civilisations conceived the fable of the Titan Atlas, who, as a punishment for revolt against the Olympian gods, was condemned to hold up the expanse of sky for ever and ever. Later on glimmerings of the true light began to break in upon men. The Greek philosophers, who busied themselves much with such matters, gradually became convinced that the earth was spherical in shape, that is to say, round like a ball. In this opinion we now know that they were right ; but in their other impor- tant belief, viz. that the earth was placed at the centre of all things, they were indeed very far from the truth. By the second century of the Christian era, the ideas of the early philosophers had become hardened into a definite theory, which, though it appears very incorrect to us to-day, nevertheless demands excep- tional notice from the fact that it was everywhere accepted as the true explanation until so late as some four centuries ago. This theory of the universe is known by the name of the Ptolemaic System, because it was first set forth in definite terms by one of the most famous of the astronomers of antiquity, Claudius Ptolemaeus Pelusinensis (100-170 A.D.), better known as Ptolemy of Alexandria. In his system the Earth occupied the centre ; while around it circled in order outwards the Moon, the planets Mercury and Venus, the Sun, and then the planets Mars, Jupiter, and Saturn. Beyond The Ancient View these again revolved the background of the heaven, upon which it was beHeved that the stars were fixed — " Stellis ardentibus aptum," as Virgil puts it (see Fig. i). Fig. I. — The Ptolemaic idea of the Universe. The Ptolemaic system persisted unshaken for about fourteen hundred years after the death of its author. Clearly men were flattered by the notion that their earth was the most important body in nature, that it stood still at the centre of the universe, and was the pivot upon which all things revolved. 19 CHAPTER II THE MODERN VIEW It is still well under four hundred years since the modern, or Copernican, theory of the universe sup- planted the Ptolemaic, which had held sway during so many centuries. In this new theory, propounded towards the middle of the sixteenth century by Nicholas Copernicus (1473-1543), a Prussian astro- nomer, the earth was dethroned from its central position and considered merely as one of a number of planetary bodies which revolve around the sun. As it is not a part of our purpose to follow in detail the history of the science, it seems advisable to begin by stating in a broad fashion the conception of the universe as accepted and believed in to-day. The Sun, the most important of the celestial bodies so far as we are concerned, occupies the central position ; not, however, in the whole universe, but only in that limited portion which is known as the Solar System. Around it, in the following order outwards, circle the planets Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, and Neptune (see Fig. 2, p. 21). At an immense distance beyond the solar system, and scattered irregularly through the depth of space, lie the stars. The two first- mentioned members of the solar system. Mercury and Venus, are known as the Inferior Planets ; 20 The Modern View Orbit of Nepfune ^-' y ^-^.^.^^^ ^ y ^\^ /" \, Orbit of I'rxxnu^ y^ Orbit of Safi,^^ \ [ M-^C. '" "T!,r?un \EarHi .."-■- Orbit/ \ Orbit C.; "fMf.rW / / \ ~ ^--^^ / Fig. 2. — The Copernican theory of the Solar System. 21 Astronomy of To-day and in their courses about the sun, they always keep well inside the path along which our earth moves. The remaining members (exclusive of the earth) are called Superior Planets, and their paths lie all outside that of the earth. The five planets, Mercury, Venus, Mars, Jupiter, and Saturn, have been known from all antiquity. Nothing then can bring home to us more strongly the immense advance which has taken place in astronomy during modern times than the fact that it is only 127 years since observation of the skies first added a planet to that time-honoured number. It was indeed on the 13th of March 1781, while en- gaged in observing the constellation of the Twins, that the justly famous Sir WilHam Herschel caught sight of an object which he did not recognise as having met with before. He at first took it for a comet, but observations of its movements during a few days showed it to be a planet. This body, which the power of the telescope alone had thus shown to belong to the solar family, has since become known to science under the name of Uranus. By its discovery the hitherto accepted hmits of the solar system were at once pushed out to twice their former extent, and the hope naturally arose that other planets would quickly reveal themselves in the immensities beyond. For a number of years prior to Herschel's great discovery, it had been noticed that the distances at which the then known planets circulated appeared to be arranged in a somewhat orderly progression outwards from the sun. This seeming plan, known to astronomers by the name of Bode's Law, was 22 The Modern View closely confirmed by the distance of the new planet Uranus. There still lay, however, a broad gap between the planets Mars and Jupiter. Had another planet indeed circuited there, the solar system would have presented an appearance of almost perfect order. But the void between Mars and Jupiter was unfilled ; the space in which one would reasonably expect to find another planet circHng was unaccount- ably empty. On the first day of the nineteenth century the mystery was however explained, a body being dis- covered 1 which revolved in the space that had hitherto been considered planetless. But it was a tiny globe hardly worthy of the name of planet. In the following year a second body was discovered re- volving in the same space ; but it was even smaller in size than the first. During the ensuing five years two more of these little planets were discovered. Then came a pause, no more such bodies being added to the system until half-way through the century, when suddenly the discovery of these so-called ^' minor planets " began anew. Since then additions to this portion of our system have rained thick and fast. The small bodies have received the name of Asteroids or Planetoids ; and up to the present time some six hundred of them are known to exist, all revolving in the previously unfilled space between Mars and Jupiter. In the year 1846 the outer boundary of the solar system was again extended by the discovery that a great planet circulated beyond Uranus. The new body, which received the name of Neptune, was * By the Italian astronomer, Piazzi, at Palermo. 23 Astronomy of To-day brought to light as the result of calculations made at the same time, though quite independently, by the Cambridge mathematician Adams, and the French astronomer Le Verrier. The discovery of Neptune differed, however, from that of Uranus in the following respect. Uranus was found merely in the course of ordinary telescopic survey of the heavens. The position of Neptune, on the other hand, was predicted as the result of rigorous mathematical investigations undertaken with the object of fixing the position of an unseen and still more distant body, the attraction of which, in passing by, was disturbing the position of Uranus in its revolution around the sun. Adams actually completed his investigation first ; but a delay at Cambridge in examining that portion of the sky, w^here he announced that the body ought just then to be, allowed France to snatch the honour of dis- covery, and the new planet was found by the observer Galle at Berlin, very near the place in the heavens which Le Verrier had mathematically predicted for it. Nearly fifty years later, that is to say, in our own time, another important planetary discovery was made. One of the recent additions to the numerous and con- stantly increasing family of the asteroids, a tiny body brought to light in 1898, turned out after all not to be circulating in the customary space between Mars and Jupiter, but actually in that betw^een our earth and Mars. This body is very small, not more than about tw^enty miles across. It has received the name of Eros (the Greek equivalent for Cupid), in allusion to its insignificant size as compared with the other leading members of the system. This completes the list of the planets which, so 24 The Modern View far, have revealed themselves to us. Whether others exist time alone will show. Two or three have been suspected to revolve beyond the path of Neptune ; and it has even been asserted, on more than one occasion, that a planet circulates nearer to the sun than Mercury. This supposed body, to which the name of '' Vulcan " was provisionally given, is said to have been 'discovered" in 1859 by a French doctor named Lescarbault, of Orgeres near Orleans ; but up to the present there has been no sufficient evidence of its existence. The reason why such uncertainty should exist upon this point is easy enough to understand, when we consider the overpowering glare which fills our atmosphere all around the sun's place in the sky. Mercury, the nearest known planet to the sun, is for this reason always very difficult to see ; and even when, in its course, it gets sufficiently far from the sun to be left for a short time above the horizon after sunset, it is by no means an easy object to observe on account of the mists which usually hang about low down near the earth. One oppor- tunity, however, offers itself from time to time to solve the riddle of an 'Mntra-Mercurial" planet, that is to say, of a planet which circulates within the path followed by Mercury. The opportunity in question is furnished by a total eclipse of the sun ; for when, during an eclipse of that kind, the body of the moon for a few minutes entirely hides the sun's face, and the dazzling glare is thus completely cut off, as- tronomers are enabled to give an unimpeded, though all too hurried, search to the region close around. A goodly number of total eclipses of the sun have, how- ever, come and gone since the days of Lescarbault, 25 Astronomy of To-day and no planet, so far, has revealed itself in the intra- Mercurial space. It seems, therefore, quite safe to affirm that no globe of sufficient size to be seen by means of our modern telescopes circulates nearer to the sun than the planet Mercury. Next in importance to the planets, as permanent members of the solar system, come the relatively small and secondary bodies known by the name of Satellites. The name satellite is derived from a Latin word signifying an attendant ; for the bodies so-called move along always in close proximity to their re- spective ^' primaries," as the planets which they accompany are technically termed. The satellites cannot be considered as allotted with any particular regularity among the various members of the system ; several of the planets, for instance, having a goodly number of these bodies accompany- ing them, while others have but one or two, and some again have none at all. Taking the planets in their order of distance outward from the Sun, we find that neither Mercury nor Venus are provided with satellites ; the Earth has only one, viz. our neighbour the Moon ; while Mars has but two tiny ones, so small indeed that one might imagine them to be merely asteroids, which had wandered out of their proper region and attached themselves to that planet. For the rest, so far as we at present know, Jupiter possesses seven,^ Saturn ten, Uranus four, and Neptune one. It is indeed possible, nay more, it is extremely probable, that the two last-named planets have a greater number of these secondary bodies revolving around them ; but, unfortunately, the Uranian and ^ Probably eight. (See note, page 232.) 26 The Modern View Neptunian systems are at such immense distances from us, that even the magnificent telescopes of to-day can extract very little information concerning them. From the distribution of the satellites, the reader will notice that the planets relatively near to the sun are provided with few or none, while the more distant planets are richly endowed. The conclusion, therefore, seems to be that nearness to the sun is in some way unfavourable either to the production, or to the continued existence, of satellites. A planet and its satelHtes form a repetition of the solar system on a tiny scale. Just as the planets re- volve around the sun, so do these secondary bodies revolve around their primaries. When Galileo, in 1610, turned his newly invented telescope upon Jupiter, he quickly recognised in the four circling moons which met his gaze, a miniature edition of the solar system. Besides the planets and their satellites, there are two other classes of bodies which claim membership of the solar system. These are Comets and Meteors. Comets differ from the bodies which we have just been describing in that they appear filmy and trans- parent, whereas the others are solid and opaque. Again, the paths of the planets around the sun and of the satellites around their primaries are not actually circles ; they are ovals, but their ovalness is not of a marked degree. The paths of comets on the other hand are usually very oval ; so that in their courses many of them pass out as far as the known limits of the solar system, and even far beyond. It should be mentioned that nowadays the tendency is to consider comets as perm.anent members of the system, though 27 Astronomy of To-day this was formerly not by any means an article of faith with astronomers. Meteors are very small bodies, as a rule perhaps no larger than pebbles, which move about unseen in space, and of which we do not become aware until they arrive very close to the earth. They are then made visible to us for a moment or two in conse- quence of being heated to a white heat by the friction of rushing through the atmosphere, and are thus usually turned into ashes and vapour long before they reach the surface of our globe. Though occa- sionally a meteoric body survives the fiery ordeal, and reaches the earth more or less in a solid state to bury itself deep in the soil, the majority of these celestial visitants constitute no source of danger whatever for us. Any one who will take the trouble to gaze at the sky for a short time on a clear night, is fairly certain to be rewarded with the view of a meteor. The impression received is as if one of the stars had suddenly left its accustomed place, and dashed across the heavens, leaving in its course a trail of light. It is for this reason that meteors are popularly known under the name of ''shooting stars." CHAPTER III THE SOLAR SYSTEM We have seen, in the course of the last chapter, that the solar system is composed as follows : — there is a central body, the sun, around which revolve along stated paths a number of important bodies known as planets. Certain of these planets, in their courses, carry along in company still smaller bodies called satellites, which revolve around them. With regard, however, to the remaining members of the system, viz. the comets and the meteors, it is not advisable at this stage to add more to what has been said in the preceding chapter. For the time being, therefore, we will devote our attention merely to the sun, the planets, and the satellites. Of what shape then are these bodies ? Of one shape, and that one alone which appears to char- acterise all solid objects in the celestial spaces : they are spherical, which means round like a ball. Each of these spherical bodies rotates ; that is to say, turns round and round, as a top does when it is spinning. This rotation is said to take place ^'upon an axis," a statement which may be explained as follows : — Imagine a ball with a knitting-needle run right through its centre. Then imagine this needle held pointing in one fixed direction while the ball is turned round and round. Well, it is the same 29 Astronomy of To-day thing with the earth. As it journeys about the sun, it keeps turning round and round continually as if pivoted upon a mighty knitting needle transfixing it fron^i North Pole to South Pole. In reality, how- ever, there is no such material axis to regulate the constant direction of the rotation, just as there are no actual supports to uphold the earth itself in space. The causes which keep the celestial spheres poised, and which control their motions, are far more wonder- ful than any mechanical device. At this juncture it will be well to emphasise the sharp distinction between the terms rotation and re- volution. The term *^ rotation" is invariably used by astronomers to signify the motion which a celestial body has upon an axis; the term ^^ revolution," on the other hand, is used for the movement of one celestial body around another. Speaking of the earth, for instance, we say, that it rotates on its axis, and that it revolves around the sun. So far, nothing has been said about the sizes of the members of our system. Is there any stock size, any pattern according to which they may be judged ? None whatever ! They vary enormously. Very much the largest of all is the Sun, which is several hundred times larger than all the planets and satellites of the system rolled together. Next comes Jupiter and afterwards the other planets in the following order of size : — Saturn, Uranus, Neptune, the Earth, Venus, Mars, and Mercury. Very much smaller than any of these are the asteroids, of which Ceres, the largest, is less than 500 miles in diameter. It is, by the way, a strange fact that the zone of asteroids should mark the separation of the small planets from the giant 30 The Solar System ones. The following table, giving roughly the various diameters of the sun and the principal planets in miles, will clearly illustrate the great discrepancy in size which prevails in the system. Sun . . 866,540 miles Mercury . 2,765 „ Venus . . 7,826 „ Earth 7,918 „ Mars 4,332 „ ZONE O F ASTEROIDS Jupiter • 87,380 „ Saturn • 73,125 „ Uranus 1 . 34,900 „ Neptune 1. 32,900 „ It does not seem possible to arrive at any general-, isation from the above data, except it be to state that there is a continuous increase in size from Mercury to the earth, and a similar decrease in size from Jupiter outwards. Were Mars greater than the earth, the planets could then with truth be said to increase in size up to Jupiter, and then to decrease. But the zone of asteroids, and the relative smallness of Mars, negative any attempt to regard the dimensions of the planets as an orderly sequence. Next with respect to relative distance from the sun, Venus circulates nearly twice as far from it as Mercury, the earth nearly three times as far, and •^ As there seems to be much difference of opinion concerning the dia- meters of Uranus and Neptune, it should here be mentioned that the above figures are taken from Professor F. R. Moulton's Introduction to Astro- no?ny (1906). They are there stated to be given on the authority of " Barnard's many measures at the Lick Observatory." 31 Astronomy of To-day Mars nearly four times. After this, just as we found a sudden increase in size, so do we meet with a sudden increase in distance. Jupiter, for instance, is about thirteen times as far as Mercury, Saturn about twenty-five times, Uranus about forty-nine times, and Neptune about seventy-seven. (See Fig. 2, p. 21.) It will thus be seen how enormously the solar system was enlarged in extent by the discovery of the outermost planets. The finding of Uranus plainly doubled its breadth ; the finding of Neptune made it more than half as broad again. Nothing indeed can better show the import of these great discoveries than to take a pair of compasses and roughly set out the above relative paths in a series of concentric circles upon a large sheet of paper, and then to consider that the path of Saturn was the supposed boundary of our solar system prior to the year 1781. We have seen that the usual shape of celestial bodies themselves is spherical. Of what form then are their paths, or oj^bitSy as these are called ? One might be inclined at a venture to answer ^^ circular," but this is not the case. The orbits of the planets cannot be regarded as true circles. They are ovals, or, to speak in technical language, '' ellipses." Their ovalness or '^ ellipticity " is, however, in each case not by any means of the same degree. Some orbits — for instance, that of the earth — differ only slightly from circles ; while others — those of Mars or Mercury, for example — are markedly elliptic. The orbit of the tiny planet Eros is, however, far and away the most elliptic of all, as we shall see when we come to deal with that little planet in detail. It has been stated that the sun and planets are 32 The Solar System always rotating. The various rates at which they do so will, however, be best appreciated by a comparison with the rate at which the earth itself rotates. But first of all, let us see what ground we have, if any, for asserting that the earth rotates at all ? If we carefully watch the heavens we notice that the background of the sky, wath all the brilliant objects which sparkle in it, appears to turn once round us in about twenty-four hours ; and that the pivot upon which this movement takes place is situated some- where near what is known to us as the Pole Star. This was one of the earliest facts noted with regard to the sky ; and to the men of old it therefore seems as if the heavens and all therein were always revolving around the earth. It w^as natural enough for them to take this view, for they had not the slightest idea of the immense distance of the celestial bodies, and in the absence of any knowledge of the kind they were inclined to imagine them compara- tively near. It was indeed only after the lapse of many centuries, when men had at last realised the enormous gulf which separated them from even the nearest object in the sky, that a more reason- able opinion began to prevail. It was then seen that this revolution of the heavens about the earth could be more easily and more satisfactorily ex- plained by supposing a mere rotation of the solid earth about a fixed axis, pointed in the direction of the polar star. The probability of such a rotation on the part of the earth itself was further strengthened by the observations made with the telescope. When the surfaces of the sun and planets were carefully studied these bodies were seen to be rotating. This 33 C Astronomy of To-day being the case, there could not surely be much hesi- tation in granting that the earth rotated also ; par- ticularly when it so simply explained the daily movement of the sky, and saved men from the almost inconceivable notion that the whole stupen- dous vaulted heaven was turning about them once in every twenty-four hours. If the sun be regularly observed through a tele- scope, it will gradually be gathered from the slow displacement of sunspots across its face, their dis- appearance at one edge and their reappearance again at the other edge, that it is rotating on an axis in a period of about twenty-six days. The movement, too, of various well-known markings on the surfaces of the planets Mars, Jupiter, and Saturn proves to us that these bodies are rotating in periods, which are about twenty-four hours for the first, and about ten hours for each of the other two. With regard, however, to Uranus and Neptune there is much more uncertainty, as these planets are at such great dis- tances that even our best telescopes give but a confused view of the markings which they display ; still a period of rotation of from ten to twelve hours appears to be accepted for them. On the other hand the constant blaze of sunlight in the neighbourhood of Mercury and Venus equally hampers astronomers in this quest. The older telescopic observers con- sidered that the rotation periods of these two planets were about the same as that of the earth ; but of recent years the opinion has been gaining ground that they turn round on their axes in exactly the same time as they revolve about the sun. This question is, how- ever, a very doubtful one, and will be again referred 34 The Solar System to later on ; but, putting it on one side, it will be seen from what we have said above, that the rotation periods of the other planets of our system are usually about twenty-four hours, or under. The fact that the rotation period of the sun should run into days need not seem extraordinary when one considers its enormous size. The periods taken by the various planets to revolve around the sun is the next point which has to be considered. Here, too, it is well to start with the earth's period of revolution as the standard, and to see how the periods taken by the other planets com- pare with it. The earth takes about 365^ days to revolve around the sun. This period of time is known to us as a ''year." The following table shows in days and years the periods taken by each of the other planets to make a complete revolution round the sun ; — Mercury about 88 days. Venus 226 „ Mars I year and 321 days. Jupiter IT years and 313 days. Saturn 29 years and 167 days. Uranus 84 years and 7 days. Neptune 164 years and 284 days. From these periods we gather an important fact, namely, that the nearer a planet is to the sun the faster it revolves. Compared with one of our years what a long time does an Uranian, or Neptunian, ''year" seem? For instance, if a " year " had commenced in Neptune about the middle of the reign of George II., that "year" 35 Astronomy of To-day would be only just coming to a close ; for the planet is but now arriving back to the position, with regard to the sun, which it then occupied. Uranus, too, has only completed a little more than ij of its ^'years'' since Herschel discovered it. Having accepted the fact that the planets are revolving around the sun, the next point to be in- quired into is : — What are the positions of their orbits, or paths, relatively to each other ? Suppose, for instance, the various planetary orbits to be represented by a set of hoops of different sizes, placed one within the other, and the sun by a small ball in the middle of the whole ; in what positions will these hoops have to be arranged so as to imitate exactly the true condition of things ? First of all let us suppose the entire arrangement, ball and hoops, to be on one level, so to speak. This may be easily compassed by imagining the hoops as floating, one surrounding the other, with the ball in the middle of all, upon the surface of still water. Such a set of objects would be described in astrono- mical parlance as being in the same plane. Suppose, on the other hand, that some of these floating hoops are tilted with regard to the others, so that one half of a hoop rises out of the water and the other half conse- quently sinks beneath the surface. This indeed is the actual case with regard to the planetary orbits. They do not by any means He all exactly in the same plane. Each one of them is tilted, or inclined, a little with respect to the plane of the earth's orbit, which astronomers, for convenience, regard as the level oi the solar system. This tilting, or '^ inclination," is, in the larger planets, greatest for the orbit of Mercury, 36 The Solar System least for that of Uranus. Mercury's orbit is inclined to that of the earth at an angle of about 7°, that of Venus at a little over 3°, that of Saturn 24° ; while in those of Mars, Neptune, and Jupiter the inclination is less than 2^ But greater than any of these is the inclination of the orbit of the tiny planet Eros, viz. nearly 11°. The systems of satellites revolving around their respective planets being, as we have already pointed out, mere miniature editions of the solar system, the considerations so far detailed, which regulate the behaviour of the planets in their relations to the sun, will of necessity apply to the satellites very closely. In one respect, however, a system of satellites differs materially from a system of planets. The central body around which planets are in motion is self-luminous, whereas the planetary body around which a satellite revolves is not. True, planets shine, and shine very brightly too ; as, for instance, Venus and Jupiter. But they do not give forth any light of their own, as the sun does ; they merely reflect the sunlight which they receive from him. Putting this one fact aside, the analogy between the planetary system and a satellite system is remarkable. The satellites are spherical in form, and differ markedly in size ; they rotate, so far as we know, upon their axes in varying times ; they re- volve around their governing planets in orbits, not circular, but elliptic ; and these orbits, furthermore, do not of necessity lie in the same plane. Last of all the satelHtes revolve around their primaries at rates which are directly comparable with those at which the planets revolve around the sun, the rule in fact holding good that the nearer a satellite is to its primary the faster it revolves. 37 CHAPTER IV CELESTIAL MECHANISM As soon as we begin to inquire closely into the actual condition of the various members of the solar system we are struck with a certain distinction. We find that there are two quite different points of view from w^hich these bodies can be regarded. For instance^ w^e may make our estimates of them either as regards volume — that is to say, the mere room w^hich they take up ; or as regards mass — that is to say, the amount of matter w^hich they contain. Let us imagine two globes of equal volume ; in other words, w'hich take up an equal amount of space. One of these globes, however, may be composed of material much more tightly put together than in the other ; or of greater density^ as the term goes. That globe is said to be the greater of the two in mass. Were such a pair of globes to be weighed in scales, one globe in each pan, we should see at once, by its weighing down the other, w^hich of the two was com- posed of the more tightly packed materials ; and we should, in astronomical parlance, say of this one that it had the greater mass. Volume being merely another word for size, the order of the members of the solar system, with regard to their volumes, will be as follows, begin- ning with the greatest : — the Sun, Jupiter, Saturn, 38 Celestial Mechanism Uranus, Neptune, the Earth, Venus, Mars, and Mercury. With regard to mass the same order strangely enough holds good. The actual densities of the bodies in question are, however, very different. The densest or closest packed body of all is the Earth, which is about five and a half times as dense as if it were composed entirely of water. Venus follows next, then Mars, and then Mercury. The remaining bodies, on the other hand, are relatively loose in struc- ture. Saturn is the least dense of all, less so than water. The density of the Sun is a little greater than that of water. This method of estimating is, however, subject to a qualification. It must be remembered that in speaking of the Sun, for instance, as being only a little denser than water, we are merely treating the question from the point of view of an average. Certain parts of it in fact will be ever so much denser than water : those are the parts in the centre. Other portions, for instance, the outside portions, will be very much less dense. It will easily be understood that in all such bodies the densest or most compressed portions are to be found towards the centre ; while the portions towards the exterior being less pressed upon, will be less dense. We now reach a very important point, the question of Gravitation. d'avitatioUy or gravity , as it is often called, is the attractive force which, for instance, causes objects to fall to the earth. Now it seems rather strange that one should say that it is owing to a certain force that things fall towards the earth. All things seem to us to fall so of their own accord, as if it 39 Astronomy of To-day were quite natural, or rather most unnatural if they did not. Why then require a '^ force" to make them fall ? The story goes that the great Sir Isaac Newton was set a-thinking on this subject by seeing an apple fall from a tree to the earth. He then carried the train of thought further ; and, by studying the movements of the moon, he reached the conclusion that a body even so far off as our satellite would be drawn towards the earth in the same manner. This being the case, one will naturally ask why the moon herself does not fall in upon the earth. The answer is indeed found to be that the moon is travelling round and round the earth at a certain rapid pace, and it is this very samxC rapid pace which keeps her from falling in upon us. Any one can test this simple fact for himself. If we tie a stone to the end of a string, and keep whirling it round and round fast enough, there will be a strong pull from the stone in an outward direction, and the string will remain tight all the time that the stone is being whirled. If, however, we gradually slacken the speed at which we are making the stone whirl, a moment will come at length when the string will become limp, and the stone will fall back towards our hand. It seems, therefore, that there are two causes which maintain the stone at a regular distance all the time it is being steadily whirled. One of these is the con- tinual pull inward towards our hand by means of the string. The other is the continual pull away from us caused by the rate at which the stone is travelling. When the rate of whirling is so regulated that these pulls exactly balance each other, the stone travels com- fortably round and round, and shows no tendency 40 Celestial Mechanism either to fall back upon our hand or to break the string and fly away into the air. It is indeed pre- cisely similar with regard to the moon. The continual pull of the earth's gravitation takes the place of the string. If the moon were to go round and round slower than it does, it would tend to fall in towards the earth ; if, on the other hand, it were to go faster, it would tend to rush away into space. The same kind of pull which the earth exerts upon the objects at its surface, or upon its satellite, the moon, exists through space so far as we know. "Every particle of matter in the universe is found in fact to attract every other particle. The moon, for instance, attracts the earth also, but the controlling force is on the side of the much greater mass of the earth. This force of gravity or attraction of gravita- tion, as it is also called, is perfectly regular in its action. Its power depends first of all exactly upon the mass of the body which exerts it. The gravita- tional pull of the sun, for instance, reaches out to an enormous distance, controlling perhaps, in their courses, unseen planets circling far beyond the orbit of Neptune. Again, the strength with which the force of gravity acts depends upon distance in a regularly diminishing proportion. Thus, the nearer an object is to the earth, for instance, the stronger is the gravitational pull which it gets from it ; the farther off it is, the weaker is this pull. If then the moon were to be brought nearer to the earth, the gravitational pull of the latter would become so much stronger that the moon's rate of motion would have also to increase in due proportion to prevent her from being drawn into the earth. Last of all, 41 Astronomy of To-day the point in a body from which the attraction of gravitation acts, is not necessarily the centre of the body, but rather what is known as its centre of gravity, that is to say, the balancing point of all the matter which the body contains. It should here be noted that the moon does not actually revolve around the centre of gravity of the earth. What really happens is that both orbs re- volve around their common centre of gravity, which is a point within the body of the earth, and situ- ated about three thousand miles from its centre. In the same manner the planets and the sun revolve around the centre of gravity of the solar system, which is a point within the body of the sun. The neatly poised movements of the planets around the sun, and of the satellites around their respective planets, will therefore be readily understood to result from a nice balance between gravitation and speed of motion. The mass of the earth is ascertained to be about eighty times that of the moon. Our knowledge of the mass of a planet is learned from comparing the revolutions of its satellite or satellites around it, with those of the moon around the earth. We are thus enabled to deduce what the mass of such a planet would be compared to the earth's mass ; that is to say, a study, for instance, of Jupiter's satellite system shows that Jupiter must have a mass nearly three hundred and eighteen times that of our earth. In the same manner we can argue out the mass of the sun from the movements of the planets and other bodies of the system around it. With regard, however, to Venus and Mercury, the problem is by 42 Celestial Mechanism no means such an eas}^ one, as these bodies have no satelHtes. For information in this latter case we have to rely upon such uncertain evidence as, for instance, the sHght disturbances caused in the motion of the earth by the attraction of these planets when they pass closest to us, or their observed effect upon the motions of such comets as may happen to pass near to them. Mass and weight, though often spoken of as one and the same thing, are by no means so. Mass, as we have seen, merely means the amount of matter which a body contains. The weight of a body, on the other hand, depends entirely upon the gravitational pull which it receives. The force of gravity at the surface of the earth is, for instance, about six times as great as that at the surface of the moon. All bodies, therefore, weigh about six times as much on the earth as they would upon the moon ; or, rather, a body transferred to the moon's surface would weigh only about one-sixth of what it did on the terrestrial surface. It will therefore be seen that if a body of given mass were to be placed upon planet after planet in turn, its weight would regularly alter according to the force of gravity at each planet's surface. Gravitation is indeed one of the greatest mysteries of nature. What it is, the means by which it acts, or why such a force should exist at all, are questions to which so far we have not had even the merest hint of an answer. Its action across space appears to be instantaneous. The intensity of gravitation is said in mathematical parlance ^^to vary inversely with the square of the distance." This means that at twice the distance the 43 Astronomy of To-day pull will become only one-quarter as strong, and not one-half as otherwise might be expected. At four times the distance, therefore; it will be one-sixteenth as strong. At the earth's surface a body is pulled by the earth's gravitation, or ''falls," as we ordinarily term it, through 1 6 feet in one second of time ; whereas at the distance of the moon the attraction of the earth is so very much weakened that a body would take as long as one minute to fall through the same space. Newton's investigations showed that if a body were to be placed at rest in space entirely away from the attraction of any other body it would remain always in a motionless condition, because there would plainly be no reason why it should move in any one direc- tion rather than in another. And, similarly, if a body were to be projected in a certain direction and at a certain speed, it would move always in the same direction and at the same speed so long as it did not come within the gravitational attraction of any other body. The possibiHty of an interaction between the celestial orbs had occurred to astronomers before the time of Newton ; for instance, in the ninth century to the Arabian Musa-ben-Shakir, to Camillus Agrippa in 1553, and to Kepler, who suspected its existence from observation of the tides. Horrox also, writing in 1635, spoke of the moon as moved by an emanation from the earth. But no one prior to Newton attempted to examine the question from a mathematical stand- point. Notwithstanding the acknowledged truth and far- reaching scope of the law of gravitation — for we find its effects exemplified in every portion of the universe — 44 Celestial Mechanism there are yet some minor movements which it does not account for. For instance, there are small irre- gularities in the movement of Mercury which cannot be explained by the influence of possible intra-Mer- curial planets, and similarly there are slight unaccount- able deviations in the motions of our neighbour the Moon. 45 CHAPTER V CELESTIAL DISTANCES Up to this we have merely taken a general view of the solar system — a bird's-eye view, so to speak, from space. In the course of our inquiry we noted in a rough way the relative distances at which the various planets move around the sun. But we have not yet stated what these distances actually are, and it were there- fore well now to turn our attention to this impor- tant matter. Each of us has a fair idea of what a mile is. It is a quarter of an hour's sharp walk, for instance ; or yonder village or building, we know, lies such and such a number of miles away. The measurements which have already been given of the diameters of the various bodies of the solar system appear very great to us, who find that a walk of a few miles at a time taxes our strength ; but they are a mere nothing wheri we consider the distances from the sun at which the various planets revolve in their orbits. The following table gives these distances in round numbers. As here stated they are what are called ''mean" distances; for, as the orbits are oval, the planets vary in their distances from the sun, and 46 Celestial Distances we are therefore obliged to strike a kind of average for each case : — miles. Mercury about 36,000,000 Venus }5 67,200,000 Earth 55 92,900,000 Mars 55 141,500,000 Jupiter 5J 483,300,000 Saturn 55 886,000,000 Uranus 55 1,781,900,000 Neptune J) 2,791,600,000 From the above it will be seen at a glance that we have entered upon a still greater scale of distance than in dealing with the diameters of the various bodies of the system. In that case the distances were limited to thousands of miles ; in this, however, we have to deal with millions. A million being ten hundred thousand, it will be noticed that even the diameter of the huge sun is well under a million miles. How indeed are we to get a grasp of such distances, when those to which we are ordinarily accustomed — the few miles' walk, the little stretch of sea or land which we gaze upon around us — are so utterly minute in comparison ? The fact is, that though men may think that they can picture in their minds such immense distances, they actually can not. In matters like these we unconsciously employ a kind of convention, and we estimate a thing as being two or three or more times the size of another. More than this we are unable to do. For instance, our ordinary experience of a mile enables us to judge, in a way, of a stretch of several miles, such 47 Astronomy of To-day as one can take in with a glance ; but in our estima- tion of a thousand miles, or even of one hundred, we are driven back upon a mental trick, so to speak. In our attempts to realise such immense distances as those in the solar system we are obliged to have recourse to analogies ; to comparisons with other and simpler facts, though this is at the best a mere self- cheating device. The analogy which seems most suited to our purpose here, and one which has often been employed by writers, is borrowed from the rate at which an express train travels. Let us imagine, for instance, that we possess an express train which is capable of running anywhere, never stops, never requires fuel, and always goes along at sixty miles an hour. Suppose we commence by employing it to gauge the size of our own planet, the earth. Let us send it on a trip around the equator, the span of which is about 24,000 miles. At its sixty- miles-an-hour rate of going, this journey will take nearly 17 days. Next let us send it from the earth lo the moon. This distance, 240,000 miles, being ten times as great as the last, will of course take ten times as long to cover, namely, 170 days ; that is to say, nearly half a year. Again, let us send it still further afield, to the sun, for example. Here, however, it enters upon a journey which is not to be measured in thousands of miles, as the others were, but in millions. The distance from the earth to the sun, as we have seen in the foregoing table, is about 93 millions of miles. Our express train would take about i']^ years to traverse this. Having arrived at the sun, let us suppose that our 48 Celestial Distances train makes a tour right round it. This will take more than five years. Supposing, finally, that our train were started from the sun, and made to run straight out to the known boundaries of the solar system, that is to say, as far as the orbit of Neptune, it would take over 5000 years to traverse the distance. That sixty miles an hour is a very great speed any one, I think, will admit who has stood upon the plat- form of a country station while one of the great mail trains has dashed past. But are not the immensities of space appaUing to contemplate, when one realises that a body moving incessantly at such a rate would take so long as 10,000 years to traverse merely the breadth of our solar system ? Ten thousand years ! Just try to conceive it. Why, it is only a little more than half that time since the Pyramids were built, and they mark for us the Dawn of History. And since then half-a-dozen mighty empires have come and gone ! Having thus concluded our general survey of the appearance and dimensions of the solar system, let us next inquire into its position and size in relation to what we call the Universe. A mere glance at the night sky, when it is free from clouds, shows us that in every direction there are stars ; and this holds good, no matter what portion of the globe we visit. The same is really true of the sky by day, though in that case we cannot actually see the stars, for their light is quite overpowered by the dazzling light of the sun. We thus reach the conclusion that our earth, that our solar system in fact, lies plunged within the midst 49 D Astronomy of To-day of a great tangle of stars. What position, by the way, do we occupy in this mighty maze ? Are we at the centre, or anywhere near the centre, or where ? It has been indeed amply proved by astronomical research that the stars are bodies giving off a light of their own, just as our sun does ; that they are in fact suns, and that our sun is merely one, perhaps indeed a very unimportant member, of this great universe of stars. Each of these stars, or suns, besides, may be the centre of a system similar to what we call our solar system, comprising planets and satellites, comets and meteors ; — or perchance indeed some further variety of attendant bodies of which we have no ex- ample in our tiny corner of space. But as to w^hether one is right in a conjecture of this kind, there is up to the present no proof whatever. No telescope has yet shown a planet in attendance upon one of these distant suns ; for such bodies, even if they do exist, are entirely out of the range of our mightiest instru- ments. On what then can we ground such an as- sumption ? Merely upon analogy ; upon the common- sense deduction that as the stars have characteristics similar to our particular star, the sun, it would seem unlikely that ours should be the only such body in the whole of space which is attended by a planetary system. ^' The Stars/' using that expression in its most general sense, do not lie at one fixed distance from us, set here and there upon a background of sky. There is in fact no background at all. The brilliant orbs are all around us in space, at different distances from us and from each other ; and w^e can gaze between them out into the blackness of the void 50 Celestial Distances which, perhaps, continues to extend unceasingly long after the very outposts of the stellar universe has been left behind. Shall we then start our imaginary express train once more, and send it out towards the nearest of the stars ? This would, however, be a use- less experiment. Our express-train method of gauging space would fail miserably in the attempt to bring home to us the mighty gulf by which we are now faced. Let us therefore halt for a moment and look back upon the orders of distance with which we have been deahng. First of all we dealt with thousands of miles. Next we saw how they shrank into insignifi- cance when we embarked upon millions. We found, indeed, that our sixty-mile-an-hour train, rushing along without ceasing, would consume nearly the whole of historical time in a journey from the sun to Neptune. In the spaces beyond the solar system we are faced, however, by a new order of distance. From sun to planets is measured in millions of miles, but from sun to sun is measured in bilHons. But does the mere stating of this fact convey anything ? I fear not. For the word ^'billion" runs as glibly off the tongue as "million," and both are so wholly unrealisable by us that the actual difference between them might easily pass unnoticed. Let us, however, make a careful comparison. What is a million ? It is a thousand thousands. But what is a billion ? It is a million millions. Consider for a moment ! A million of millions. That means a million, each unit of which is again a million. In fact every separate ** i " in this million is itself a million. Here is a way of trying to realise this 51 Astronomy of To-day gigantic number. A million seconds make only eleven and a half days and nights. But a billion seconds will make actually more than thirty thousand years ! Having accepted this, let us try and probe with our express train even a little of the new gulf which now lies before us. At our old rate of going it took almost two years to cover a million miles. To cover a billion miles — that is to say, a million times this distance — would thus take of course nearly two million years. Alpha Centauri, the nearest star to our earth, is some twenty-five billions of miles away. Our express train would thus take about fifty millions of years to reach it I This shows how useless our illustration, appropriate though it seemed for interplanetary space, becomes when applied to the interstellar spaces. It merely gives us millions in return for billions ; and so the mind, driven in upon itself, whirls round and round Hke a squirrel in its revolving cage. There is, how- ever, a useful illustration still left us, and it is the one which astronomers usually employ in dealing with the distances of the stars. The illustration in question is taken from the velocity of light. Light travels at the tremendous speed of about 186,000 miles a second. It therefore takes only about a second and a quarter to come to us from the moon. It traverses the 93,000,000 of miles which separate us from the sun in about eight minutes. It travels from the sun out to Neptune in about four hours, which means that it would cross the solar system from end to end in eight. To pass, however, across the distance which separates us from Alpha Centauri 52 Celestial Distances it would take so long as about four and a quarter years ! Astronomers, therefore, agree in estimating the distances of the stars from the point of view of the time which light would take to pass from them to our earth. They speak of that distance which light takes a year to traverse as a ^^ light year." According to this notation, Alpha Centauri is spoken of as being about four and a quarter light years distant from us. Now as the rays of light coming from Alpha Cen- tauri to us are chasing one another incessantly across the gulf of space, and as each ray left that star some four years before it reaches us, our view of the star itself must therefore be always some four years old. Were then this star to be suddenly removed from the universe at any moment, we should continue to see it still in its place in the sky for some four years more, after which it would suddenly disappear. The rays which had already started upon their journey towards our earth must indeed continue travelling, and reach- ing us in their turn until the last one had arrived ; after which no more would come. We have drawn attention to Alpha Centauri as the nearest of the stars. The majority of the others in- deed are ever so much farther. We can only hazard a guess at the time it takes for the rays from many of them to reach our globe. Suppose, for instance, we see a sudden change in the light of any of these re- mote stars, we are inclined to ask ourselves when that change did actually occur. Was it in the days of Queen Elizabeth, or at the time of the Norman Con- quest; or was it when Rome was at the height of her glory, or perhaps ages before that when the Pyramids 53 Astronomy of To-day of Egypt were being built ? Even the last of these suppositions cannot be treated lightly. We have indeed no real knowledge of the distance from us of those stars which our giant telescopes have brought into view out of the depths of the celestial spaces. 54 CHAPTER VI CELESTIAL MEASUREMENT Had the telescope never been invented our knowledge of astronomy would be trifling indeed. Prior to the year 1610, when Galileo first turned the new instrument upon the sky, all that men knew of the starry realms was gathered from observation with their own eyes unaided by any artificial means. In such researches they had been very much at a disadvantage. The sun and moon, in their opinion, were no doubt the largest bodies in the heavens, for the mere reason that they looked so ! The mighty solar disturbances, which are now such common- places to us, were then quite undreamed of. The moon displayed a patchy surface, and that was all ; her craters and ring-mountains were surprises as yet in store for men. Nothing of course was known about the surfaces of the planets. These objects had indeed no particular characteristics to distinguish them from the great host of the stars, except that they con- tinually changed their positions in the sky while the rest did not. The stars themselves were considered as fixed inalterably upon the vault of heaven. The sun, moon, and planets apparently moved about in the intermediate space, supported in their courses by strange and fanciful devices. The idea of satellites was as yet unknown. Comets were regarded as 55 Astronomy of To-day celestial portents, and meteors as small conflagrations taking place in the upper air. In the entire absence of any knowledge with regard to the actual sizes ^and distances of the various celes- tial bodies, men naturally considered them as small ; and, concluding that they \vere comparatively near, assigned to them in consequence a permanent con- nection with terrestial affairs. Thus arose the quaint and erroneous beliefs of astrology, according to which the events which took place upon our earth were con- sidered to depend upon the various positions in which the planets, for instance, found themselves from time to time. It must, however, be acknowledged that the study of astrology, fallacious though its conclusions were, indirectly performed a great service to astronomy by reason of the accurate observations and diligent study of the stars which it entailed. We will now inquire into the means by which the distances and sizes of the celestial orbs have been ascertained, and see how it was that the ancients were so entirely in the dark in this matter. There are two distinct methods of finding out the distance at which any object happens to be situated from us. One method is by actual measurement. The other is by moving oneself a little to the right or left, and observing whether the distant object appears in any degree altered in position by our own change of place. One of the best illustrations of this relative change of position which objects undergo as a result of our own change of place, is to observe the landscape from the 56 Celestial Measurement window of a moving railway carriage. As we are borne rapidly along we notice that the telegraph posts which are set close to the line appear to fly past us in the contrary direction ; the trees, houses, and other things beyond go by too, but not so fast; objects a good way off displace slowly ; while some spire, or tall landmark, in the far distance appears to remain unmoved during a comparatively long time. Actual change of position on our own part is found indeed to be invariably accompanied by an apparent displacement of the objects about us, such apparent displacement as a result of our own change of posi- tion being known as ^^ parallax." The dependence between the two is so mathematically exact, that if we know the amount of our own change of place, and if we observe the amount of the consequent displacement of any object, we are enabled to cal- culate its precise distance from us. Thus it comes to pass that distances can be measured without the necessity of moving over them; and the breadth of a river, for instance, or the distance from us of a ship at sea, can be found merely by such means. It is by the application of this principle to the wider field of the sky that we are able to ascertain the distance of celestial bodies. We have noted that it requires a goodly change of place on our own part to shift the position in which some object in the far distance is seen by us. To two persons separated by, say, a few hundred yards, a ship upon the horizon will appear pretty much in the same direction. They would require, in fact, to be much farther apart in order to displace it sufficiently for the purpose of estimating their distance from it. It 57 Astronomy of To-day is the same with regard to the moon. Two observers, standing upon our earth, will require to be some thousands of miles apart in order to see the position of our satellite sufficiently altered with regard to the starry background, to give the necessary data upon which to ground their calculations. The change of position thus offered by one side of the earth's surface at a time is, however, not suffi- cient to displace any but the nearest celestial bodies. When we have occasion to go farther afield we have to seek a greater change of place. This we can get as a consequence of the earth's movement around the sun. Observations, taken several days apart, will show the effect of the earth's change of place during the interval upon the positions of the other bodies of our system. But when we desire to sound the depths of space beyond, and to reach out to measure the distance of the nearest star, we find ourselves at once thrown upon the greatest change of place which we can possibly hope for ; and this we get during the long journey of many millions of miles which our earth performs around the sun during the course of each year. But even this last change of place, great as it seems in comparison with ter- restial measurements, is insufficient to show any- thing more than the tiniest displacements in a paltry forty-three out of the entire host of the stars. We can thus realise at what a disadvantage the ancients were. The measuring instruments at their command were utterly inadequate to detect such small displacements. It was reserved for the tele- scope to reveal them ; and even then it required the great telescopes of recent times to show the 58 Celestial Measurement slight changes in the position of the nearer stars, which were caused by the earth's being at one time at one end of its orbit, and some six months later at the other end — stations separated from each other by a gulf of about one hundred and eighty-six millions of miles. The actual distances of certain celestial bodies being thus ascertainable, it becomes a matter of no great difficulty to determine the actual sizes of the measurable ones. It is a matter of every-day ex- perience that the size which any object appears to have, depends exactly upon the distance it is from us. The farther off it is the smaller it looks ; the nearer it is the bigger. If, then, an object which lies at a known distance from us looks such and such a size, we can of course ascertain its real dimensions. Take the moon, for instance. As we have already shown, we are able to ascertain its distance. We observe also that it looks a certain size. It is therefore only a matter of calculation to find what its actual dimensions should be, in order that it may look that size at that distance away. Similarly we can ascertain the real dimensions of the sun. The planets, appearing to us as points of light, seem at first to offer a difficulty ; but, by means of the telescope, we can bring them, as it were, so much nearer to us, that their broad ex- panses may be seen. We fail, however, signally with regard to the stars ; for they are so very distant, and therefore such tiny points of light, that our mightiest telescopes cannot magnify them sufficiently to show any breadth of surface. Instead of saying that an object looks a certain 59 Astronomy of To-day breadth across, such as a yard or a foot, a statement which would really mean nothing, astronomers speak of it as measuring a certain angle. Such angles are estimated in what are called '' degrees of arc " ; each degree being divided into sixty minutes, and each minute again into sixty seconds. Popularly con- sidered the moon and sun look about the same size, or, as an astronomer would put it, they measure about the same angle. This is an angle, roughly, of thirty-two minutes of arc ; that is to say, slightly more than half a degree. The broad expanse of surface which a celestial body shows to us, whether to the naked eye, as in the case of the sun and moon, or in the telescope, as in the case of other members of our system, is technically known as its '' disc." 60 CHAPTER VII ECLIPSES AND KINDRED PHENOMENA Since some members of the solar system are nearer to us than* others, and all are again much nearer than any of the stars, it must often happen that one celestial body will pass between us and another, and thus intercept its light for a while. The moon, being the nearest object in the universe, will, of course, during its motion across the sky, temporarily blot out every one of the others which happen to lie in its path. When it passes in this manner across the face of the sun, it is said to eclipse it. When it thus hides a planet or star, it is said to occult it. The reason .why a separate term is used for what is merely a case ;of obscuring light in exactly the same way, will be plain when one considers that the disc of the sun is almost of the same apparent size as that of the moon, and so the complete hiding of the sun can last but a few minutes at the most ; whereas a planet or a star looks so very small in comparison, that it is always entirely swallowed up for so7ne time when it passes behind the body of our satellite. The sun, of course, occults planets and stars in exactly the same manner as the moon does, but we cannot see these occupations on account of the blaze of sunlight. By reason of the small size which the planets look 6i Astronomy of To-day when viewed with the naked eye, we are not able to note them in the act of passing over stars and so blotting them out ; but such occurrences may be seen in the telescope, for the planetary bodies then display broad discs. There is yet another occurrence of the same class which is known as a transit. This takes place when an apparently small body passes across the face of an apparently large one, the phenomenon being in fact the exact reverse of an occultation. As there is no appreciable body nearer to us than the moon, we can never see anything in transit across her disc. But since the planets Venus and Mercury are both nearer to us than the sun, they will occasionally be seen to pass across his face, and thus we get the well-known phenomena called Transits of Venus and Transits of Mercury. As the satellites of Jupiter are continually revolving around him, they will often pass behind or across his disc. Such occultations and transits of satellites can be well observed in the telescope. There is, however, a way in which the light of a celestial body may be obscured without the necessity of its being hidden from us by one nearer. It will no doubt be granted that any opaque object casts a shadow when a strong light falls directly upon it. Thus the earth, under the powerful light which is directed upon it from the sun, casts an extensive shadow, though we are not aware of the existence of this shadow until it falls upon something. The shadow which the earth casts is indeed not noticeable to us until some celestial body passes into it. As the sun is very large, and the earth in comparison very 62 Eclipses and Kindred Phenomena small; the shadow thrown by the earth is compara- tively short, and reaches out in space for only about a million miles. There is no visible object except the moon, which circulates within that distance from our globe, and therefore she is the only body which can pass into this shadow. Whenever such a thing happens, her surface at once becomes dark, for the reason that she never emits any light of her own, but merely reflects that of the sun. As the moon is continually revolving around the earth, one would be inclined to imagine that once in every month, namely at what is C2L\led full moon, when she is on the other side of the earth with respect to the sun, she ought to pass through the shadow in question. But this does not occur every time, because the moon's orbit is not quite zipon the same plane with the earth's. It thus happens that time after time the moon passes clear of the earth's shadow, sometimes above it, and sometimes below it. It is indeed only at intervals of about six months that the moon can be thus obscured. This darkening of her light is known as an eclipse of the moon. It seems a great pity that custom should oblige us to employ the one term ^'eclipse" for this and also for the quite different occurrence, an eclipse of the sun ; in which the sun's face is hidden as a consequence of the moon's body coming directly between it and our eyes. The popular mind seems always to have found it more difficult to grasp the causes of an eclipse of the moon than an eclipse of the sun. As Mr. J. E. Gore^ puts it: '^The darkening of the sun's light by the interposition of the moon's body seems more ^ Astronomical Essays {'^. 40), London, 1907. 63 Astronomy of To-day obvious than the passing of the moon through the earth's shadow." Eclipses of the moon furnish striking spectacles, but really add little to our knowledge. They exhibit, however, one of the most remarkable evidences of the globular shape of our earth ; for the outline of its shadow when seen creeping over the moon's surface is always circular. Eclipses of the Moon, or Lunar Eclipses, as they are also called, are of two kinds — Totals and Partial. Earlh Fig. 3. — Total and Partial Eclipses of the Moon. The Moon is here shown in two positions ; i.e. entirely plunged in the earth's shadow and therefore totally eclipsed, and only partly plunged in it or partially eclipsed. In a total lunar eclipse the moon passes entirely into the earth's shadow, and the whole of her surface is consequently darkened. This darkening lasts for about two hours. In a partial lunar eclipse, a portion only of the moon passes through the shadow^, and so only part of her surface is darkened (see Fig. 3). A very striking phenomenon during a total eclipse of the moon^ is that the darkening of the lunar surface is usually by no means so intense as one would expect, w^hen one considers that the sunlight at that time should be wholly cut off from it. The occasions indeed upon which the moon has com- 64 Eclipses and Kindred Phenomena pletely disappeared from view during the progress of a total lunar eclipse are very rare. On the majority of these occasions she has appeared of a coppery-red colour, while sometimes she has assumed an ashen hue. The explanations of these variations of colour is to be found in the then state of the atmosphere which surrounds our earth. When those portions of our earth's atmosphere through which the sun's rays have to filter on their way towards the moon are free from watery vapour, the lunar surface will be tinged with a reddish light, such as we ordinarily experience at sunset when our air is dry. The ashen colour is the result of our atmosphere being laden with watery vapour, and is similar to what we see at sunset when rain is about. Lastly, when the air around the earth is thickly charged with cloud, no light at all can pass ; and on such occasions the moon disappears altogether for the time being from the night sky. Eclipses of the Sufiy otherwise known as Solar Eclipses, are divided into Totals Partial, and Annular. A total eclipse of the sun takes place when the moon comes between the sun and the earth, in such a manner that it cuts off the sunlight entirely for the time being from 2, portion of the earth's surface. A person situated in the region in question will, there- fore, at that moment find the sun temporarily blotted out from his view by the body of the moon. Since the moon is a very much smaller body than the sun, and also very much the nearer to us of the two, it will readily be understood that the portion of the earth from which the sun is seen thus totally eclipsed will be of small extent. In places not very distant 65 E Astronomy of To-day from this region, the moon will appear so much shifted in the sky that the sun will be seen onl}^ partially eclipsed. The moon being in constant movement round the earth, the portion of the earth's surface from which an eclipse is seen as total will be always a comparatively narrow band lying roughly from west to east. This band, known as the track of totality^ can, at the utmost, never be more than about 165 miles in width, and as a rule is very much less. For about 2000 miles on either side of it the sun is seen partially eclipsed. Outside these limits no eclipse of any kind is visible, as from such regions the moon is not seen to come in the way of the sun (see Fig. 4 (i.), p. 6^), It may occur to the reader that eclipses can also take place in the course of which the positions, where the eclipse would ordinarily be seen as total, will lie outside the surface of the earth. Such an eclipse is thus not dignified with the name of total eclipse, but is called 2, partial eclipse, because from the earth's surface the sun is only seen partly eclipsed at the utmost (see Fig. 4 (ii.), p. 67). An Annular eclipse is an eclipse which just fails to become total for yet another reason. We have pointed out that the orbits of the various members of the solar system are not circular, but oval. Such oval figures, it will be remembered, are technically known as ellipses. In an elliptic orbit the controlling body is situated not in the middle of the figure, but rather towards one of the ends ; the actual point which it occupies being known as the focus. The sun being at the focus of the earth's orbit, it follows that the earth is, at times, a little nearer to him than 66 Eclipses and Kindred Phenomena at others. The sun will therefore appear to us to vary a little in size, looking sometimes slightly larger than at other times. It is so, too, with the moon, at the focus of whose orbit the earth is situated. She therefore also appears to us at times to vary slightly in size. The result is that when the sun is eclipsed (i.) Total Eclipse of the Sun. Moon (ii, ) Partial Eclipse of the Sun. Fig. 4. — ^Total and Partial Eclipses of the Sun. From the position A the Sun cannot be seen, as it is entirely blotted out by the Moon. From B it is seen partially blotted out, because the Moon is to a certain degree in the way. From C no eclipse is seen, because the Moon does not come in the way. It is to be noted that in a Partial Eclipse of the Sun, the position A lies outside the surface of the Earth. by the moon, and the moon at the time appears the larger of the two, she is able to blot out the sun completely, and so we can get a total eclipse. But when, on the other hand, the sun appears the larger, the eclipse will not be quite total, for a portion of the sun's disc will be seen protruding all around the moon like a ring of light. This is what is known as 67 Astronomy of To-day an annular eclipse, from the Latin word annulusj which means a ring. The term is consecrated by long usage, but it seems an unfortunate one on account of its similarity to the word '^ annual." The Germans speak of this kind of eclipse as ^' ring- formed," which is certainly much more to the point. There can never be a year without an eclipse of the sun. Indeed there must be always two such eclipses at least during that period, though there need be no eclipse of the moon at all. On the other hand, the greatest number of eclipses which can ever take place during a year are seven ; that is to say, either five solar eclipses and two lunar, or four solar and three lunar. This general statement refers merely to eclipses in their broadest significance, and informs us in no way whether they will be total or partial. Of all the phenomena which arise from the hiding of any celestial body by one nearer coming in the way, a total eclipse of the sun is far the most im- portant. It is, indeed, interesting to consider how much poorer modern astronomy would be but for the extraordinary coincidence which makes a total solar eclipse just possible. The sun is about 400 times farther off from us than the moon, and enormously greater than her in bulk. Yet the two are relatively so distanced from us as to look about the same size. The result of this is that the moon, as has been seen, can often blot out the sun entirely from our view for a short time. When this takes place the great blaze of sunlight which ordinarily dazzles our eyes is completely cut off, and we are thus enabled, unimpeded, to note what is going on in the immediate vicinity of the sun itself. 68 Eclipses and Kindred Phenomena In a total solar eclipse, the time which elapses from the moment when the moon's disc first begins to impinge upon that of the sun at his western edge until the eclipse becomes total, lasts about an hour. During all this time the black lunar disc may be watched making its way steadily across the solar face. Notwithstanding the gradual obscuration of the sun, one does not notice much diminution of light until about three-quarters of his disc are covered. Then a wan, unearthly appearance begins to pervade all things, the temperature falls noticeably, and nature seems to halt in expectation of the coming of something unusual. The decreasing portion of sun becomes more and more narrow, until at length it is reduced to a crescent-shaped strip of exceeding fine- ness. Strange, ill-defined, flickering shadows (known as " Shadow Bands ") may at this moment be seen chasing each other across any white expanse such as a wall, a building, or a sheet stretched upon the ground. The western side of the sky has now assumed an appearance dark and lowering, as if a rainstorm of great violence were approaching. This is caused by the mighty mass of the lunar shadow sweeping rapidly along. It flies onward at the terrific velocity, of about half a mile a second. If the gradually diminishing crescent of sun be now watched through a telescope, the observer will notice that it does not eventually vanish all at once, as he might have expected. Rather, it breaks up first of all along its length into a series of brilliant dots, known as '^ Baily's Beads." The reason of this phenomenon is perhaps not entirely agreed upon, but the majority of astronomers incline to the opinion 69 Astronomy of To-day that the so-called ''beads" are merely the last rem- nants of sunlight peeping between those lunar mountain peaks which happen at the moment to fringe the advancing edge of the moon. The beads are no sooner formed than they rapidly disappear one after the other, after which no portion of the solar surface is left to view, and the eclipse is now total (see Fig. 5). /n a total Eclipse /n an annu/arFclipse Fig. 5.—" Baily's Beads." But with the disappearance of the sun there springs into view a new and strange appearance, ordinarily unseen because of the blaze of sunlight. It is a kind of aureole, or halo, pearly white in colour, which is seen to surround the black disc of the moon. This white radiance is none other than the celebrated phenomenon widely known as the Solar Corona. It was once upon a time thought to belong to the moon, and to be perhaps a lunar atmosphere illumi- nated by the sunlight shining through it from behind. But the suddenness with which the moon always blots out stars when occulting them, has amply 70 Eclipses and Kindred Phenomena proved that she possesses no atmosphere worth speaking about. It is now, however, satisfactorily determined that the corona belongs to the sun, for during the short time that it remains in view the black body of the moon can be seen creeping across it. All the time that the total phase (as it is called) lasts, the corona glows with its pale unearthly light, shedding upon the earth's surface an illumination somewhat akin to full moonlight. Usually the planet Venus and a few stars shine out the while in the darkened heaven. Meantime around the observer animal and plant life behave as at nightfall. Birds go to roost, bats fly out, worms come to the surface of the ground, flowers close up. In the Norwegian eclipse of 1896 fish were seen rising to the surface of the water. When the total phase at length is over, and the moon in her progress across the sky has allowed the brilliant disc of the sun to spring into view once more at the other side, the corona disappears. There is another famous accompaniment of the sun which partly reveals itself during total solar eclipses. This is a layer of red flame which closely envelops the body of the sun and lies between it and the corona. This layer is known by the name of the Chromosphere. Just as at ordinary times we cannot see the corona on account of the blaze of sunlight, so are we likewise unable to see the chromo- sphere because of the dazzling white light which shines through from the body of the sun underneath and completely overpowers it. When, however, during a solar eclipse, the lunar disc has entirely hidden the brilliant face of the sun, we are still able for a few moments to see an edgewise portion of the 71 Astronomy of To-day chromosphere in the form of a narrow red strip, fringing the advancing border of the moon. Later on, just before the moon begins to uncover the face of the sun from the other side, we may again get a view of a strip of chromosphere. The outer surface of the chromosphere is not by any means even. It is rough and billowy, like the sur- face of a storm-tossed sea. Portions of it, indeed, rise at times to such heights that they may be seen stand- ing out like blood-red points around the black disc of the moon, and remain thus during a good part of the total phase. These projections are known as the Solar Prominences. In the same way as the corona, the chromosphere and prominences were for a time supposed to belong to the moon. This, however, was soon found not to be the case, for the lunar disc was noticed to creep slowly across them also. The total phase, or "totality," as it is also called, lasts for different lengths of time in different eclipses. It is usually of about two or three minutes' duration, and at the utmost it can never last longer than about eight minutes. When totality is over and the corona has faded away, the moon's disc creeps little by little from the face of the sun, light and heat returns once more to the earth, and nature recovers gradually from the gloom in which she has been plunged. About an hour after totality, the last remnant of moon draws away from the solar disc, and the eclipse is entirely at an end. The corona, the chromosphere, and the promi- nences are the most important of these accompani- ments of the sun which a total eclipse reveals to us. 72 Eclipses and Kindred Phenomena Our further consideration of them must, however, be reserved for a subsequent chapter, in which the sun will be treated of at length. Every one who has had the good fortune to see a total eclipse of the sun will, the writer feels sure, agree with the verdict of Sir Norman Lockyer that it is at once one of the "grandest and most awe- inspiring sights " which man can witness. Needless to say, such an occurrence used to cause great con- sternation in less civilised ages ; and that it has not in modern times quite parted with its terrors for some persons, is shown by the fact that in Iowa, in the United States, a woman died from fright during the eclipse of 1869. To the serious observer of a total solar eclipse every instant is extremely precious. Many distinct observations have to be crowded into a time all too limited, and this in an eclipse-party necessitates constant rehearsals in order that not a moment may be wasted when the longed-for totality arrives. Such preparation is very necessary ; for the rarity and uncommon nature of a total eclipse of the sun, coupled with its exceeding short duration, tends to flurry the mind, and to render it slow to seize upon salient points of detail. And, even after every pre- caution has been taken, weather possibilities remain to be reckoned with, so that success is rather a lottery. Above all things, therefore, a total solar eclipse is an occurrence for the proper utilisation of which personal experience is of absolute necessity. It was manifestly out of the question that such experience could be gained by any individual in early times, 73 Astronomy of To-day as the imperfection of astronomical theory and geographical knowledge rendered the predicting of the exact position of the track of totality well-nigh impossible. Thus chance alone would have enabled one in those days to witness a total phase, and the probabilities, of course, were much against a second such experience in the span of a life-time. And even in more modern times, when the celestial motions had come to be better understood, the difficulties of foreign travel still were in the way ; for it is, indeed, a notable fact that during many years following the invention of the telescope the tracks were placed for the most part in far-off regions of the earth, and Europe was visited by singularly few total solar eclipses. Thus it came to pass that the building up of a body of organised knowledge upon this subject was greatly delayed. Nothing perhaps better shows the soundness of modern astronomical theory than the almost exact agreement of the time predicted for an eclipse with its actual occurrence. Similarly, by calculating back- wards, astronomers have discovered the times and seasons at which many ancient eclipses took place, and valuable opportunities have thus arisen for checking certain disputed dates in history. It should not be omitted here that the ancients were actually able, in a rough way, to predict eclipses. The Chaldean astronomers had indeed noticed very early a curious circumstance, i.e, that eclipses tend to repeat themselves after a lapse of slightly more than eighteen years. In this connection it must, however, be pointed out, in the first instance, that the eclipses which 74 Eclipses and Kindred Phenomena occur in any particular year are in no way associated with those which occurred in the previous year. In other words, the mere fact that an eclipse takes place upon a certain day this year will not bring about a repetition of it at the same time next year. However, the nicely balanced behaviour of the solar system, an equilibrium resulting from aeons of orbital ebb and flow, naturally tends to make the members which compose that family repeat their ancient com- binations again and again ; so that after definite lapses of time the same order of things will almost exactly recur. Thus, as a consequence of their beautifully poised motions, the sun, the moon, and the earth tend, after a period of i8 years and io\ days,^ to occupy very nearly the same positions with regard to each other. The result of this is that, during each recurring period, the eclipses comprised within it will be repeated in their order. To give examples : — The total solar eclipse of August 30, 1905, was a repetition of that of August 19, 1887. The partial solar eclipse of February 23, 1906, corresponded to that which took place on February II, 1888. The annular eclipse of July 10, 1907, was a recur- rence of that of June 28, 1889. In this way we can go on until the eighteen year cycle has run out, and we come upon a total solar 1 In some cases the periods between the dates of the corresponding eclipses appem- to include a greater number of days than ten ; but this is easily explained when allowance is made for intervening leap years (in each of which an extra day has of course been added), and also for variations in local time. 75 Astronomy of To-day eclipse predicted for September lo, 1923, which will repeat the above-mentioned ones of 1905 and 1887 ; and so on too with the others. From mere observation alone, extending no doubt over many ages, those time-honoured watchers of the sky, the early Chaldeans, had arrived at this remarkable generalisation ; and they used it for the rough prediction of eclipses. To the period of re- currence they give the name of ^^ Saros." And here we find ourselves led into one of the most interesting and fascinating by-paths in as- tronomy, to which writers, as a rule, pay all too little heed. In order not to complicate matters unduly, the recurrence of solar eclipses alone will first be dealt with. This limitation will, however, not affect the arguments in the slightest, and it will be all the more easy in consequence to show their application to the case of eclipses of the moon. The reader will perhaps have noticed that, with regard to the repetition of an eclipse, it has been stated that the conditions which bring it on at each recurrence are reproduced almost exactly. Here, then, lies the crux of the situation. For it is quite evident that were the conditions exactly reproduced, the re- currences of each eclipse would go on for an in- definite period. For instance, if the lapse of a saros period found the sun, moon, and earth again in the precise relative situations which they had previously occupied, the recurrences of a solar eclipse would tend to duplicate its forerunner with regard to the position of the shadow upon the terrestrial surface. But the conditions not being exactly reproduced, the 76 Eclipses and Kindred Phenomena shadow-track does not pass across the earth in quite the same regions. It is shifted a little, so to speak ; and each time the eclipse comes round it is found to be shifted a little farther. Every solar eclipse has therefore a definite ^Mife" of its own upon the earth, lasting about 1150 years, or 64 saros returns, and working its way little by little across our globe from north to south, or from south to north, as the case may be. Let us take an imaginary example. A partial eclipse occurs, say, somewhere near the North Pole, the edge of the ^' partial " shadow just grazing the earth, and the ^' track of totality" being as yet cast into space. Here we have the beginning of a series. At each saros recurrence the partial shadow encroaches upon a greater extent of earth-surface. At length, in its turn, the track of totality begins to impinge upon the earth. This track streaks across our globe at each return of the eclipse, repeating itself every time in a slightly more southerly latitude. South and south it moves, pass- ing in turn the Tropic of Cancer, the Equator, the Tropic of Capricorn, until it reaches the South Pole ; after which it touches the earth no longer, but is cast into space. The rear portion of the partial shadow, in its turn, grows less and less in extent ; and it too in time finally passes off. Our imaginary eclipse series is now no more — its '' life " has ended. We have taken, as an example, an eclipse series moving from north to south. We might have taken one moving from south to north, for they progress in either direction. From the description just given the reader might 77 Astronomy of To-day suppose that, if the tracks of totality of an eclipse series were plotted upon a chart of the world, they would lie one beneath another like a set of steps. This is, however, not the case, and the reason is easily found. It depends upon the fact that the saros does not comprise an exact number of days, but includes, as we have seen, one-third of a day in addition. It will be granted, of course, that if the number of days was exact, the same parts of the earth would always be brought round by the axial rotation to front the sun at the moment of the recurrence of the eclipse. But as there is still one-third of a day to complete the saros period, the earth has yet to make one-third of a rotation upon its axis before the eclipse takes place. Thus at every recurrence the track of totality finds itself placed one-third of the earth's circumference to the westward. Three of the recur- rences will, of course, complete the circuit of the globe ; and so the fourth recurrence will duplicate the one which preceded it, three saros returns, or 54 years and i month before. This duplication, as we have already seen, will, however, be situated in a latitude to the south or north of its predecessor, according as the eclipse series is progressing in a southerly or northerly direction. Lastly, every eclipse series, after working its way across the earth, will return^ again to go through the same process after some 12,000 years ; so that, at the end of that great lapse of time, the entire ^' life " of every eclipse should repeat itself, provided that the conditions of the solar system have not altered ap- preciably during the interval. 78 Eclipses and Kindred Phenomena We are now in a position to consider this gradual southerly or northerly progress of eclipse recurrences in its application to the case of eclipses of the moon. It should be evident that, just as in solar eclipses the lunar shadow is lowered or raised (as the case may be) each time it strikes the terrestrial surface, so in lunar eclipses will the body of the moon shift its place at each recurrence relatively to the position of the earth's shadow. Every lunar eclipse, therefore, will commence on our satellite's disc as a partial eclipse at the northern or southern extremity, as the case may be. Let us take, as an example, an imagi- nary series of eclipses of the moon progressing from north to south. At each recurrence the partial phase will grow greater, its boundary encroaching more and more to the southward, until eventually the whole disc is enveloped by the shadow, and the eclipse becomes total. It will then repeat itself as total during a number of recurrences, until the entire breadth of the shadow has been passed through, and the northern edge of the moon at length springs out into sunlight. This illuminated portion will grow more and more extensive at each succeeding return, the edge of the shadow appearing to recede from it until it finally passes off at the south. Similarly, when a lunar eclipse commences as partial at the south of the moon, the edge of the shadow at each subsequent recurrence finds itself more and more to the northward. In due course the total phase will supervene, and will persist during a number of re- currences until the southerly trend of the moon results in the uncovering of the lunar surface at the south. Thus, as the boundary of the shadow is left 79 Astronomy of To-day more and more to the northward, the illuminated portion on the southern side of the moon becomes at each recurrence greater and the darkened portion on the northern side less, until the shadow eventually passes off at the north. The ^Mife" of an eclipse of the moon happens, for certain reasons, to be much shorter than that of an eclipse of the sun. It lasts during only about 860 years, or 48 saros returns. Fig. 6, p. 81, is a map of the world on Mercator's Projection, showing a portion of the march of the total solar eclipse of August 30, 1905, across the sur- face of the earth. The projection in question has been employed because it is the one with which people are most familiar. This eclipse began by striking the neighbourhood of the North Pole in the guise of a partial eclipse during the latter part of the reign of Queen Elizabeth, and became total on the earth for the first 'time on the 24th of June 1797. Its next appearance was on the 6th of July 181 5. It has not been possible to show the tracks of totality of these two early visitations on account of the distortion of the polar regions consequent on the fiction of Mer- cator's Projection. It is therefore made to commence with the track of its third appearance, viz. on July 17, 1833. In consequence of those variations in the apparent sizes of the sun and moon, which result, as we have seen, from the variations in their distances from the earth, this eclipse will change from a total into an annular eclipse towards the end of the twenty- first century. By that time the track will have passed to the southern side of the equator. The track 80 Eclipses and Kindred Phenomena 8i F Astronomy of To-day will eventually leave the earth near the South Pole about the beginning of the twenty-sixth century, and the rear portion of the partial shadow will in its turn be clear of the terrestrial surface by about 2700 A.D., when the series comes to an end. 82 CHAPTER VIII FAMOUS ECLIPSES OF THE SUN What is thought to be the earhest reference to an edipse comes down to us from the ancient Chinese records^ and is over four thousand years old. The ecHpse in question was a solar one, and occurred, so far as can be ascertained, during the twenty-second century B.C. The story runs that the two state astronomers, Ho and Hi by name, being exceedingly intoxicated, were unable to perform their required duties, which consisted in superintending the custo- mary rites of beating drums, shooting arrows, and the like, in order to frighten away the mighty dragon which it was believed was about to swallow up the Lord of Day. This eclipse seems to have been only partial ; nevertheless a great turmoil ensued, and the two astronomers were put to death, no doubt with the usual celestial cruelty. The next eclipse mentioned in the Chinese annals is also a solar eclipse, and appears to have taken place more than a thousand years later, namely in 776 B.C. Records of similar eclipses follow from the same source ; but as they are mere notes of the events, and do not enter into any detail, they are of little interest. Curiously enough the Chinese have taken practically no notice of eclipses of the moon, but have left us a comparatively careful record of 83 Astronomy of To-day comets, which has been of value to modern astro- nomy. The earliest mention of a total eclipse of the sun (for it should be noted that the ancient Chinese eclipse above-mentioned was merely partial) was de- ciphered in 1905, on a very ancient Babylonian tablet, by Mr. L. W. King of the British Museum. This eclipse took place in the year 1063 B.C. Assyrian tablets record three solar eclipses which occurred between three and four hundred years later than this. The first of these was in 763 B.C. ; the total phase being visible near Nineveh. The next record of an eclipse of the sun comes to us from a Grecian source. This eclipse took place in 585 B.C.; and has been the subject of much in- vestigation. Herodotus, to whom we are indebted for the account, tells us that it occurred during a battle in a war which had been waging for some years between the Lydians and Medes. The sudden coming on of darkness led to a termination of the contest, and peace was afterwards made between the combatants. The historian goes on to state that the eclipse had been foretold by Thales, who is looked upon as the Founder of Grecian astronomy. This eclipse is in consequence known as the ^'Eclipse of Thales." It would seem as if that philosopher were acquainted with the Chaldean saros. The next solar eclipse worthy of note was an annular one, and occurred in 431 B.C., the first year of the Peloponnesian War. Plutarch relates that the pilot of the ship, which was about to convey Pericles to the Peloponnesus, was very much frightened by it ; but Pericles calmed him by holding up a cloak 84 Famous Eclipses of the Sun before his eyes, and saying that the only difference between this and the ecHpse was that something larger than the cloak prevented his seeing the sun for the time being. An eclipse of great historical interest is that known as the ^' Eclipse of Agathocles/' which occurred on the morning of the 15th of August, 310. B.C. Aga- thocles, Tyrant of Syracuse, had been blockaded in the harbour of that town by the Carthaginian fleet, but effected the escape of his squadron under cover of night, and sailed for Africa in order to invade the enemy's territory. During the following day he and his vessels experienced a total eclipse, in which 'May wholly put on the appearance of night, and the stars were seen in all parts of the sky." A few solar eclipses are supposed to be referred to in early Roman history, but their identity is very doubtful in comparison with those which the Greeks have recorded. Additional doubt is cast upon them by the fact that they are usually associated with famous events. The birth and death of Romulus, and the Passage of the Rubicon by Julius Caesar, are stated indeed to have been accompanied by these marks of the approval or disapproval of the gods ! Reference to our subject in the Bible is scanty. Amos viii. 9 is thought to refer to the Nineveh eclipse of 763 B.C., to which allusion has already been made ; while the famous episode of Hezekiah and the shadow on the dial of Ahaz has been connected with an eclipse which was partial at Jerusalem in 689 B.C. The first solar eclipse, recorded during the Christian Era, is known as the '^ Eclipse of Phlegon,"from the fact that we are indebted for the account to a pagan writer 8s Astronomy of To-day of that name. This eclipse took place in a.d. 29, and the total phase was visible a little to the north of Palestine. It has sometimes been confounded with the ^' darkness of the Crucifixion/' which event took place near the date in question ; but it is sufficient here to say that the Crucifixion is well known to have occurred during the Passover of the Jews, which is always celebrated at ih.Qfull moon, whereas an eclipse of the sun can only take place at new moon. Dion Cassius, commenting on the Emperor Claudius about the year A.D. 45, writes as follows : — ^'As there was going to be an eclipse on his birth- day, through fear of a disturbance, as there had been other prodigies, he put forth a public notice, not only that the obscuration would take place, and about the time and magnitude of it, but also about the causes that produce such an event." This is a remarkable piece of information ; for the Romans, an essentially military nation, appear hitherto to have troubled themselves very little about astro- nomical matters, and were content, as we have seen, to look upon phenomena, like eclipses, as mere celestial prodigies. What is thought to be the first definite mention of the solar corona occurs in a passage of Plutarch. The eclipse to which he refers is probably one which took place in A.D. 71. He says that the obscuration caused by the moon ^^ has no time to last and no ex- tensiveness, but some light shows itself round the sun's circumference, which does not allow the dark- ness to become deep and complete." No further reference to this phenomenon occurs until near the end of the sixteenth century. It should, however, be here 86 Famous Eclipses of the Sun mentioned that Mr. E. W. Maunder has pointed out the probability 1 that we have a very ^ancient symboUc representation of the corona in the ^^ winged circle," " winged disc/' or '^ ring with wings/' as it is variously called, which appears so often upon Assyrian and Fig. 7. — The " Ring with Wings." The upper is the Assyrian form of the symbol, the lower the Egyptian. (From Knoidedge.) Compare the form of the corona on Plate VII. (B), p. 142. Egyptian monuments, as the symbol of the Deity (Fig- 7)- The first solar eclipse recorded to have been seen in England is that of a.d. 538, mention of which is found in the Anglo-Saxon Chronicle. The track of totality did not, however, come near our islands, for only two-thirds of the sun's disc were eclipsed at London. 1 Knoivledge^ vol. xx. p. 9, January 1897. 87 Astronomy of To-day In 840 a great eclipse took place in Europe, which was total for more than five minutes across what is now Bavaria. Terror at this eclipse is said to have hastened the death of Louis le Debonnaire, Emperor of the West, who lay ill at Worms. In 878 — temp. King Alfred — an eclipse of the sun took place which was total at London. From this until 171 5 no other eclipse was total at London itself ; though this does not apply to other portions of England. An eclipse, generally known as the ^' Eclipse of Stiklastad," is said to have taken place in 1030, during the sea-fight in which Olaf of Norway is supposed to have been slain. Longfellow, in his Saga of King Olaf, has it that "The Sun hung red As a drop of blood," but, as in the case of most poets, the dramatic value of an eclipse seems to have escaped his notice. In the year 1140 there occurred a total eclipse of the sun, the last to be visible in England for more than five centuries. Indeed there have been only two such since — namely, those of 171 5 and 1724, to which we shall allude in due course. The eclipse of 1140 took place on the 20th March, and is thus referred to in the Anglo-Saxon Chronicle : — ^' In the Lent, the sun and the day darkened, about the noon-tide of the day, when men were eating, and they lighted candles to eat by. That was the 13th day before the calends of April. Men were very much struck with wonder." Several of the older historians speak of a ^'fearful eclipse " as having taken place on the morning of the Famous Eclipses of the Sun Battle of Crecy, 1346. Lingard, for instance, in his History of England, has as follows : — " Never, perhaps, were preparations for battle made under circumstances so truly awful. On that very day the sun suffered a partial eclipse : birds, in clouds, the precursors of a storm, flew screaming over the two armies, and the rain fell in torrents, accompanied by incessant thunder and lightning. About five in the afternoon the weather cleared up ; the sun in full splendour darted his rays in the eyes of the enemy." Calculations, however, show that no eclipse of the sun took place in Europe during that year. This error is found to have arisen from the mistranslation of an obsolete French word esclistre (lightning), which is employed by Froissart in his description of the battle. In 1598 an eclipse was total over Scotland and part of North Germany. It was observed at Torgau by Jessenius, an Hungarian physician, who noticed a bright light around the moon during the time of totality. This is said to be the first reference to the corona since that of Plutarch, to which we have already drawn attention. Mention of Scotland recalls the fact that an unusual number of eclipses happen to have been visible in that country, and the occult bent natural to the Scottish character has traditionalised a few of them in such terms as the "Black Hour" (an eclipse of 1433), "Black Saturday" (the eclipse of 1598 which has been alluded to above), and "Mirk Monday" (1652). The track of the last-named also passed over Carrickfergus in Ireland, where it was observed by a certain Dr. Wybord, in whose account the 89 Astronomy of To-day term ^'corona" is first employed. This eclipse is the last which has been total in Scotland, and it is calculated that there will not be another eclipse seen as total there until the twenty-second century. An eclipse of the sun which took place on May 30, 1612, is recorded as having been seen "through a tube." This probably refers to the then recent invention — the telescope. The eclipses which we have been describing are chiefly interesting from an historical point of view. The old mystery and confusion to the beholders seem to have lingered even into comparatively enlightened times, for we see how late it is before the corona attracts definite attention for the sake of itself alone. It is not a far cry from notice of the corona to that of other accompaniments of a solar eclipse. Thus the eclipse of 1706, the total phase of which was visible in Switzerland, is of great interest ; for it was on this occasion that the famous red prominences seem first to have been noted. A certain Captain Stannyan observed this eclipse from Berne in Switzer- land, and described it in a letter to Flamsteed, the then Astronomer Royal. He says the sun's " getting out of his eclipse was preceded by a blood-red streak of light from its left limb, which continued not longer than six or seven seconds of time ; then part of the Sun's disc appeared all of a sudden, as bright as Venus was ever seen in the night, nay brighter ; and in that very instant gave a Light and Shadow to things as strong as Moonlight uses to do." How little was then expected of the sun is, however, shown by Flamsteed's words, when communicating this in- formation to the Royal Society : — 90 Famous Eclipses of the Sun ''The Captain is the first man I ever heard of that took notice of a Red Streak of Light preceding the Emersion of the Sun's body from a total EcHpse. And I take notice of it to you because it infers that the Moon has mi atmosphere ; and its short continu- ance of only six or seven seconds of time, tells us that its height is not more than the five or six hundredth part of her diameter ^ What a change has since come over the ideas of men ! The sun has proved a veritable mine of dis- covery, while the moon has 37^ielded up nothing new. The eclipse of 1715, the first total at London since that of 878, was observed by the famous astronomer, Edmund Halley, from the rooms of the Royal Society, then in Crane Court, Fleet Street. On this occasion both the corona and a red projection were noted. Halley further makes allusion to that curious phenomenon, which later on became cele- brated under the name of '' Baily's beads." It was also on the occasion of this eclipse that the earliest recorded drawings of the corona were made. Cam- bridge happened to be within the track of totality; and a certain Professor Cotes of that University, who is responsible for one of the drawings in ques- tion, forwarded them to Sir Isaac Newton together with a letter describing his observations. In 1724 there occurred an eclipse, the total phase of which was visible from the south-west of England, but not from London. The weather was unfavour- able, and the eclipse consequently appears to have been seen by only one person, a certain Dr. Stukeley, who observed it from Haraden Hill near Salisbur}^ Plain. This is the last eclipse of which the total 91 Astronomy of To-day phase was seen in any part of England. The next will not be until June 29, 1927, and will be visible along a line across North Wales and Lancashire. The discs of the sun and moon will just then be almost of the same apparent size, and so totality will be of extremely short duration ; in fact only a few seconds. London itself will not see a totality until the year 2151 — a circumstance which need hardly distress any of us personally ! It is only from the early part of the nineteenth century that serious scientific attention to eclipses of the sun can be dated. An annular eclipse, visible in 1836 in the south of Scotland, drew the careful notice of Francis Baily of Jedburgh in Roxburgh- shire to that curious phenomenon which we have already described, and which has ever since been known by the name of ^' Daily's beads." Spurred by his observation, the leading astronomers of the day determined to pay particular attention to a total eclipse, which in the year 1842 was to be visible in the south of France and the north of Italy. The public interest aroused on this occasion was also very great, for the region across which the track of totality was to pass was very populous, and inhabited by races of a high degree of culture. This eclipse occurred on the morning of the 8th July, and from it may be dated that great enthu- siasm with which total eclipses of the sun have ever since been received. Airy, our then Astronomer Royal, observed it from Turin ; Arago, the celebrated director of the Paris Observatory, from Perpignan in the south of France ; Francis Baily from Pavia ; and Sir John Herschel from Milan. The corona 92 Famous Eclipses of the Sun and three large red prominences were not only well observed by the astronomers, but drew tremendous applause from the watching multitudes. The success of the observations made during this eclipse prompted astronomers to pay similar attention to that of July 28, 1851, the total phase of which was to be visible in the south of Norway and Sweden, and across the east of Prussia. This eclipse was also a success, and it was now ascertained that the red prominences belonged to the sun and not to the moon ; for the lunar disc, as it moved onward, was seen to cover and to uncover them in turn. It was also noted that these prominences were merely uprushes from a layer of glowing gaseous matter, which was seen closely to envelop the sun. The total eclipse of July 18, i860, was observed in Spain, and photography was for the first time systematically employed in its observation.^ In the photographs taken the stationary appearance of both the corona and prominences with respect to the moving moon, definitely confirmed the view already put forward that they were actual appendages of the sun. The eclipse of August 18, 1868, the total phase of which lasted nearly six minutes, was visible in India, and drew thither a large concourse of astronomers. In this eclipse the spectroscope came to the front, and showed that both the prominences, and the chromospheric layer from which they rise, are com- posed of glowing vapours — chief among which is the ^ The first photographic representation of the coro/ia had, however, been made during the eclipse of 1851. This was a daguerreotype taken by Dr. Eusch at Konigsberg in Prussia, 93 Astronomy of To-day vapour of hydrogen. The dhxct result of the obser- vations made on this occasion was the spectroscopic method of examining prominences at any time in full daylight, and without a total eclipse. This method, which has given such an immense impetus to the study of the sun, was the outcome of independent and simultaneous investigation on the part of the French astronomer, the late M. Janssen, and the Eng- lish astronomer. Professor (now Sir Norman) Lockyer, a circumstance strangely reminiscent of the discovery of Neptune. The principles on which the method was founded seem, however, to have occurred to Dr. (now Sir William) Huggins some time previously. The eclipse of December 22, 1870, was total for a little more than two minutes, and its track passed across the Mediterranean. M. Janssen, of whom mention has just been made, escaped in a balloon from then besieged Paris, taking his instruments with him, and made his way to Gran, in Algeria, in order to observe it ; but his expectations were disappointed by cloudy weather. The expedition sent out from England had the misfortune to be shipwrecked oiT the coast of Sicily. But the occasion was redeemed by a memorable observation made by the American astronomer, the late Professor Young, which revealed the existence of what is now known as the ^' Reversing Layer." This is a shallow layer of gases which lies immediately beneath the chromosphere. An illustra- tion of the corona, as it was seen during the above eclipse, will be found on Plate VII. (A), p. 142. In the eclipse of December 12, 1871, total across Southern India, the photographs of the corona ob- tained by Mr. Davis, assistant to Lord Lindsay (now 94 Famous Eclipses of the Sun the Earl of Crawford), displayed a wealth of detail hitherto unapproached. The eclipse of July 29, 1878, total across the western states of North America, was a remarkable success, and a magnificent view of the corona was obtained by the well-known American astronomer and physicist, the late Professor Langley, from the summit of Pike's Peak, Colorado, over 14,000 feet above the level of the sea. The coronal streamers were seen to extend to a much greater distance at this altitude than at points less elevated, and the corona itself remained visible during more than four minutes after the end of totality. It was, however, not entirely a question of altitude ; the coronal streamers were actually very much longer on this occasion than in most of the eclipses which had previously been observed. The eclipse of May 17, 1882, observed in Upper Egypt, is notable- from the fact that, in one of the photographs taken by Dr. Schuster at Sohag, a bright comet appeared near the outer limit of the corona (see Plate I., p. 96). The comet in question had not been seen before the eclipse, and was never seen after- wards. This is the third occasion on which attention has been drawn to a comet merely by a total eclipse. The first is mentioned by Seneca ; and the second by Philostorgius, in an account of an eclipse observed at Constantinople in A.D. 418. A fourth case of the kind occurred in 1893, when faint evidences of one of these filmy objects were found on photographs of the corona taken by the American astronomer, Professor Schae- berle, during the total eclipse of April 16 of that year. The eclipse of May 6, 1883, had a totality of over 95 Astronomy of To-day five minutes, but the central track unfortunately- passed across the Pacific Ocean, and the sole point of land available for observing it from was one of the Marquesas Group, Caroline Island, a coral atoll seven and a half miles long by one and a half broad. Never- theless astronomers did not hesitate to take up their posts upon that little spot, and were rewarded with good weather. The next eclipse of importance was that of April 1 6, 1893. It stretched from Chili across South America and the Atlantic Ocean to the West Coast of Africa, and, as the weather was fine, many good results were obtained. Photographs were taken at both ends of the track, and these showed that the appearance of the corona remained unchanged during the interval of time occupied by the passage of the shadow across the earth. It was on the occasion of this eclipse that Professor Schaeberle found upon his photographs those traces of the presence of a comet, to which allusion has already been made. Extensive preparations were made to observe the eclipse of August 9, 1896. Totality lasted from two to three minutes, and the track stretched from Norway to Japan. Bad weather disappointed the observers, with the exception of those taken to Nova Zembla by Sir George Baden Powell in his yacht Otaria. The eclipse of January 22, 1898, across India vid Bombay and Benares, was favoured with good weather, and is notable for a photograph obtained by Mrs. E. W. Maunder, which showed a ray of the corona extending to a most unusual distance. Of very great influence in the growth of our know- ledge with regard to the sun, is the remarkable piece q6 Plate I. The Total Eclipse of the Sun of May i7th, 1SS2 A comet is here shown in the immediate neighbourhood of the corona. Drawn by Mr. W. H. Wesley from the photographs. (Page 05) Famous Eclipses of the Sun of good fortune by which the countries around the Mediterranean, so easy of access, have been favoured with a comparatively large number of total eclipses during the past sixty years. Tracks of totality have, for instance, traversed the Spanish peninsula on no less than five occasions during that period. Two of these are among the most notable eclipses of recent years, namely, those of May 28, 1900, and of August 30, 1905. In the former the track of totality stretched from the western seaboard of Mexico, through the Southern States of America, and across the Atlantic Ocean, after which it passed over Portugal and Spain into North Africa. The total phase lasted for about a minute and a half, and the eclipse was well observed from a great many points along the line. A representation of the corona, as it appeared on this occasion, will be found on Plate VII. (B), p. 142. The track of the other eclipse to which we have alluded, i.e. that of August 30, 1905, crossed Spain about 200 miles to the northward of that of 1900. It stretched from Winnipeg in Canada, through Labrador, *and over the Atlantic ; then traversing Spain, it passed across the Balearic Islands, North Africa, and Egypt, and ended in Arabia (see Fig. 6, p. 81). Much was to be expected from a comparison between the photographs taken in Labrador and Egypt on the question as to whether the corona would show any alteration in shape during the time that the shadow was traversing the intervening space — some 6000 miles. The duration of the total phase in this eclipse was nearly four minutes. Bad weather, however, interfered a good deal with the observations. It was not possible, for instance, to do anything at all 97 ^ Astronomy of To-day in Labrador. In Spain the weather conditions were by no means favourable ; though at Burgos, where an immense number of people had assembled, the total phase was, fortunately, well seen. On the whole, the best results were obtained at Guelma in Algeria. The corona on the occasion of this eclipse was a very fine one, and some magnificent groups of prominences were plainly visible to the naked eye (see the Frontispiece). The next total eclipse after that of 1905 was one which occurred on January 14, 1907. It passed across Central Asia and Siberia, and had a totality lasting two and a half minutes at most ; but it was not observed as the weather was extremely bad, a circumstance not surprising with regard to those regions at that time of year. The eclipse of January 3, 1908, passed across the Pacific Ocean. Only tw^o small coral islands — Hull Island in the Phoenix Group, and Flint Island about 400 miles north of Tahiti — lay in the track. Two expeditions set out to observe it, i.e, a combined American party from the Lick Observatory and the Smithsonian Institution of Washington, and a private one from England under Mr. F. K. McClean. As Hull Island afforded few facilities, both parties installed their instruments on Flint Island, although it was very little better. The duration of the total phase was fairly long — about four minutes, and the sun very favourably placed, being nearly overhead. Heavy rain and clouds, however, marred observation during the first minute of totality, but the remaining three minutes were successfully utilised, good photo- graphs of the corona being obtained. 98 Famous Eclipses of the Sun The next few years to come are unfortunately by no means favourable from the point of view of the eclipse observer. An eclipse will take place on June 17, 1909; the track stretching from Greenland across the North Polar regions into Siberia. The geogra- phical situation is, however, a very awkward one, and totality will be extremely short — only six seconds in Greenland and twenty-three seconds in Siberia. The eclipse of May 9, 1910, will be visible in Tasmania. Totality will last so long as four minutes, but the sun will be at the time much too low in the sky for good observation. The eclipse of the following year, April 28, 191 1, will also be confined, roughly speaking, to the same quarter of the earth, the track passing across the old convict settlement of Norfolk Island, and then out into the Pacific. The eclipse of April 17, 191 2, will stretch from Portugal, through France and Belgium into North Germany. It will, however, be of practically no service to astronomy. Totality, for instance, will last for only three seconds in Portugal ; and, though Paris lies in the central track, the eclipse, which begins as barely total, will have changed into an annular one by the time it passes over that city. The first really favourable eclipse in the near future will be that of August 21, 1914. Its track will stretch from Greenland across Norway, Sweden, and Russia. This eclipse is a return, after one saros, of the eclipse of August 9, 1896. The last solar eclipse which we will touch upon is that predicted for June 29, 1927. It has been already alluded to as the first of those in the future 99 Astronomy of To-day to be total in England. The central line will stretch from Wales in a north-easterly direction. Stonyhurst Observatory, in Lancashire, will lie in the track ; but totality there will be very short, only about twenty seconds in duration. lOO CHAPTER IX FAMOUS ECLIPSES OF THE MOON The earliest lunar eclipse, of which we have any trustworthy information, was a total one w^hich took place on the 19th March, 721 B.C., and was observed from Babylon. For our knowledge of this eclipse we are indebted to Ptolemy, the astronomer, who copied it, along with two others, from the records of the reign of the Chaldean king, Merodach-Baladan. The next eclipse of the moon worth noting was a total one, which took place some three hundred years later, namely, in 425 B.C. This eclipse was observed at Athens, and is mentioned by Aristophanes in his play, The Clouds. Plutarch relates that a total eclipse of the moon, which occurred in 413 B.C., so greatly frightened Nicias, the general of the Athenians, then warring in Sicily, as to cause a delay in his retreat from Syracuse w^hich led to the destruction of his whole army. Seven years later — namely, in 406 B.C., the twenty- sixth year of the Peloponnesian War — there took place another total lunar eclipse of which mention is made by Xenophon. Omitting a number of other eclipses alluded to by ancient writers, we come to one recorded by Josephus as having occurred a little before the death of Herod the Great. It is probable that the eclipse in question lOI Astronomy of To-day was the total lunar one, which calculation shows to have taken place on the 15th September 5 B.C., and to have been visible in Western Asia. This is very important, for we are thus enabled to fix that year as the date of the birth of Christ, for Herod is known to have died in the early part of the year following the Nativity. In those accounts of total lunar eclipses, which have come down to us from the Dark and Middle Ages, the colour of the moon is nearly ahvays likened to '' blood." On the other hand, in an account of the eclipse of January 23, A.D. 753, our satellite is de- scribed as "covered with a horrid black shield." We thus have examples of the two distinct ap- pearances alluded to in Chapter VII., i.e. when the moon appears of a coppery-red colour, and when it is entirely darkened. It appears, indeed, that, in the majority of lunar eclipses on record, the moon has appeared of a ruddy, or rather of a coppery hue, and the details on its surface have been thus rendered visible. One of the best examples of a bright eclipse of this kind is that of the 19th March 1848, when the illumination of our satellite was so great that many persons could not believe that an eclipse was actually taking place. A certain Mr. Foster, who observed this eclipse from Bruges, states that the marki igs on the lunar disc were alm^ost as visible as on an "ordinary dull moonlight night." He goes on to say that the British Consul at Ghent, not knowing that there had been any eclipse, wrote to him for an explanation of the red colour of the moon on that evening. Out of the dark eclipses recorded, perhaps the 102 Famous Eclipses of the Moon best example is that of May i8, 1761, observed by Wargentin at Stockholm. On this occasion the lunar disc is said to have disappeared so completely, that it could not be discovered even with the telescope. Another such instance is the eclipse of June 10, 1816, observed from London. The summer of that year was particularly wet — a point worthy of notice in connection with the theory that these different ap- pearances are due to the varying state of our earth's atmosphere. Sometimes, indeed, it has happened that an eclipse of the moon has partaken of both appearances, part of the disc being visible and part invisible. An in- stance of this occurred in the eclipse of July 12, 1870, when the late Rev. S. J. Johnson, one of the leading authorities on eclipses, who observed it, states that he found one-half the moon's surface quite invis- ible, both with the naked eye and with the telescope. In addition to the examples given above, there are three total lunar eclipses which deserve especial mention. 1. A.D. 755, November 23. During the progress of this eclipse the moon occulted the star Aldebaran in the constellation of Taurus. 2. A.D. 1493, April 2. This is the celebrated eclipse which is said to have so well served the purposes of Christopher Columbus. Certain natives having re- fused to supply him with provisions when in sore straits, he announced to them that the moon would be darkened as a sign of the anger of heaven. When the event duly came to pass, the savages were so terrified that they brought him provisions as much as he needed. 103 Astronomy of To-day 3. A.D. 1610, July 6. The eclipse in question is notable as having been seen through the telescope, then a recent invention. It was without doubt the first so observed, but unfortunately the name of the observer has not come down to us. 104 CHAPTER X THE GROWTH OF OBSERVATION The earliest astronomical observations must have been made in the Dav^n of Historic Time by the men who tended their flocks upon the great plains. As they watched the clear night sky they no doubt soon noticed that, with the exception of the moon and those brilliant wandering objects known to us as the planets, the individual stars in the heaven remained apparently fixed with reference to each other. These seemingly changeless points of light came in time to be regarded as sign-posts to guide the wanderer across the trackless desert, or the voyager upon the wide sea. Just as when looking into the red coals of a fire, or when watching the clouds, our imagination conjures up strange and grotesque forms, so did the men of old see in the grouping of the stars the outlines of weird and curious shapes. Fed with mythological lore, they imagined these to be rough representations of ancient heroes and fabled beasts, whom they sup- posed to have been elevated to the heavens as a reward for great deeds done upon the earth. We know these groupings of stars to-day under the name of the Constellations. Looking up at them we find it extremely difficult to fit in the majority with the figures which the ancients believed them to represent. 105 Astronomy of To-day Nevertheless, astronomy has accepted the arrange- ment, for want of a better method of fixing the leading stars in the memory. Our early ancestors lived the greater part of their lives in the open air, and so came to pay more atten- tion in general to the heavenly orbs then we do. Their clock and their calendar was, so to speak, in the celestial vault. They regulated their hours, their days, and their nights by the changing positions of the sun, the moon, and the stars ; and recognised the periods of seed-time and harvest, of calm and stormy weather, by the rising or setting of certain well-known constellations. Students of the classics will recall many allusions to this, especially in the Odes of Horace. As time went on and civilisation progressed, men soon devised measuring instruments, by means of which they could note the positions of the celestial bodies in the sky with respect to each other ; and, from observations thus made, they constructed charts of the stars. The earliest complete survey of this kind, of which we have a record, is the great Cata- logue of stars which was made, in the second century B.C., by the celebrated Greek astronomer, Hipparchus, and in which he is said to have noted down about 1080 stars. It is unnecessary to follow in detail the tedious progress of astronomical discovery prior to the advent of the telescope. Certain it is that, as time went on, the measuring instruments to which we have alluded had become greatly improved ; but, had they even been perfect, they would have been utterly inadequate to reveal those minute displacements, from which we have learned the actual distance of the nearest of the 106 The Growth of Observation celestial orbs. From the early times, therefore, until the mediaeval period of our own era, astronomy grew up upon a faulty basis, for the earth ever seemed so much the largest body in the universe, that it con- tinued from century to century to be regarded as the very centre of things. To the Arabians is due the credit of having kept alive the study of the stars during the dark ages of European history. They erected some fine observa- tories, notably in Spain and in the neighbourhood of Bagdad. Following them, some of the Oriental peoples embraced the science in earnest ; Ulugh Beigh, grandson of the famous Tamerlane, founding, for instance, a great observatory at Samarcand in Central Asia. The Mongol emperors of India also established large astronomical instruments in the chief cities of their empire. When the revival of learning took place in the West, the Europeans came to the front once more in science, and rapidly forged ahead of those who had so assiduously kept alight the lamp of knowledge through the long centuries. The dethronement of the older theories by the Copernican system, in which the earth was relegated to its true place, was fortunately soon followed by an invention of immense import, the invention of the Telescope. It is to this instrument, indeed, that we are indebted for our knowledge of the actual scale of the celestial distances. It penetrated the depths of space ; it brought the distant orbs so near, that men could note the detail on the planets, or measure the small changes in their positions in the sky which resulted from the movement of our own globe. It was in the year 1609 that the telescope was first 107 Astronomy of To-day constructed. A year or so previous to this a spectacle- maker of Middleburgh in Holland; one Hans Lipper- shey, had, it appears, hit upon the fact that distant objects, when viewed through certain glass lenses suitably arranged, looked nearer.^ News of this dis- covery reached the ears of Galileo Galilei, of Florence, the foremost philosopher of the day, and he at once applied his great scientific attainments to the con- struction of an instrument based upon this principle. The result was what was called an ^' optick tube," which magnified distant objects some few times. It was not much larger than what we nowadays contemptuously refer to as a ^^ spy-glass," yet its employment upon the leading celestial objects in- stantly sent astronomical science onward with a bound. In rapid succession Galileo announced world-moving discoveries ; large spots upon the face of the sun ; crater-like mountains upon the moon ; four subordinate bodies, or satellites, circling around the planet Jupiter ; and a strange appearance in connection with Saturn, which later telescopic ob- servers found to be a broad flat ring encircling that planet. And more important still, the magnified image of Venus showed itself in the telescope at certain periods in crescent and other forms ; a result which Copernicus is said to have announced should of necessity follow if his system were the true one. ^ The principle upon which the telescope is based appears to have been known theoretically for a long time previous to this. The monk Roger Bacon, who lived in the thirteenth century* describes it very clearly ; and several writers of the sixteenth century have also dealt with the idea. Even Lippershey's claims to a practical solution of the question were hotly contested at the time by two of his own countrym^en, i.e. a certain Jacob Metius, and another spectacle-maker of Middleburgh, named Jansen. I08 The Growth of Observation The discoveries made with the telescope produced, as time went on, a great alteration in the notions of men with regard to the imiverse at large. It must have been, indeed, a revelation to find that those points of light which they called the planets, were, after all, globes of a size comparable with the earth, and peopled perchance with sentient beings. Even to us, who have been accustomed since our early youth to such an idea, it still requires a certain stretch of imagination to enlarge, say, the Bright Star of Eve, into a body similar in size to our earth. The reader will perhaps recollect Tennyson's allusion to this in Locksley Hall, Sixty Years After : — " Hesper — Venus — were we native to that splendour or in Mars, We should see the Globe we groan in, fairest of their evening stars. "Could we dream of wars and carnage, craft and madness, lust and spite, Roaring London, raving Paris, in that point of peaceful light?" The form of instrument as devised by Galileo is called the Refracting Telescope, or '' Refractor." As we know it to-day it is the same in principle as his ^^optick tube," but it is not quite the same in construction. The early object-glass^ or large glass at the end, was a single convex lens (see Fig. 8, p. 113, ^^ Galilean") ; the modern one is, on the other hand, composed of two lenses fitted together. The attempts to construct large telescopes of the Galilean type met in course of time with a great difficulty. The magnified image of the object observed was not quite pure ; its edges, indeed, were fringed with rainbow-like colours. This defect was found to be aggravated with increase in the size of object-glasses. A method was, however, 109 Astronomy of To-day discovered of diminishing this colouration, or chromatic aberration as it is called from the Greek word %pwyua (chroma)^ which means colour, viz. by making tele- scopes of great length and only a few inches in width. But the remedy was, in a way, worse than the disease ; for telescopes thus became of such huge proportions as to be too unwieldy for use. Attempts were made to evade this unwieldiness by constructing them with skeleton tubes (see Plate II., p. no), or, indeed, even without tubes at all ; the object-glass in the tubeless or ^'aerial" telescope being fixed at the top of a high post, and the eye-piece^ that small lens or combination of lenses, which the eye looks directly into, being kept in line with it by means of a string and manoeuvred about near the ground (Plate IIL, p. 112). The idea of a telescope without a tube may appear a contradic- tion in terms ; but it is not really so, for the tube adds nothing to the magnifying power of the instru- ment, and is, in fact, no more than a m.ere device for keeping the object-glass and eye-piece in a straight line, and for preventing the observer from being hindered by stray lights in his neighbourhood. It goes without saying, of course, that the image of a celestial object will be more clear and defined when examined in the darkness of a tube. The ancients, though they knew nothing of tele- scopes, had, however, found out the merit of a tube in this respect ; for they employed simple tubes, blackened on the inside, in order to obtain a clearer view of distant objects. It is said that Julius Caesar, before crossing the Channel, surveyed the opposite coast of Britain through a tube of this kind. A few of the most famous of the immensely long no The Growth of Observation telescopes above alluded to are worthy of mention. One of these, 123 feet in length, was presented to the Royal Society of London by the Dutch astronomer Huyghens. Hevelius of Danzig constructed a skele- ton one of 150 feet in length (see Plate II., p. no). Bradley used a tubeless one 212 feet long to measure the diameter of Venus in 1722 ; while one of 600 feet is said to have been constructed, but to have proved quite unworkable ! Such difficulties, however, produced their natural result. They set men at work to devise another kind of telescope. In the new form, called the Reflecting Telescope, or ^' Reflector," the light coming from the object under observation was reflected into the eye-piece from the surface of a highly polished concave metallic mirror, or speculum^ as it was called. It is to Sir Isaac Newton that the world is indebted for the re- flecting telescope in its best form. That philosopher had set himself to investigate the causes of the rainbow- like, or prismatic colours which for a long time had been such a source of annoyance to telescopic ob- servers ; and he pointed out that, as the colours were produced in the passage of the rays of light through the glass, they would be entirely absent if the light were reflected from the surface of a mirror instead. The reflecting telescope, however, had in turn cer- tain drawbacks of its own. A mirror, for instance, can plainly never be polished to such a high degree as to reflect as much light as a piece of transparent glass will let through. Further, the position of the eye-piece is by no means so convenient. It cannot, of course, be pointed directly towards the mirror, for the observer would then have to place his head right III Astronomy of To-day in the way of the Hght coming from the celestial object, and would thus, of course, cut it off. In order to obviate this difficulty^ the following device was employed by Newton in his telescope, of which he constructed his first example in 1668. A small, flat mirror was fixed by thin wires in the centre of the tube of the telescope, and near to its open end. It was set slant-wise, so that it reflected the rays of light directly into the eye-piece, which was screwed into a hole at the side of the tube (see Fig. 8, p. 113, "New- tonian "). Although the Newtonian form of telescope had the immense advantage of doing away with the prismatic colours, yet it wasted a great deal of light ; for the objection in this respect with regard to loss of light by reflection from the large mirror applied, of course, to the small mirror also. In addition, the position of the "flat," as the small mirror is called, had the further effect of excluding from the great mirror a certain proportion of light. But the reflector had the advantage, on the other hand, of costing less to m_ake than the refractor, as it was not necessary to procure flawless glass for the purpose. A disc of a certain metallic composition, an alloy of copper and tin, known in consequence as speculum metaly had merely to be cast ; and this had to be ground and polished upon one side only, whereas a lens has to be thus treated upon both its sides. It was, therefore, pos- sible to make a much larger instrument at a great deal less labour and expense. We have given the Newtonian form as an example of the principle of the reflecting telescope. A some- what similar instrument had, however, been projected, 112 Plate III. A Tubeless, or "Aerial" Telescope From an illustration in the Opera Varia of Christian Huyghens. (Page no) The Growth of Observation though not actually constructed, by James Gregory a few years earlier than Newton's, i.e, in 1663. In /iefracitn^ Telescopes ^ Galilean Achromatic fiefiectin^ Telescopes f^' 'M% A ObjeclGlass B.Eyepicce 4 ¥} Aleirtonfan, /ferscAellan ^ Gregorian ... 1^ ] A Speculum, mijwr B. Eyepiece C.flal A, Speculum mlrwr B. Eyepiece i^^A. Speculupt mirror B. Eyepiece C. Small CcMove mmr '\{_B A. Speculum nilnvr .!pi3l^ B. Eyepiece J] C. Small Convermiiivr Cassegraniaa Fig. 8. — The various types of Telescope. All the above telescopes are pointed in the same direction ; that is to say, the rays of light from the object are coming from the left-hand side. this form of reflector, known as the ''Gregorian" telescope, a hole was made in the big concave mirror ; and a small mirror,- also concave, which faced it at a 113 H Astronomy of To-day certain distance, received the reflected rays, and re- flected them back again through the hole in question into the eye-piece, which was fixed just behind (see Fig. 8, p. 113, '^Gregorian "). The Gregorian had thus the sentimental advantage of being pointed directly at the object. The hole in the big mirror did not cause any loss of light, for the central portion in which it was made was anyway unable to receive light through the small mirror being directly in front of it. An adaptation of the Gregorian was the " Cassegrainian " telescope, devised byCassegrain in 1672, which'differed from it chiefly in the small mirror being convex instead of concave (see Fig. 8, p. 113, ^^ Cassegrainian "). These direct-view forms of the reflecting telescope were much in vogue about the middle of the eighteenth century, when many beautiful examples of Gregorians were made by the famous optician, James Short, of Edinburgh. An adaptation of the Newtonian type of telescope is known as the '^ Herschelian," from being the kind favoured by Sir William Herschel. It is, however, only suitable in immense instruments, such as Hers- chel was in the habit of employing. In this form the object-glass is set at a slight slant, so that the light coming from the object is reflected straight into the eye-piece, which is fixed facing it in the side of the tube (see Fig. 8, p. 113, "Herschelian"). This telescope has an advantage over the other forms of reflector through the saving of light consequent on doing away with the second reflection. There is, how- ever, the objection that the slant of the object-glass is productive of some distortion in the appearance of the object observed ; but this slant is of necessity slight when the length of the telescope is very great, 114 The Growth of Observation The principle of this type of telescope had been described to the French Academy of Sciences as early as 1728 by Le Maire, but no one availed himself of the idea until 1776, when Herschel tried it. At first, however, he rejected it ; but in 1786 he seems to have found that it suited the huge instruments which he was then making. Herschel's largest telescope, con- structed in 1789, was about four feet in diameter and forty feet in length. It is generally spoken of as the ^' Forty-foot Telescope," though all other instruments have been known by their diameters^ rather than by their lengths. To return to the refracting telescope. A solution of the colour difficulty was arrived at in 1729 (two years after Newton's death) by an Essex gentleman named Chester Moor Hall. He discovered that by making a double object-glass, composed of an outer convex lens and an inner concave lens, made respec- tively of different kinds of glass, i.e, crown glass and flint glass, the troublesome colour effects could be, to a very great extent^ removed. Hall's investigations appear to have been rather of an academic nature ; and, although he is believed to have constructed a small telescope upon these lines, yet he seems to have kept the matter so much to himself that it was not until the year 1758 that the first example of the new instrument was given to the world. This was done by John DoUond, founder of the well-known optical firm of Dollond, of Ludgate Hill, London, who had, quite independently, re-discovered the principle. This ^'Achromatic" telescope, or telescope ''free from colour effects," is the kind ordinarily in use at present, whether for astronomical or for terrestrial 115 Astronomy of To-day purposes (see Fig. 8, p. 113, ^^ Achromatic "). The ex- pense of making large instruments of this type is very great, for, in the object-glass alone, no less than four surfaces have to be ground and polished to the re- quired curves ; and, usually, the two lenses of which it is composed have to fit quite close together. With the object of evading the expense referred to, and of securing complete freedom from colour effects, telescopes have even been made, the object- glasses of which w^ere composed of various transparent liquids placed between thin lenses ; but leakages, and currents set up within them by changes of tempera- ture, have defeated the ingenuity of those who devised these substitutes. The solution of the colour difficulty by means of Dollond's achromatic refractor has not, however, ousted the reflecting telescope in its best, or New- tonian form, for which great concave mirrors made of glass, covered wdth a thin coating of silver and highly polished, have been used since about 1870 instead of metal mirrors. They are very much lighter in w^eight and cheaper to make than the old specula ; and though the silvering, needless to say, deteriorates with time, it can be renew^ed at a comparatively trifling cost. Also these mirrors reflect much more light, and give a clearer view, than did the old metallic ones. When an object is viewed through the type of astronomical telescope ordinarily in use, it is seen upside down. This is, however, a matter of very small moment in dealing with celestial objects ; for, as they are usually round, it is really not of much consequence w^hich part w^e regard as top and w^hich as bottom. Such an inversion would, of course, be 116 The Growth of Observation most inconvenient when viewing terrestrial objects. In order to observe the latter we therefore employ what is called a terrestrial telescope, which is merely a refractor with some extra lenses added in the eye portion for the purpose of turning the inverted image the right way up again. These extra lenses, needless to say, absorb a certain amount of light ; wherefore it is better in astronomical observation to save light by doing away with them, and putting up with the slight inconvenience of seeing the object inverted. This inversion of images by the astronomical telescope must be specially borne in mind with regard to the photographs of the moon in Chapter XVI. In the year 1825 the largest achromatic refractor in existence was one of nine and a half inches in diameter constructed by Fraunhofer for the Observa- tory of Dorpat in Russia. The largest refractors in the world to-day are in the United States, Le. the forty- inch of the Yerkes Observatory (see Plate IV., p. 118), and the thirty-six inch of the Lick. The object- glasses of these and of the thirty-inch telescope of the Observatory of Pulkowa, in Russia, were made by the great optical house of Alvan Clark & Sons, of Cambridge, Massachusetts, U.S.A. The tubes and other portions of the Yerkes and Lick telescopes were, however, constructed by the Warner and Swasey Co., of Cleveland, Ohio. The largest reflector, and so the largest telescope in the world, is still the six-foot erected by the late Lord Rosse at Parsonstown in Ireland, and completed in the year 1845. It is about fifty-six feet in length. Next come two of five feet, with mirrors of silver on 117 Astronomy of To-day glass ; one of them made by the late Dr. Common, of Ealing, and the other by the American astronomer, Professor G. W. Ritchey. The latter of these is in- stalled in the Solar Observatory belonging to Carnegie Institution of Washington, which is situated on Mount Wilson in California. The former is now at the Harvard College Observatory, and is considered by Professor Moulton to be probably the most efficient reflector in use at present. Another large reflector is the three-foot made by Dr. Common. It came into the possession of Mr. Crossley of Halifax, who pre- sented it to the Lick Observatory, where it is now known as the ^' Crossley Reflector." Although to the house of Clark belongs, as we have seen, the credit of constructing the object-glasses of the largest refracting telescopes of our time, it has nevertheless keen competitors in Sir Howard Grubb, of Dublin, and such well-known firms as Cooke of York and Steinheil of Munich. In the four-foot reflector, made in 1870 for the Observatory of Melbourne by the firm of Grubb, the Cassegrainian principle was employed. With regard to the various merits of refractors and reflectors much might be said. Each kind of instru- ment has, indeed, its special advantages; though perhaps, on the whole, the most perfect type of telescope is the achromatic refractor. In connection with telescopes certain devices have from time to time been introduced, but these merely aim at the convenience of the observer and do not supplant the broad principles upon which are based the various types of instrument above described. Such, for instance, are the '^ Siderostat," and another 118 Plate IV. The Great Verkes Telescoi^e Great telescope at the Yerkes Observatory of the University of Chicago, Williams Bay, Wisconsin,_U.S.x'\. It was erected in 1896-7, and is the largest refracting telescope in the world. Diameter of object-glass, 40 inches ; length of telescope, about 60 feet. The object-glass was made by the firm of Alvan Clark and Sons, of Cambridge, Massachusetts ; the other portions of the instalment by the Warner and Swasey Co., of Cleveland. Ohio. (Page 117') The Growth of Observation form of it called the ''Coelostat/' in which a plane mirror is made to revolve in a certain manner, so as to reflect those portions of the sky which are to be observed, into the tube of a telescope kept fixed. Such too are the '^ Equatorial Coude" of the late M. Loewy, Director of the Paris Observatory, and the " Sheepshanks Telescope " of the Observatory of Cambridge, in which a telescope is separated into two portions, the eye-piece portion being fixed upon a downward slant, and the object-glass portion jointed to it at an angle and pointed up at the sky. In these two instruments (which, by the way, differ materially) an arrangement of slanting mirrors in the tubes directs the journey of the rays of light from the object-glass to the eye-piece. The observer can thus sit at the eye-end of his telescope in the warmth and comfort of his room, and observe the stars in the same unconstrained manner as if he were merely looking down into a microscope. Needless to say, devices such as these are subject to the drawback that the mirrors employed sap a certain proportion of the rays of light. It will be remembered that we made allusion to loss of light in this way, when pointing out the advantage in light grasp of the Herschelian form of telescope, where only one reflection takes place, over the Newtonian in which there are two. It is an interesting question as to whether tele- scopes can be made much larger. The American astronomer, Professor G. E. Hale, concludes that the limit of refractors is about five feet in diameter, but he thinks that reflectors as large as nine feet in diameter might now be made. As regards refractors 119 Astronomy of To-day there are several strong reasons against augmenting their proportions. First of all comes the great cost. Secondly, since the lenses are held in position merely round their rims, they will bend by their weight in the centres if they are made much larger. On the other hand; attempts to obviate this, by making the lenses thicker, would cause a decrease in the amount of light let through. But perhaps the greatest stumbling-block to the construction of larger telescopes is the fact that the unsteadiness of the air will be increasingly magnified. And further, the larger the tubes become, the more difficult will it be to keep the air within them at one constant temperature throughout their lengths. It would, indeed, seem as if telescopes are not destined greatly to increase in size, but that the means of observation will break out in some new direction, as it has already done in the case of photography and the spectroscope. The direct use of the eye is gradually giving place to indirect methods. We are, in fact, now feeling rather than seeing our way about the universe. Up to the present, for instance, we have not the slightest proof that life exists else- where than upon our earth. But who shall say that the twentieth century has not that in store for us, by which the presence of life in other orbs may be perceived through some form of vibration trans- mitted across illimitable space ? There is no use speaking of the impossible or the inconceivable. After the extraordinary revelations of the spectroscope — nay, after the astounding discovery of Rontgen — the word impossible should be cast aside, and incon- ceivability cease to be regarded as any criterion. 1 20 CHAPTER XI SPECTRUM ANALYSIS If white light (that of the sun, for instance) be passed through a glass prism, namely, a piece of glass of triangular shape, it will issue from it in rainbow-tinted colours. It is a common experience with any of us to notice this when the sunlight shines through cut- glass, as in the pendant of a chandelier, or in the stopper of a wine-decanter. The same effect may be produced when light passes through water. The Rainbow, which we all know so well, is merely the result of the sunlight passing through drops of falling rain. White light is composed of rays of various colours. Red, orange, yellow, green, blue, indigo, and violet, taken all together, go, in fact, to make up that effect which we call white. It is in the course of the refraction, or bending of a beam of light, when it passes in certain conditions through a transparent and denser medium, such as glass or water, that the constituent rays are sorted out and spread in a row according to their various colours. This production of colour takes place usually near the edges of a lens ; and, as will be recollected, proved very obnoxious to the users of the old form of refracting telescope. It is, indeed, a strange irony of fate that this very 121 Astronomy of To-day same production of colour, which so hindered as- tronomy in the past, should have aided it in recent years to a remarkable degree. If sunlight, for instance, be admitted through a narrow slit before it falls upon a glass prism, it will issue from the latter in the form of a band of variegated colour, each colour blending insensibly with the next. The colours arrange them- selves always in the order which we have mentioned. This seeming band is, in reality, an array of countless coloured images of the original slit ranged side by side ; the colour of each image being the slightest possible shade different from that next to it. This strip of colour when produced by sunlight is called the ^^ Solar Spectrum" (see Fig. 9, p. 123). A similar strip, or spectrum^ will be produced by any other light ; but the appearance of the strip, with regard to pre- ponderance of particular colours, will depend upon the character of that light. Electric light and gas light yield spectra not unlike that of sunlight ; but that of gas is less rich in blue and violet than that of the sun. The Spectroscope, an instrument devised for the examination of spectra, is, in its simplest form, com- posed of a small tube with a narrow slit and prism at one end, and an eye-piece at the other. If we drop ordinary table salt into the flame of a gas light, the flame becomes strongly yellow. If, then, we observe this yellow flame with the spectroscope, we find that its spectrum consists almost entirely of two bright yellow transverse lines. Chemically considered ordi- nary table salt is sodium chloride ; that is to say, a compound of the metal sodium and the gas chlorine. Now if other compounds of sodium be experimented 122 Speetrum Analysis t I a. 'o m a H I o ^ga^^^:^5HgO 123 Astronomy of To-day with in the same manner, it will soon be found that these two yellow lines are characteristic of sodium when turned into vapour by great heat. In the same manner it can be ascertained that every element, when heated to a condition of vapour, gives as its spectrum a set of lines peculiar to itself. Thus the spectroscope enables us to find out the composition of substances by reducing them to vapour in the laboratory. In order to increase the power of a spectroscope, it is necessary to add to the number of prisms. Each, extra prism has the effect of lengthening the coloured strip still more, so that lines, which at first appeared to be single merely through being crowded together, are eventually drawn apart and become separately distinguishable. On this principle it has gradually been determined that the sun is composed of elements similar to those which go to make up our earth. Further, the com- position of the stars can be ascertained in the same manner ; and we find them formed on a like pattern, though with certain elements in greater or less pro- portion as the case may be. It is in consequence of our thus definitely ascertaining that the stars are self-luminous, and of a sun-like character, that we are enabled to speak of them as suns, or to call the sun a star. In endeavouring to discover the elements of which the planets and satellites of our system are composed, we, however, find ourselves baffled, for the simple reason that these bodies emit no real light of their own. The light which reaches us from them, being merely reflected sunlight, gives only the ordinary 124 I Spectrum Analjrsis solar spectrum when examined with the spectroscope. But in certain cases we find that the solar spectrum thus^ viewed shows traces of being weakened, or rather of suffering absorption ; and it is concluded that this may be due to the sunlight having had to pass through an atmosphere on its way to and from the surface of the planet from which it is reflected to us. Since the sun is found to be composed of elements similar to those which go to make up our earth, we need not be disheartened at this failure of the spec- troscope to inform us of the composition of the planets and satellites. We are justified, indeed, in assuming that more or less the same constituents run through our solar system ; and that the elements of which these bodies are composed are similar to those which are found upon our earth and in the sun. The spectroscope supplies us with even more in- formation. It tells us, indeed, whether the sunlike body which we are observing is moving away from us ./or towards us. A certain slight shifting of the lines towards the red or violet end of the spectrum re- spectively, is found to follow such movement. This method of observation is known by the name of Doppler's Method^ and by it we are enabled to confirm the evidence which the sunspots give us of the rota- tion of the sun ; for we find thus that one edge ^ The idea, initiated by Cliristian Doppler at Prague in 1842, was originally applied to sound. The approach or recession of a source from which sound is coming is invariably accompanied by alterations of pitch, as the reader has no doubt noticed when a whistling railway-engine has approached him or receded from him. It is to Sir William Huggins, however, that we are indebted for the application of the principle to spectroscopy. This he gave experimental proof of in the year 1S68. Astronomy of To-day of that body is continually approaching us, and the other edge is continually receding from us. Also, we can ascertain in the same manner that certain of the stars are moving towards us, and certain of them away from us. 126 CHAPTER XII THE SUN The sun is the chief member of our system. It controls the motions of the planets by its immense gravitative power. Besides this it is the most im- portant body in the entire universe, so far as we are concerned ; for it pours out continually that flood of light and heat, without which life, as we know it, would quickly become extinct upon our globe. Light and heat, though not precisely the same thing, may be regarded, however, as next-door neighbours. The light rays are those which directly affect the eye and are comprised in the visible spectrum. We feel the heat rays, the chief of which are beyond the red portion of the spectrum. They may be investi- gated with the bolometer^ an instrument invented by the late Professor Langley. Chemical rays — for in- stance, those radiations which affect the photographic plate — are for the most part also outside the visible spectrum. They are, however, at the other end of it, namely, beyond the violet. Such a scale of radiations may be compared to the keyboard of an imaginary piano, the sound from only one of whose octaves is audible to us. The brightest light we know on the earth is dull compared with the light of the sun. It would, indeed, look quite dark if held up against it. 127 Astronomy of To-day It is extremely difficult to arrive at a precise notion of the temperature of the body of the sun. However, it is far in excess of any temperature which we can obtain here, even in the most powerful electric fur- nace. A rough idea of the solar heat may be gathered from the calculation that if the sun's surface w^ere coated all over with a layer of ice 4000 feet thick, it would melt through this completely in one hour. The sun cannot be a hot body merely cooling ; for the rate at which it is at present giving off heat could not in such circumstances be kept up, according to Professor Moulton, for more than 3000 years. Further, it is not a mere burning mass, like a coal fire, for instance ; as in that case about a thousand years w^ould show a certain drop in temperature. No per- ceptible diminution of solar heat having taken place within historic experience, so far as can be ascer- tained, we are driven to seek some more abstruse explanation. The theory which seems to have received most acceptance is that put forward by Helmholtz in 1854. His idea was that gravitation produces continual con- traction, or falling in of the outer parts of the sun ; and that this falling in, in its turn, generates enough heat to compensate for what is being given off. The calculations of Helmholtz showed that a contraction of about 100 feet a year from the surface towards the centre would suffice for the purpose. In recent years, however, this estimate has been extended to about 180 feet. Nevertheless, even with this increased figure, the shrinkage required is so slight in com- parison with the immense girth of the sun, that it 128 The Sun would take a continual contraction at this rate for about 6000 years, to show even in our finest tele- scopes that any change in the size of that body was taking place at all. Upon this assumption of con- tinuous contraction, a time should, however, eventually be reached when the sun will have shrunk to such a degree of solidity, that it will not be able to shrink any further. Then, the loss of heat not being made up for any longer, the body of the sun should begin to m^ow cold. But we need not be distressed on this account ; for it will take some 10,000,000 years, according to the above theory, before the solar orb becomes too cold to support life upon our earth. Since the discovery of radium it has, on the other hand, been suggested, and not unreasonably, that radio-active matter may possibly play an important part in keeping up the heat of the sun. But the body of scientific opinion appears to consider the theory of contraction as a result of gravitation, which has been outlined above, to be of itself quite a sound explana- tion. Indeed, the late Lord Kelvin is said to have held to the last that it was amply sufficient to account for the underground heat of the earth, the heat of the sun, and that of all the stars in the universe. One great difficulty in forming theories with regard to the sun, is the fact that the temperature and gravi- tation there are enormously in excess of anything \\q meet with upon our earth. The force of gravity at the sun's surface is, indeed, about twenty-seven times that at the surface of our globe. The earth's atmosphere appears to absorb about one-half of the radiations which come to us from the sun. This absorptive effect is very noticeable when 129 I Astronomy of To-day the solar orb is low down in our sky, for iits light and heat are then clearly much reduced. Of the light rays, the blue ones are the most easily absorbed in this way ; which explains why the sun looks red when near the horizon. It has then, of course, to shine through a much greater thickness of atmosphere than when high up in the heavens. What astonishes one most about the solar radiation, is the immense amount of it that is apparently wasted into space in comparison with what falls directly upon the bodies of the solar system. Only about the one-hundred-millionth is caught by all the planets together. What becomes of the rest we cannot tell. That brilliant white body of the sun, which we see, is enveloped by several layers of gases and vaporous matter, in the same manner as our globe is enveloped by its atmosphere (see Fig. lo, p. 131). These are transparent, just as our atmosphere is transparent ; and so we see the white bright body of the sun right through them. This white bright portion is called the Photosphere, From it comes most of that light and heat which we see and feel. We do not know what lies under the photosphere, but, no doubt, the more solid portions of the sun are there situated. Just above the photosphere, and lying close upon it, is a veil of smoke-like haze. Next upon this is what is known as the Reversing Layer, which is between 500 and 1000 miles in thick- ness. It is cooler than the underlying photosphere, and is composed of glowing gases. Many of, the elements which go to make up our earth are present in the reversing layer in the form of vapour. The Chromosphere^ of which especial mention has 130 The Sun already been made in dealing with eclipses of the sun, is another layer lying immediately upon the last one. It is between 5000 and 10,000 miles in thickness. Like the reversing layer, it is composed of glowing gaseS; chief among which is the vapour of hydrogen. The colour of the chromosphere is, in reality, a tos frh^re Fig. 10. — A section through the Sun, showing how the prominences rise from the chromosphere. brilliant scarlet ; but, as we have already said, the intensely white light of the photosphere shines through it from behind, and entirely overpowers its redness. The upper portion of the chromosphere is in violent agitation, like the waves of a stormy sea, and from it rise those red prominences which, it will be re- collected, are such a notable feature in total solar eclipses. 131 Astronomy of To=day The Corona lies next in order outside the chromo- sphere, and is, so far as we know, the outermost of the accompaniments of the sun. This halo of pearly- white light is irregular in outline, and fades away into the surrounding sky. It extends outwards from the sun to several millions of miles. As has been stated, we can never see the corona unless, when during a total solar eclipse, the moon has, for the time being, hidden the brilliant photosphere com- pletely from our view. The solar spectrum is really composed of three separate spectra commingled, i.e. those of the photo- sphere, of the reversing layer, and of the chromo- sphere respectively. If, therefore, the photosphere could be entirely removed, or covered up, we should see only the spectra of those layers which lie upon it. Such a state of things actually occurs in a total eclipse of the sun. When the moon's body has crept across the solar disc, and hidden the last piece of photo- sphere, the solar spectrum suddenly becomes what is technically called ^'reversed," — the dark lines cross- ing it changing into bright lines. This occurs because a strip of those layers which lie immediately upon the photosphere remains still uncovered. The lower of these layers has therefore been called the ^^re- versing layer," for want of a better name. After a second or two this reversed spectrum mostly vanishes, and an altered spectrum is left to view. Taking into consideration the rate at which the moon is moving across the face of the sun, and the very short time during which the spectrum of the reversing layer lasts, the thickness of that layer is estimated to be 132 The Sun not more than a few hundred miles. In the same way the last of the three spectra — namely, that of the chromosphere — remains visible for such a time as allows us to estimate its depth at about ten times that of the reversing layer, or several thousand miles. When the chromosphere, in its turn during a total eclipse, has been covered by the moon, the corona alone is left. This has a distinct spectrum of its own also ; wherein is seen a strange line in the green portion, which does not tally with that of any element we are acquainted with upon the earth. This un- known element has received for the time being the name of '< Coronium." 133 CHAPTER XIII THE SUN—conf'muecI The various parts of the Sun will now be treated of in detail. I. Photosphere. The photosphere, or " light-sphere/' from the Greek m (p^os), which means lz£-kt, is, as we have already said, the innermost portion of the sun which can be seen. Examined through a good telescope it shows a finely mottled structure, as of brilliant granules, some- what like rice grains, with small dark spaces lying in between them. It has been supposed that we have here the process of some system of circulation by which the sun keeps sending forth its radiations. In the bright granules we perhaps see masses of intensely heated matter, rising from the interior of the sun. The dark interspaces may represent matter which has become cooled and darkened through having parted with its heat and light, and is falling back again into the solar furnace. The sun spots, so familiar to every one nowadays, are dark patches which are often seen to break out in the photosphere (see Plate V., p. 134). They last during various periods of time ; sometimes only for a few days, sometimes so long as a month or more. A spot is usually composed of a dark central, portion called the umbra^ 134 Plate V. The Sun, showing several groups of Spots From a photograph taken at the Royal Observatory, Greenwich. The cross-lines seen on the disc are in no way connected with the Sun, but belong to the telescope through which the photograph was taken. (Page 134) The Sun and a less dark fringe around this called Wx^ penumbra (see Plate VI., p. 136). The umbra ordinarily has the appearance of a deep hole in the photosphere ; but, that it is a hole at all, has by no means been definitely proved. Sun spots are, as a rule, some thousands of miles across. The umbra of a good-sized spot could indeed engulf at once many bodies the size of our earth. Sun spots do not usually appear singly, but in groups. The total area of a group of this kind may be of immense extent ; even so great as to cover the one-hundredth part of the whole surface of the sun. Very large spots, when such are present, may be seen without any telescope ; either through a piece of smoked glass, or merely with the naked eye when the air is misty, or the sun low on the horizon. The umbra of a spot is not actually dark. It only appears so in contrast with the brilliant photosphere around. Spots form, grow to a large size in comparatively short periods of time, and then quickly disappear. They seem to shrink away as a consequence of the photosphere closing in upon them. That the sun is rotating upon an axis, is shown by the continual change of position of all spots in one constant direction across his disc. The time in which a spot is carried completely round depends, however, upon the position which it occupies upon the sun's surface. A spot situated near the equator of the sun goes round once in about twenty-five days. The further a spot is situated from this equator, the longer it takes. About twenty-seven days is the time taken by a spot situated midway between the equator and 135 Astronomy of To-day the solar poles. Spots occur to the north of the sun's equator, as well as to the south ; though, since regular observations have been made — that is to say, during the past fifty years or so — they appear to have broken out a little more frequently in the southern parts. From these considerations it will be seen that the sun does not rotate as the earth does, but that different portions appear to move at different speeds. Whether in the neighbourhood of the solar poles the time of rotation exceeds twenty-seven days we are unable to ascertain, for spots are not seen in those regions. No explanation has yet been given of this peculiar rotation ; and the most we can say on the subject is that the sun is not by any means a solid body. Faculcs (Latin, little torches) are brilliant patches which appear here and there upon the sun's surface, and are in some way associated with spots. Their displacement, too, across the solar face confirms the evidence which the spots give us of the sun's rotation. Our proofs of this rotation are still further strength- ened by the Doppler spectroscopic method of obser- vation alluded to in Chapter XL As was then stated, one edge of the sun is thus found to be continually approaching us, and the other side continually re- ceding from us. The varying rates of rotation, which the spots and faculse give us, are duly confirmed by this method. The first attempt to bring some regularity into the question of sun-spots was the discovery by Schwabe, in 1852, that they were subject to a regular variation. As a matter of fact they wax and wane in their number, and the total area which they cover, in 136 Plate VI. Photograph ok a Sunspot This fine picture was taken by the late M. Janssen. The granular structure of the Sun' surface is here well represented. (From A'/io-a'/i-cft^w) (Page 135) The Sun the course of a period, or cycle, of on an average about II J years; being at one part of this period large and abundant, and at another few and small. This period of iij years is known as the sun spot cycle. No explanation has yet been given of the curious round of change, but the period in question seems to govern most of the phenomena connected with the sun. II. Reversing Layer. This is a layer of relatively cool gases lying im- mediately upon the photosphere. We never see it directly ; and the only proof we have of its presence is that remarkable reversal of the spectrum already described, when during an instant or two in a total eclipse, the advancing edge of the moon, having just hidden the brilliant photosphere, is moving across the fine strip which the layer then presents edge-wise towards us. The fleeting moments during which this reversed spectrum lasts, informs us that the layer is comparatively shallow ; little more indeed than about 500 miles in depth. The spectrum of the reversing layer, or *^ flash spectrum," as it is sometimes called on account of the instantaneous character with which the change takes place, was, as we have seen, first noticed by Young in 1870 ; and has been successfully photographed since then during several eclipses. The layer itself appears to be in a fairly quiescent state ; a marked contrast to the seething photosphere beneath, and the agitated chromosphere above. 137 Astronomy of To-day III. The Chromosphere. The Chromosphere— so called from the Greek ^/owyLta {chroma)y which signifies colour — is a layer of gases lying immediately upon the preceding one. Its thickness is, however, plainly much the greater of the two ; for whereas the reversing layer is only revealed to us indirectly by the spectroscope, a portion of the chromosphere may clearly be seen in a total eclipse in the form of a strip of scarlet light. The time which the moon's edge takes to traverse it tells us that it must be about ten times as deep as the revers- ing layer, namely, from 5000 to 10,000 miles in depth. Its spectrum shows that it is composed chiefly of hydrogen, calcium and helium, in the state of vapour. Its red colour is mainly due to glowing hydrogen. The element helium, which it also contains, has received its appellation from r]kio<^ {kelios), the Greek name for the sun ; because, at the time when it first attracted attention, there appeared to be no element corresponding to it upon our earth, and it was consequently imagined to be confined to the sun alone. Sir William Ramsay, however, dis- covered it to be also a terrestrial element in 1895, and since then it has come into much prominence as one of the products given off by radium. Taking into consideration the excessive force of gravity on the sun, one would expect to find the chromosphere and reversing layer growing gradually thicker in the direction of the photosphere. This, however, is not the case. Both these layers are strangely enough of the same densities all through ; 138 The Sun which makes it suspected that, in these regionS; the force of gravity may be counteracted by some other force or forces, exerting a powerful pressure out- wards from the sun. IV. The Prominences. We have already seen, in dealing with total eclipses, that the exterior surface of the chromo- sphere is agitated like a stormy sea, and from it billows of flame are tossed up to gigantic heights. These flaming jets are known under the name of pro- minences, because they were first noticed in the form of brilliant points projecting from behind the rim of the moon when the sun was totally eclipsed. Prominences are of two kinds, eruptive and quiescent. The eruptive prominences spurt up directly from the chromosphere with immense speeds, and change their shape with great rapidity. Quiescent promi- nences, on the other hand, have a form somewhat like trees, and alter their shape but slowly. In the eruptive prominences glowing masses of gas are shot up to altitudes sometimes as high as 300,000 miles,^ with velocities even so great as from 500 to 600 miles a second. It has been noticed that the eruptive prominences are mostly found in those portions of the sun where spots usually appear, namely, in the regions near the solar equator. The quiescent prominences, on the other hand, are con- fined, as a rule, to the neighbourhood of the sun's poles. 1 On November 15, 1907, Dr. A. Rambaut, Radcliffe Observer at Oxford University, noted a prominence which rose to a height of 324,600 miles. Astronomy of To-day Prominences were at first never visible except during total eclipses of the sun. But in the year 1868, as we have already seen, a method of employing the spectroscope was devised, by means of which they could be observed and studied at any time, without the necessity of waiting for an eclipse. A still further development of the spectroscope, the Spectroheliograph, an instrument invented almost simultaneously by Professor Hale and the French astronomer, M. Deslandres, permits of photographs being taken of the sun, with the light emanating from only one of its glowing gases at a time. For instance, we can thus obtain a record of what the glowing hydrogen alone is doing, on the solar body at any particular moment. With this instrument it is also possible to obtain a series of photographs, showing what is taking place upon the sun at various levels. This is very useful in connection with the study of the spots ; for we are, in consequence, enabled to gather more evidence on the subject of their actual form than is given us by their highly foreshortened appearances when observed directly in the telescope. V. Corona. (Latin, a Crown.) This marvellous halo of pearly-white, light, which displays itself to our view only during the total phase of an eclipse of the sun, is by no means a layer like those other envelopments of the sun of which we have just been treating. It appears, on the other hand, to be composed of filmy matter, radiating outwards in every direction, and fading 140 The Sun away gradually into space. Its structure is noted to bear a strong resemblance to the tails of comets, or the streamers of the aurora borealis. Our knowledge concerning the corona has, how- ever, advanced very slowly. We have not, so far, been as fortunate with regard to it as with regard to the prominences ; and, for all we can gather concerning it, we are still entirely dependent upon the changes and chances of total solar eclipses. All attempts, in fact, to apply the spectroscopic method, so as to observe the corona at leisure in full sunlight in the way in which the prominences can be observed, have up to the present met with failure. The general form under which the corona appears to our eyes varies markedly at different eclipses. Sometimes its streamers are many, and radiate all round ; at other times they are confined only to the middle portions of the sun, and are very elongated, with short feathery-looking wisps adorning the solar poles. It is noticed that this change of shape varies in close accordance with that ii| year period during which the sun spots wax and wane ; the many- streamered regular type corresponding to the time of great sun-spot activity, while the irregular type with the long streamers is present only when the spots are few (see Plate VII., p. 142). Streamers have often been noted to issue from those regions of the sun where active prominences are at the moment in existence ; but it cannot be laid down that this is always the case. No hypothesis has yet been formulated which will account for the structure of the corona, or for its variation in shape. The great difficulty with regard 141 Astronomy of To-day to theorising upon this subject, is the fact that we see so much of the corona under conditions of marked foreshortening. Assuming, what indeed seems natural, that the rays of which it is composed issue in every direction from the solar body, in a manner which may be roughly imitated by sticking pins all over a ball ; it is plainly impossible to form any definite idea concerning .streamers, which actually may owe most of the shape they present to us, to the mixing up of multitudes of rays at all kinds of angles to the line of sight. In a word, w^e have* to try and form an opinion concerning an arrange- ment which, broadly speaking, is spherical^ but which, on account of its distance, must needs appear to us as absolutely yf<3:^. The most known about the composition of the corona is that it is made up of particles of matter, mingled with a glowing gas. It is an element in the composition of this gas which, as has been stated, is not found to tally with any known terrestrial element, and has, therefore, received the name of coronium for want of a better designation. One definite conclusion appears to be reached with regard to the corona, Le, that the matter of which it is composed, must be exceedingly rarefied ; as it is not found, for instance, to retard appreciably the speed of comets, on occasions when these bodies pass very close to the sun. A calculation has indeed been made which would tend to show that the particles composing the coronal matter, are separated from each other by a distance of perhaps between two and three yards 1 The density of the corona is found not to increase inwards towards the sun. 142 The Total Eclipse of the Sun of December 22nd, 1870 Drawn by Mr. W. H. Wesley from a photograph taken at Syracuse by Mr. Brothers. This is the type of corona seen at the time oi greatest sunspot activity. The coronas of 1882 (Plate I., p. 96) and of 1905 (Frontispiece) are of the same type. (B.) The Total Eclipse of the Sun of May 2Sth, 1900 _ Drawn by Mr. W. H. Wesley from photographs taken by Mr. E. W. Maunder. This IS the type of corona seen when thesunspotsare least active. Compare the " Ring with Wings," Fig. 7, p. 87. Plate VII. Forms of the Solar Corona at the epochs of Sunspot INIaximum and Sunspot Minimum, respectively (.Page 141 The Sun This is what has already been noted with regard to the layers lying beneath it. Powerful forces, acting in opposition to gravity, must hold sway here also. The II J year period, during which the sun spots vary in number and size, appears to govern the ac- tivities of the sun much in the same way that our year does the changing seasonal conditions of our earth. Not only, as we have seen, does the corona vary its shape in accordance with the said period, but the activity of the prominences, and of the faculae, follow suit. Further, this constant round of ebb and flow is not confined to the sun itself, but, strangely enough, affects the earth also. The displays of the aurora borealis, which we experience here, coincide closely with it, as does also the varying state of the earth's magnetism. The connection may be still better appreciated when a great spot, or group of spots, has made its appearance upon the sun. It has, for example, often been noted that when the solar rotation carries a spot, or group of spots, across the middle of the visible surface of the sun, our mag- netic and electrical arrangements are disturbed for the time being. The magnetic needles in our obser- vatories are, for instance, seen to oscillate violently, telegraphic communication is for a while upset, and magnificent displays of the aurora borealis illumine our night skies. Mr. E. W. Maunder, of Greenwich Observatory, who has made a very careful investiga- tion of this subject, suspects that, when elongated coronal streamers are whirled round in our direction by the solar rotation, powerful magnetic impulses may be projected upon us at the moments when such streamers are pointing towards the earth. 143 Astronomy of To-day Some interesting investigations with regard to sun- spots have recently been published by Mrs. E. W. Maunder. In an able paper, communicated to the Royal Astronomical Society on May lo, 1907; she reviews the Greenwich Observatory statistics dealing with the number and extent of the spots which have appeared during the period from 1889 to 1901 — a whole sun-spot cycle. From a detailed study of the dates in question, she finds that the number of those spots which are formed on the side of the sun turned away from us, and die out upon the side turned to- wards us, is much greater than the number of those which are formed on the side turned towards us and die out upon the side turned away. It used, for in- stance, to be considered that the influence of a planet might prodtcce sunspots ; but these investigations make it look rather as if some influence on the part of the earth tends, on the contrary, to extinguish them. Mrs. Maunder, so far, prefers to call the influence thus traced an apparent influence only, for, as she very fairly points out, it seems difficult to attribute a real influence in this matter to the earth, which is so small a thing in comparison not only with the sun, but even with many individual spots. The above investigation was to a certain degree anticipated by Mr. Henry Corder in 1895 ; but Mrs. Maunder's researches cover a much longer period, and the conclusions deduced are 'of a wider and more defined nature. With regard to its chemical composition, the spec- troscope shows us that thirty-nine of the elements which are found upon our earth are also to be found in the sun. Of these the best 'known are hydrogen, 144 The Sun oxygen^ helium, carbon, calcium, aluminium, iron, copper, zinc, silver, tin, and lead. Some elements of the metallic order have, however, not been found there, as, for instance, gold and mercury ; while a few of the other class of element, such as nitrogen, chlorine, and sulphur, are also absent. It must not, indeed, be concluded that the elements apparently missing do not exist at all in the solar body. Gold and mercury have, in consequence of their great atomic weight, perhaps sunk away into the centre. Again, the fact that we cannot find traces of certain other elements, is no real proof of their entire absence. Some of them may, for instance, be resolved into even simpler forms, under the unusual conditions which exist in the sun ; and so we are unable to trace them with the spectroscope, the experience of which rests on laboratory experiments conducted, at best, in con- ditions which obtain upon the earth. H5 CHAPTER XIV THE INFERIOR PLANETS Starting from the centre of the solar system, the first body we meet with is the planet Mercury. It circulates at an average distance from the sun of about thirty-six millions of miles. The next body to it is the planet Venus, at about sixty-seven millions of miles, namely, about double the distance of Mercury from the sun. Since our earth comes next again, astronomers call those planets which circulate within its orbit, i.e. Mercury and Venus, the Inferior Planets, while those which circulate outside it they call the Superior Planets.^ In studying the inferior planets, the circumstances in which we make our observations are so very similar with regard to each, that it is best to take them together. Let us begin by considering the various positions of an inferior planet, as seen from the earth, during the course of its journeys round the sun. When furthest from us it is at the other side of the sun, and cannot then be seen owing to the blaze of light. As it continues its journey it passes to the left of the sun, and is then sufficiently away from the glare to be plainly seen. It next draws in again to- ^ In employing the terms Inferior and Superior the writer bows to astronomical custom, though he cannot help feeling that, in the circum- stances, Interior and Exterior would be much more appropriate. 146 The Inferior Planets wards the sun, and is once more lost to view in the blaze at the time of its passing nearest to us. Then it gradually comes out to view on the right hand, separates from the sun up to a certain distance as before, and again recedes beyond the sun, and is for the time being once more lost to view. To these various positions technical names are given. When the inferior planet is on the far side of the sun from us, it is said to be in Superior Con- junction. When it has drawn as far as it can to the left hand, and is then as east as possible of the sun, it is said to be at its Greatest Eastern Elongation. Again, when it is passing nearest to us, it is said to be in Inferior Conjunction; and, finally, when it has drawn as far as it can to the right hand, it is spoken of as being at its Greatest Western Elongation (see Fig. ii, p. 148). The continual variation in the distance of an in- terior planet from us, during its revolution around the sun, will of course be productive of great alterations in its apparent size. At superior conjunction it ought, being then farthest away, to show the smallest disc ; while at inferior conjunction, being the nearest, it should look much larger. When at greatest elonga- tion, whether eastern or western, it should naturally present an appearance midway in size between the two. From the above considerations one would be in- clined to assume that the best time for studying the surface of an interior planet with the telescope is when it is at inferior conjunction, or, nearest to us. But that this is not the case will at once appear if we consider that the sunlight is then falling upon the side 147 Astronomy of To-day away from us, leaving the side which is towards us unillumined. In superior conjunction, on the other hand, the light falls full upon the side of the planet Various positions, and illumination by the Sun, of an Inferior Planet in the course of its orbit. 0€i)#(13C)6 Corresponding views of the same situations of an Inferior Planet as seen from the Earth, showing consequent phases and alterations in apparent size. Fig. II. — Orbit and Phases of an Inferior Planet. facing us ; but the disc is then so small-looking, and our view besides is so dazzled by the proximity of the sun, that observations are of little avail. In the elon- gations, however, the sunlight comes from the side, 148 The Inferior Planets and so we see one half of the^planet lit up ; the right half at eastern elongation, and the left half at western elongation. Piecing together the results given us at these more favourable views, we are enabled, bit by- bit, to gather some small knowledge concerning the surface of an inferior planet. From these considerations it will be seen at once that the inferior planets show various phases com- parable to the waxing and waning of our moon in its monthly round. Superior conjunction is, in fact, similar to full moon, and inferior conjunction to new moon ; while the eastern and western elongations may be compared respectively to the moon's first and last quarters. It will be recollected how, when these phases were first seen by the early telescopic ob- servers, the Copernican theory was felt to be im- mensely strengthened ; for it had been pointed out that if this system were the correct one, the planets Venus and Mercury, were it possible to see them more distinctly, would of necessity present phases like these when viewed from the earth. It should here be noted that the telescope was not invented until nearly seventy years after the death of Copernicus. The apparent swing of an inferior planet from side to side of the sun, at one time on the east side, then passing into and lost in the sun's rays to appear once more on the west side, is the explanation of what is meant when we speak of an evening or a morning star. An inferior planet is called an evening star when it is at its eastern elongation, that is to say, on the left-hand of the sun ; for, being then on the eastern side, it will set after the sun sets, as both sink in their turn below the western horizon at the close of 149 Astronomy of To-day day. Similarly, when such a planet is at its western elongation, that is to say, to the right-hand of the sun, it will go in advance of him, and so will rise above the eastern horizon before the sun rises, receiving therefore the designation of morning star. In very early times, Jiowever, before any definite ideas had been come to with regard to the celestial motions, it was generally believed that the morning and evening stars were quite distinct bodies. Thus Venus, when a morning star, w^as known to the ancients under the name of Phosphorus, or Lucifer ; v\^hereas they called it Hesperus when it was an evening star. Since an inferior planet circulates between us and the sun, one would be inclined to expect that such a body, each time it passed on the side nearest to the earth, should be seen as a black spot against the bright solar disc. Now this would most certainly be the case were the orbit of an inferior planet in the same plane with the orbit of the earth. But w^e have already seen how the orbits in the solar system, whether those of planets or of satellites, are by no means in the one plane ; and that it is for this very reason that the moon is able to pass time after time in the direction of the sun, at the epoch known as new moon, and yet not to eclipse him save after the lapse of several such passages. Transits, then, as the passages of an inferior planet across the sun's disc are called, take place, for the same reason, only after certain regular lapses of time ; and, as regards the circumstances of their occurrence, are on a par with eclipses of the sun. The latter, however, happen much more frequently, because the moon passes in the neighbourhood of the sun, roughly speaking, once 150 The Inferior Planets a month, whereas Venus conies to each inferior con- junction at intervals so long apart as a year and a half, and Mercury only about every four months. From this it will be further gathered that transits of Mercury take place much oftener than transits of Venus. Until recent years Transits of Venus wxre pheno- mena of great importance to astronomers, for they furnished the best means then available of calculat- ing the distance of the sun from the earth. This was arrived at through comparing the amount of apparent displacement in the planet's path across the solar disc, when the transit was observed from widely separated stations on the earth's surface. The last transit of Venus took place in 1882, and there will not be another until the year 2004. Transits of Mercury^ on the other hand, are not of much scientific importance. They are of no interest as a popular spectacle ; for the dimensions of the planet are so small, that it can be seen only with the aid of a telescope when it is in the act of crossing the sun's disc. The last transit of Mercury took place on November 14, 1907, and there will be another on November 6, -1914. The first person known to have observed a transit of an inferior planet was the celebrated French philo- sopher, Gassendi. This was the transit of Mercury which took place on the 7th of December 1631. The first time a transit of Venus was ever seen, so far as is known, was on the 24th of November 1639. The observer was a certain Jeremiah Horrox, curate of Hoole, near Preston, in Lancashire. The transit in question commenced shortly before sunset, and his 151 Astronomy of To-day observations in consequence were limited to only about half-an-hour. Horrox happened to have a great friend, one William Crabtree, of Manchester, whom he had advised by letter to be on the look out for the phenomenon. The weather in Crabtree's neighbourhood was cloudy, with the result that he only got a view of the transit for about ten minutes before the sun set. That this transit was observed at all is due entirely to the remarkable ability of Horrox. According to the calculations of the great Kepler, no transit could take place that year (1639), as the planet would just pass clear of the lower edge of the sun. Horrox, however, not being satisfied with this, worked the question out for himself, and came to the conclusion that the planet would actually traverse the lower portion of the sun's disc. The event, as we have seen, proved him to be quite in the right. Horrox is said to have been a veritable prodigy of astro- nomical skill ; and had he lived longer would, no doubt, have become very famous. Unfortunately he died about two years after his celebrated transit, in his twenty-second ytdiV only, according to the accounts. His friend Crabtree, who was then also a young man, is said to have been killed at the battle of Naseby in 1645. There is an interesting phenomenon in connection with transits which is known as the ''Black Drop." When an inferior planet has just made its way on to the face of the sun, it is usually seen to remain for a short time as if attached to the sun's edge by what looks like a dark ligament (see Fig. 12, p. 153). This gives to the planet for the time being an elongated 152 The Inferior Planets appearance, something like that of a pear ; but when the Hgament, which all the while keeps getting thinner and thinner, has at last broken, the black body of the planet is seen to stand outu'ound against the solar disc. This appearance may be roughly compared to the manner in which a drop of liquid (or, preferably, of Fig. 12.— The "Black Drop, some glutinous substance) tends for a while to adhere to an object from which it is falling. When the planet is in turn making its way off the face of the sun, the ligament is again seen to form and to attach it to the sun's edge before its due time. The phenomenon of the black drop, or ligament, is entirely an illusion, and, broadly speaking, of an optical origin. Something very similar will be noticed if one 153 Astronomy of To-day brings one's thumb and forefinger slowly together against a very bright background. This peculiar phenomenon has proved one of the greatest drawbacks to the proper observation of tran- sits, for it is quite impossible to note the exact instant of the planet's entrance upon and departure from the solar disc in conditions such as these. The black drop seems to bear a family relation, so to speak, to the phenomenon of Daily's beads. In the latter instance the lunar peaks, as they approach the sun's edge, appear to lengthen out in a similar manner and bridge the intervening space before their time, thus giving prominence to an effect which otherwise should scarcely be noticeable. The last transit of Mercury, which, as has been already stated, took place on November 14, 1907, was not successfully observed by astronomers in England, on account of the cloudiness of the weather. In France, however. Professor Moye, of Montpellier, saw it under good conditions, and mentions that the black drop remained very conspicuous for fully a minute. The transit was also observed in the United States, the reports from which speak of the black drop as very ^^troublesome." Before leaving the subject of transits it should be mentioned that it was in the capacity of commander of an expedition to Otaheite, in the Pacific, to observe the transit of Venus of June 3, 1769, that Captain Cook embarked upon the first of his celebrated voyages. In studying the surfaces of Venus and Mercury with the telescope, observers are, needless to say, very much hindered by the proximity of the sun. Venus, 154 The Inferior Planets when at the greatest elongations, certainly draws some distance out of the glare ; but her surface is, even then, so dazzlingly bright, that the markings upon it are difficult to see. Mercury, on the other hand, is much duller in contrast, but the disc it shows in the telescope is exceedingly small ; and, in addition, when that planet is left above the horizon for a short time after sunset, as necessarily happens after certain in- tervals, the mists near the earth's surface render observation of it very difficult. Until about twenty-five years ago, it was generally believed that both these planets rotated on their axes in about twenty-four hours, a notion, no doubt, originally founded upon an unconscious desire to bring them into some conformity with our earth. But Schiaparelli, observing in Italy, and Percival Lowell, in the clear skies of Arizona and Mexico, have lately come to the conclusion that both planets rotate upon their axes in the same time as they revolve in their orbits,^ the result being that they turn one face ever towards the sun in the same manner that the moon turns one face ever towards the earth — a curious state of things, which will be dealt with more fully when we come to treat of our satellite. The marked difference in the brightness between the two planets has already been alluded to. The surface of Venus is, indeed, about five times as bright as that of Mercury. The actual brightness of Mercury is about equivalent to that of our moon, and astro- nomers are, therefore, inclined to think that it may 1 This question is, however, uncertain, for some very recent spectro- scopic observations of Venus seem to show a rotation period of about twenty-four hours. Astronomy of To-day resemble her in having a very rugged surface and practically no atmosphere. This probable lack of atmosphere is further corroborated by two circum- stances. One of these is that when Mercury is just about to transit the face of the sun, no ring of diffused light is seen to encircle its disc as would be the case if it possessed an atmosphere. Such a lack of atmosphere is, indeed, only to be expected from what is known as the Kinetic Theory of Gases, According to this theory, which is based upon the behaviour of various kinds of gas, it is found that these elements tend to escape into space from the surface of bodies whose force of gravitation is weak. Hydrogen gas, for example, tends to fly away from our earth, as any one may see for himself when a balloon rises into the air. The gravitation of the earth seems, however, powerful enough to hold down other gases, as, for instance, those of which the air is chiefly composed, namely, oxygen and nitrogen. In due accordance with the Kinetic theory, we find the moon and Mercury, which are much about the same size, destitute of atmospheres. Mars, too, .whose diameter is only about double that of the moon, has very little atmosphere. We find, on the other hand, that Venus, which is about the same size as our earth, clearly possesses an atmosphere, as just before the planet is in transit across the sun, the outline of its dark body is seen to be surrounded by a bright ring of light. The results of telescopic observation show that more markings are visible on Mercury than on Venus. The intense brilliancy of Venus is, indeed, about the same as that of our white clouds when the sun is 156 The Inferior Planets shining directly upon them. It has, therefore, been supposed that the planet is thickly enveloped in cloud, and that we do not ever see any part of its surface, except perchance the summit of some lofty mountain projecting through the fleecy mass. With regard to the great brilliancy of Venus, it may be mentioned that she has frequently been seen in England, with the naked eye in full sunshine, when at the time of her greatest brightness. The writer has seen her thus at noonday. Needless to say, the sky at the moment was intensely blue and clear. The orbit of Mercury is very oval, and much more so than that of any other planet. The consequence is that, when Mercury is nearest to the sun, the heat which it receives is twice as great as when it is farthest away. The orbit of Venus, on the other hand, is in marked contrast with that of Mercury, and is, besides, more nearly of a circular shape than that of any of the other planets. Venus, there- fore, always keeps about the same distance from the sun, and so the heat which she receives during the course of her year can only be subject to very slight variations. 157 CHAPTER XV THE EARTH We have already seen (in Chapter I.) how, in very early times, men naturally enough considered the earth to be a flat plane extending to a very great distance in every direction ; but that, as years went on, certain of the Greek philosophers suspected it to be a sphere. One or two. of the latter are, indeed, said to have further believed in its rotation about an axis, and even in its revolution around the sun ; but, as the ideas in question were founded upon fancy, rather than upon any direct evidence, they did not generally attract attention. The small effect, there- fore, which these theories had upon astronomy, may well be gathered from the fact that in the Ptolemaic system the earth was considered as fixed and at the centre of things; and this belief, as we have seen, continued unaltered down to the days of Copernicus. It was, indeed, quite impossible to be certain of the real shape of the earth or the reality of its motions until knowledge became more extended and scientific instruments much greater in precision. We will now consider in detail a few of the more obvious arguments which can be put forward to show that our earth is a sphere. If, for instance, the earth were a plane surface, a ship sailing away from us over the sea would appear 158 The Earth to grow smaller and smaller as it receded into the distance, becoming eventually a tiny speck, and fading gradually from our view. This, however, is not at all what actually takes place. As we watch a vessel receding, its hull appears bit by bit to slip gently down over the horizon, leaving the masts alone visible. Then, in their turn, the masts are seen to slip down in the same manner, until eventually every trace of the vessel is gone. On the other hand, when a ship comes into view, the masts are the first portions to appear. They gradually rise up from below the horizon, and the hull follows in its turn, until the whole vessel is visible. Again, when one is upon a ship at sea, a set of masts will often be seen sticking up alone above the horizon, and these may shorten and gradually disappear from view without the body of the ship to which they belong becoming visible at all. Since one knows from experience that there is no edge at the horizon over which a vessel can drop down, the appearance which we have been describing can only be explained by supposing that the surface of the earth is always curving gradually in every direction. The distance at which what is known as the horizon lies away from us depends entirely upon the height above the earth's surface where we happen at the moment to be. A ship which has appeared to sink below the horizon for a person standing on the beach, will be found to come back again into view if he at once ascends a high hill. Experiment shows that the horizon line lies at about three miles away for a person standing at the water's edge. The curving of the earth's surface is found, indeed, to be at the rate ^59 Astronomy of To-day of eight inches in every mile. Now it can be ascer- tained, by calculation, that a body curving at this rate in every direction must be a globe about 8000 miles in diameter. Again, the fact that, if not stopped by such insuper- able obstacles as the polar ice and snow, those who travel continually in any one direction upon the earth's surface always find themselves back again at the regions from which ithey originally set out, is additional ground for concluding that the earth is a globe. We can find still further evidence. For instance, in an eclipse of the moon the earth's shadow, when seen creeping across the moon's face, is noted to be always circular in shape. One cannot imagine how such a thing could take place unless the earth were a sphere. Also, it is found from observation that the sun, the planets, and the satellites are, all of them, round. This roundness cannot be the roundness of a flat plate, for instance, for then the objects in question would sometimes present their thin sides to our view. It happens, also, that upon the discs which these bodies show, we see certain markings shifting along continually in one direction, to disappear at one side and to reappear again at the other. Such bodies must, indeed, be spheres in rotation. The crescent and other phases, shown by the moon and the inferior planets, should further impress the truth of the matter upon us, as such appearances can only be caused by the sunlight falling from various directions upon the surfaces of spherical bodies. Another proof, perhaps indeed the weightiest of all, is the continuous manner in which the stars over- 160 The Earth head give place to others as one travels about the surface of the earth. When in northern regions the Pole Star and its neighbours — the stars composing the Plough, for instance — are over our heads. As one journeys south these gradually sink towards the northern horizon, while other stars take their place, and yet others are uncovered to view from the south. The regularity with which these changes occur shows that every point on the earth's surface faces a different direction of the sky, and such an arrangement would only be possible if the earth were a sphere. The celebrated Greek philosopher, Aristotle, is known to have believed in the globular shape of the earth, and it was by this very argument that he had convinced himself that it was so. The idea of the sphericity of the earth does not appear, hawever, to have been generally accepted until the voyages of the great navigators showed that it could be sailed round. The next point we have to consider is the rota- tion of the earth about its axis. From the earliest times men noticed that the sky and everything in it appeared to revolve around the earth in one fixed direction, namely, towards what is called the West, and that it made one complete revolution in the period of time which we know as twenty-four hours. The stars were seen to come up, one after another, from below ^-he eastern horizon, to mount the sky, and then to sink in turn below the western horizon. The sun was seen to perform exactly the same journey, and the moon, too, whenever she was visible. One or two of the ancient Greek philosophers perceived that this might be explained, either by a movement i6i L Astronomy of To-day of the entire heavens around the earth, or by a turning motion on the part of the earth itself. Of these diverse explanations, that which supposed an actual movement of the heavens appealed to them the most, for they could hardly conceive that the earth should continually rotate and men not be aware of its movement. The question may be com- pared to what we experience when borne along in a railway train. We see the telegraph posts and the trees and buildings near the line fly past us one after another in the contrary direction. Either these must be moving, or we must be moving ; and as w^e happen to kjiow that it is, indeed, we who are moving, there can be no question therefore about the matter. But it would not be at all so easy to be sure of this movement were one unable to see the objects close at hand displacing themselves. For instance, if one is shut up in a railway carriage at night with the blinds down, there is really nothing to show that one is moving, except the jolting of the train. And even then it is hard to be sure in w^hich direction one is actually travelling. The way w^e are situated upon the earth is therefore as follows. There are no other bodies sufficiently near to be seen flying past us in turn ; our earth spins without a jolt ; we and all things around us, including the atmosphere itself, are borne along to- gether with precisely the same impetus, just as all the objects scattered about a railway carriage share in the forward movement of the train. Such being the case, what wonder that we are unconscious of the earth's rotation, of which we should know nothing at all, were it not for that slow displacement 162 The Earth of the distant objects in the heavens, as we are borne past them in turn. If the night sky be watched, it will be soon found that its apparent turning movement seems to take place around a certain point, which appears as if fixed. This point is known as the north pole of the heavens ; and a rather bright star, which is situated very close to this hub of movement, is in consequence called the Pole Star. For the dwellers in southern latitudes there is also a point in their sky which appears to remain similarly fixed, and this is known as the south pole of the heavens. Since, however, the heavens do not turn round at all, but the earth does, it will easily be seen that these apparently stationary regions in the sky are really the points towards which the axis of the earth is directed. The positions on the earth's surface itself, known as the North and South Poles, are merely the places where the earth's axis, if there were actually such a thing, would be expected to jut out. The north pole of the earth will thus be situated exactly beneath the north pole of the heavens, and the south pole of the earth exactly beneath the south pole of the heavens. We have seen that the earth rotates upon its imaginary axis once in about every twenty-four hours. This means that everything upon the surface of the earth is carried round once during that time. The measurement around the earth's equator is about 24,000 miles ; and, therefore, an object situated at the equator must be carried round through a distance of about 24,000 miles in each twenty-four hours. Every- thing at the equator is thus moving along at the rapid rate of about 1000 miles an hour, or between sixteen 163 Astronomy of To-day and seventeen times as fast as an express train. If, however, one were to take measurements around the earth parallel to the equator, one would find these measurements becoming less and less, according as the poles were approached. It is plain, therefore, that the speed with which any point moves, in conse- quence of the earth's rotation, will be greatest at the equator, and less and less in the direction of the poles ; while at the poles themselves there will be practically no movement, and objects there situated will merely turn round. The considerations above set forth, with regard to the different speeds at which different portions of a rotating globe will necessarily be moving, is the foundation of an interesting experiment, which gives us further evidence of the rotation of our earth. The measurement around the earth at any distance below the surface, say, for instance, at the depth of a mile, will clearly be less than a similar measurement at the surface itself. The speed of a point at the bottom of a mine, which results from the actual rotation of the earth, must therefore be less than the speed of a point at the surface overhead. This can be definitely proved by dropping a heavy object down a mine shaft. The object, which starts with the greater speed of the surface, will, when it reaches the bottom of the mine, be found, as might be indeed expected, to be a little ahead {i,e. to the east) of the point which originally lay exactly underneath it. The distance by which the object gains upon this point is, how- ever, very small. In our latitudes it amounts to about an inch in a fall of 500 feet. The great speed at which, as we have seen, the 164 The Earth equatorial regions of the earth are moving, should result in giving to the matter there situated a certain tendency to fly outwards. Sir Isaac Newton was the first to appreciate this point, and he concluded from it that the earth must be bulged a little all round the equator. This is, indeed, found to be the case, the diameter at the equator being nearly twenty-seven miles greater than it is from pole to pole. The reader will, no doubt, be here reminded of the familiar com- parison in geographies between the shape of the earth and that of an orange. In this connection it is interesting to consider that, w^ere the earth to rotate seventeen times as fast as it does (i.e. in one hour twenty-five minutes, instead of twenty-four hours), bodies at the equator would have such a strong tendency to fly outwards that the force of terrestrial gravity acting upon them would just be counterpoised, and they would virtually have no zueight. And, further, were the earth to rotate a little faster still, objects lying loose upon its surface would be shot off into space. The earth is, therefore, what is technically known as an oblate spheroid ; that is, a body of spherical shape flattened at the poles. It follows of course from this, that objects at the polar regions are slightly nearer to the earth's centre than objects at the equatorial regions. We have already seen that gravitation acts from the central parts of a body, and that its force is greater the nearer are those central parts. The result of this upon our earth will plainly be that objects in the polar regions will be pulled with a slightly stronger pull, and will therefore weigh a trifle more than objects in the equatorial regions. This is, 165 Astronomy of To-day indeed, found by actual experiment to be the case. As an example of the difference in question, Professor Young, in his Manual of Astronomy y points out that a man who weighs 190 pounds at the equator would weigh 191 at the pole. In such an experiment the weighing would, however, have to be made with a spring balance^ and not with scales ; for, in the latter case, the ^^ weights " used would alter in their weight in exactly the same degree as the objects to be weighed. It used to be thought that the earth was composed of a relatively thin crust, with a molten interior. Scientific men now believe, on the other hand, that such a condition cannot after all prevail, and that the earth must be more or less solid all through, except perhaps in certain isolated places where col- lections of molten matter may exist. The atmosphere, or air which we breathe, is in the form of a layer of limited depth which closely en- velops the earth. Actually, it is a mixture of several gases, the most important being nitrogen and oxygen, which between them practically make up the air, for the proportion of the other gases, the chief of which is carbonic acid gas, is exceedingly small. It is hard to picture our earth, as we know it, without this atmosphere. Deprived of it, men at once would die ; but even if they could be made to go on living without it by any miraculous means, they would be like unto deaf beings, for they would never hear any sound. What we call sounds are merely vibrations set up in the air, which travel along and strike upon the drum of the ear. The atmosphere is densest near the surface of the 166 The Earth earth, and becomes less and less dense away from it, as a result of diminishing pressure of air from above. The greater portion of it is accumulated within four or five miles of the earth's surface. It is impossible to determine exactly at what distance from the earth's surface the air ceases altogether, for it grows continually more and more rarefied. There are, however, two distinct methods of ascertain- ing the distance beyond which it can be said practi- cally not to exist. One of these methods we get from twilight. Twilight is, in fact, merely light reflected to us from those upper regions of the air, which still continue to be illuminated by the sun after it has disappeared from our view below the horizon. The time during which twilight lasts, shows us that the atmosphere must be at least fifty miles high. But the most satisfactory method of ascertaining the height to which the atmosphere extends is from the observation of meteors. It is found that these bodies become ignited, by the friction of passing into the atmosphere, at a height of about loo miles above the surface of the earth. We thus gather that the atmosphere has a certain degree of density even at this height. It may, indeed, extend as far as about 150 miles. The layer of atmosphere surrounding our earth acts somewhat in the manner of the glass covering of a greenhouse, bottling in the sun's rays, and thus storing up their warmth for our benefit. Were this not so, the heat which we get from the sun would, after falling upon the earth, be quickly radiated again into space. It is owing to the unsteadiness of the air that stars 167 Astronomy of To-day are seen to twinkle. A night when this takes place, though it may please the average person, is worse than useless to the astronomer, for the unsteadi- ness is greatly magnified in the telescope. This twinkling is, no doubt, in a great measure responsible for the conventional '' points " with which Art has elected to embellish stars, and which, of course, have no existence in fact. The phenomena of Refraction, ^ namely, that bending which rays of light undergo, when passing slantwise from a rare into a dense transparent medium, are very marked with regard to the atmosphere. The denser the medium into which such rays pass, the greater is this bending found to be. Since the layer of air around us becomes denser and denser towards the surface of the earth, it will readily be granted that the rays of light reaching our eyes from a celestial object, will suffer the greater bending the lower the object happens to be in the sky. Celestial objects, unless situated directly overhead, are thus not seen in their true places, and when nearest to the horizon are most out of place. The bending alluded to is upwards. Thus the sun and the moon, for instance, when we see them resting upon the horizon, are actually entirely beneath it. When the sun, too, is sinking towards the horizon, the lower edge of its disc will, for the above reason, ^ Every one knows the simple experiment in which a coin lying at the bottom of an empty basin, and hidden from the eye by its side, becomes visible when a certain quantity of water has been poured in. This is an example of refraction. The rays of light coming from the coin ought not to reach the eye, on account of the basin's side being in the way ; yet by the action of the water they are refracted^ or bent over its edge, in such a manner that they do. i68 The Earth look somewhat more raised than the upper. The result is a certain appearance of flattening ; which may plainly be seen by any one who watches the orb at setting. In observations to determine the exact positions of celestial objects correction has to be made for the effects of refraction, according to the apparent eleva- tion of these objects in the sky. Such effects are least when the objects in question are directly over- head, for then the rays of light, coming from them to the eye, enter the atmosphere perpendicularly, and not at any slant. A very curious effect, due to refraction, has occa- sionally been observed during a total eclipse of the moon. To produce an eclipse of this kind, the earth must, of course^ lie directly between the sun and the moon. Therefore, when we see the shadow creeping over the moon's surface, the sun should actually be well below the horizon. But when a lunar eclipse happens to come on just about sunset, the sun, although really sunk below the horizon, appears still above it through refraction, and the eclipsed moon, situated, of course, exactly opposite to it in the sky, is also lifted up above the horizon by the same cause. Pliny, writing in the first century of the Christian era, describes an eclipse of this kind, and refers to it as a ^'prodigy." The phenomenon is known as a "horizontal eclipse." It was, no doubt, partly owing to it that the ancients took so long to decide that an eclipse of the moon was really caused by the shadow cast by the earth. Plutarch, indeed, remarks that it was easy enough to understand that a solar eclipse was caused by the interposition of the moon, but that one could not 169 Astronomy of To-day imagine by the interposition of what body the moon itself could be eclipsed. In that apparent movement of the heavens about the earth, which men now know to be caused by the mere rotation of the earth itself, a slight change is observed to be continually taking place. The stars, indeed, are always found to be gradually drawing westward, i,e. towards the sun, and losing themselves one after the other in the blaze of his light, only to reappear, however, on the other side of him after a certain lapse of time. This is equivalent to saying that the sun itself seems always creeping slowly eastward in the heaven. The rate at which this appears to take place is such that the sun finds itself back again to its original position, with regard to the starry background, at the end of a year's time. In other words, the sun seems to make a complete tour of the heavens in the course of a year. Here, how- ever, we have another illusion, just as the daily move- ment of the sky around the earth was an illusion. The truth indeed is, that this apparent movement of the sun eastward among the stars during a year, arises merely from a continuous displacement of his position caused by an actual motion of the earth itself around him in that very time. In a word, it is the earth which really moves around the sun, and not the sun around the earth. The stress laid upon this fundamental point by Copernicus, marks the separation of the modern from the ancient view. Not that Copernicus, indeed, had obtained any real proof that the earth is merely a planet revolving around the sun ; but it seemed to his profound intellect that a movement of this kind 170 The Earth on the part of oar globe was the more Hkely explana- tion of the celestial riddle. The idea was not new ; for, as we have already seen, certain of the ancient Greeks (Aristarchus of Samos, for example) had held such a view ; but their notions on the subject were very fanciful, and unsupported by any good argument. What Copernicus, however, really seems to have done was to insist upon the idea that the sun occu- pied the centre^ as being more consonant with common sense. No doubt, he was led to take up this position by the fact that the sun appeared entirely of a different character from, the other members of the system. The one body in the scheme, which performed the important function of dispenser of light and heat, would indeed be more likely to occupy a position apart from the rest ; and what position more appro- priate for its purposes than the centre ! But here Copernicus only partially solved the diffi- cult question. He unfortunately still clung to an ancient belief, which as yet remained unquestioned ; i.e, the great virtue, one might almost say, the divine- nesSf of circular motion. The ancients had been hag- ridden, so to speak, by the circle ; and it appeared to them that such a perfectly formed curve was alone fitted for the celestial motions. Ptolemy employed it throughout his system. According to him the ^' planets " (which included, under the ancient view, both the sun and the moon), moved around the earth in circles ; but, as their changing positions in the sky could not be altogether accounted for in this way, it was further supposed that they performed addi- tional circular movements, around peculiarly placed centres, during the course of their orbital revolutions. 171 Astronomy of To-day Thus the Ptolemaic system grew to be extremely complicated ; for astronomers did not hesitate to add new circular movements whenever the celestial positions calculated for the planets w^ere found not to tally with the positions observed. In this manner, indeed, they succeeded in doctoring the theory, so that it fairly satisfied the observations made w4th the rough instruments of pre-telescopic times. Although Copernicus performed the immense ser- vice to astronomy of boldly directing general attention to the central position of the sun, he unfortunately took over for the new scheme the circular machinery of the Ptolemaic system. It therefore remained for the famous Kepler, who lived about a century after him, to find the complete solution. Just as Coper- nicus, for instance, had broken free from tradition with regard to the place of the sun ; so did Kepler, in turn, break free from the spell of circular motion, and thus set the coping-stone to the new astronomical edifice. This astronomer showed, in fact, that if the paths of the planets around the sun, and of the moon around the earth, were not circles, but ellipsesy the movements of these bodies about the sky could be correctly accounted for. The extreme simplicity of such an arrangement was far more acceptable than the bewildering intricacy of movement required by the Ptolemaic theory. The Copernican system, as amended by Kepler, therefore carried the day ; and was further strengthened, as we have already seen, by the telescopic observations of Galileo and the researches of Newton into the effects of gravi- tation. And here a word on the circle, now fallen from 172 The Earth its high estate. The ancients were in error in sup- posing that it stood entirely apart — the curve of curves. As a matter of fact it is merely a special kind of ellipse. To put it paradoxically^ it is an ellipse which has no ellipticity, an oval without any ovalness ! Notwithstanding all this, astronomy had to wait yet a long time for a definite proof of the revolution of the earth around the sun. The leading argument advanced by Aristotle, against the reality of any movement of the earth, still held good up to about seventy years ago. That philosopher had pointed out that the earth could not move about in space to any great extent, or the stars would be found to alter their apparent places in the sky, a thing which had never been observed to happen. Centuries ran on, and instruments became more and more perfect, yet no displacements of stars were noted. In accept- ing the Copernican theory men were therefore obliged to suppose these objects as immeasurably distant. At length, however, between the years 1835 and 1840, it was discovered by the Prussian astronomer, Bessel, that a star known as 61 Cygni — that is to say, the star marked in celestial atlases as No. 61 in the constellation of the Swan — appeared, during the course of a year, to perform a tiny circle in the heavens, such as would result from a movement on our own part around the sun. Since then about forty-three stars have been found to show minute displacements of a similar kind, which cannot be accounted for upon any other supposition than that of a continuous revolution of the earth around the sun. The triumph of the Copernican system is now at last supreme. 173 Astronomy of To-day If the axis of the earth stood ^' straight up/' so to speak, while the earth revolved in its orbit, the sun would plainly keep always on a level with the equator. This is equivalent to stating that, in such circum- stances, a person at the equator would see it rise each morning exactly in the east, pass through the zenith^ that is, the point directly overhead of him, at midday, and set in the evening due in the west. As this would go on unchangingly at the equator every day throughout the year, it should be clear that, at any particular place upon the earth, the sun would in these conditions always be seen to move in an unvarying manner across the sky at a certain alti- tude depending upon the latitude of the place. Thus the more north one went upon the earth's surface, the more southerly in the sky would the sun's path lie ; while at the north pole itself, the sun would always run round and round the horizon. Similarly, the more south one went from the equator the more northerly would the path of the sun lie, while at the south pole it would be seen to skirt the horizon in the same manner as at the north pole. The result of such an arrangement would be, that each place upon the earth would always have one unvarying climate ; in which case there would not exist any of those beneficial changes of season to which we owe so much. The changes of season, which we fortunately ex- perience, are due, however, to the fact that the sun does not appear to move across the sky each day at one unvarying altitude, but is continually altering the position of its path ; so that at one period of the year it passes across the sky low down, and remains 174 The Earth above the horizon for a short time only, while at another it moves high up across the heavens, and is above the horizon for a much longer time. Actually, the sun seems little by little to creep up the sky during one half of the year, namely, from mid-winter to mid-summer, and then, just as gradually, to slip down it again during the other half, namely, from mid-summer to mid-winter. It will therefore be clear that every region of the earth is much more thoroughly warmed during one portion of the year than during another, i,e, when the sun's path is high in the heavens than when it is low down. Once more we find appearances exactly the con- trary from the truth. The earth is in this case the real cause of the deception, just as it was in the other cases. The sun does not actually creep slowly up the sky, and then slowly dip down it again, but, owing to the earth's axis being set aslant, different regions of the earth's surface are presented to the sun at different times. Thus, in one portion of its orbit, the northerly regions of the earth are presented to the sun, and in the other portion the southerly. It follows of course from this, that when it is summer in the northern hemisphere it is winter in the southern, and vice versd (see Fig. 13, p. 176). The fact that, in consequence of this slant of the earth's axis, the sun is for part of the year on the north side of the equator and part of the year on the south side, leads to a very peculiar result. The path of the moon around the earth is nearly on the same plane with the earth's path around the sun. The moon, therefore, always keeps to the same regions of the sky as the sun. The slant of the earth's axis 175 Astronomy of To-day thus regularly displaces the position of both the sun and the moon to the north and south sides of the equator respectively in the manner we have been de- scribing. Were the earth, however, a perfect sphere, such change of position would not produce any effect. We have shown, however, that the earth is not a perfect sphere, but that it is bulged out all round the equator. The result is that this bulged-out portion swings slowly under the pulls of solar and lunar Orbit ofEarf/i ijune. Af fiale/i/^senled ti'fi/vrds Sun and S. Me turned , ' aimi/ /rem it. , ' Xly/J.fhh S.PotS. Summer there fore in ~ , the .Northern Nemts^/nA- ■ , cmdWinterin the boutA^n ' December. S Po'e pi'esented tcmrt/i Sun andiVJhte turned - ^ a nay from it. S.Po. , ' Summer thure/bre in ^ ' the Soulher/i Memisp/uirs and Wintayin the A/urthem, Orfaitof F.orlh Fig. 13. — Summer and Winter. gravitation, in response to the displacements of the sun and moon to the north and to the south of it. This slow swing of the equatorial regions results, of course, in a certain slow change of the direction of the earth's axis, so that the north pole does not go on pointing continually to the same region of the sky. The change in the direction of the axis is, however, so extremely slight, that it shows up only after the lapse of ages. The north pole of the heavens, that is, the region of the sky towards which the north pole of the earth's axis points, displaces 176 The Earth therefore extremely slowly, tracing out a wide circle, and arriving back again to the same position in the sky only after a period of about 25,000 years. At present the north pole of the heavens is quite close to a bright star in the tail of the constellation of the Little Bear, which is consequently known as the Pole Star ; but in early Greek times it was at least ten times as far away from this star as it is now. After some 12,000 years the pole will point to the constellation of Lyra, and Vega, the most brilliant star in that constellation, '^ill then be considered as the pole star. This slow twisting of the earth's axis is technically known as Precession, or the Precession of the Equinoxes (see Plate XIX., p. 292). The slow displacement of the celestial pole appears to have attracted the attention of men in very early times, but it was not until the second century B.C. that precession was established as a fact by the cele- brated Greek astronomer, Hipparchus. For the ancients this strange cyclical movement had a mystic significance ; and they looked towards the end of the period as the end, so to speak, of a ^' dispensation," after which the life of the universe would begin anew : — " Magnus ab integro s£eclorum nascitur ordo. Jam redit et Virgo, redeunt Saturnia regna ; Alter erit turn Tiphys, et altera quae vehat Argo Delectos heroas ; erunt etiam altera bella, Atque iterum ad Trojam magnus mittetur Achilles." We have seen that the orbit of the earth is an ellipse, and that the sun is situated at what is called the focusy a point not in the middle of the ellipse, 177 M Astronomy of To-day but rather towards one of its ends. Therefore, during the course of the year the distance of the earth from the sun varies. The sun, in consequence of this, is about 3,000,000 miles nearer to us in our northern winter than it is in our northern summer, a statement which sounds somewhat paradoxical. This variation in distance, large as it appears in figures, can, how- ever, not be productive of much alteration in the amount of solar heat which we receive,- for during the first week in January, when the distance is least, the sun only looks about one-eighteenth broader than at the commencement of July, when the distance is greatest. The great disparity in temperature between winter and summer depends, as we have seen, upon causes of quite another kind, and varies between such wide limits that the effects of this slight altera- tion in the distance of the sun from the earth may be neglected for practical purposes. The Tides are caused by the gravitational pull of the sun and moon upon the water of the earth's surface. Of the two, the moon, being so much the nearer, exerts the stronger pull, and therefore may be regarded as the chief cause of the tides. This pull always draws that portion of the water, which happens to be right underneath the moon at the time, into a heap ; and there is also a second heaping of water at the same moment at the contrary side of the earth, the reasons for which can be shown mathe- matically, but cannot be conveniently dealt with here. As the earth rotates on its axis each portion of its surface passes beneath the moon, and is swelled up by this pull ; the watery portions being, however, the 178 The Earth only ones to yield visibly. A similar swelling up, as we have seen, takes place at the point exactly away from the moon. Thus each portion of our globe is borne by the rotation through two "tide-areas" every day, and this is the reason why there are two tides during every twenty-four hours. The crest of the watery swelling is known as high tide. The journey of the moon around the earth takes about a month, and this brings her past each place in turn by about fifty minutes later each day, which is the reason why high tide is usually about twenty- five minutes later each time. The moon is, however, not the sole cause of the tides, but the sun, as we have said, has a part in the matter also. When it is new moon the gravitational attractions of both sun and moon are clearly acting together from precisely the same direction, and, there- fore, the tide will be pulled up higher than at other times. At full moon, too, the same thing happens ; for, although the bodies are now acting from opposite directions, they do not neutralise each other's pulls as one might imagine, since the sun, in the same manner as the moon, produces a tide both under it and also at the opposite side of the earth. Thus both these tides are actually increased in height. The ex- ceptionally high tides which we experience at new and full moons are known as Spring TideSf in contra- distinction to the minimum high tides, which are known as Neap Tides. The ancients appear to have had some idea of the cause of the tides. It is said that as early as looo B.C. the Chinese noticed that the moon exerted an influ- ence upon the waters of the sea. The Greeks and 179 Astronomy of To-day Romans, too, had noticed the same thing ; and Caesar tells us that when he was embarking his troops for Britain the tide was high because the moon was full. Pliny went even further than this, in recognising a similar connection between the waters and the sun. From casual observation one is inclined to suppose that the high tide always rises many feet. But that this is not the case is evidenced by the fact that the tides in the midst of the great oceans are only from three to four feet high. However, in the seas and straits around our Isles, for instance, the tides rise very many feet indeed, but this is merely owing to the extra heaping up which the large volumes of water undergo in forcing their passage through narrow channels. As the earth, in rotating, is continually passing through these tide-areas, one might expect that the friction thus set up would tend to slow down the rotation itself. Such a slowing down, or '4idd drag," as it is called, is indeed continually going on ; but the effects produced are so exceedingly minute that it will take many millions of years to make the rotation appreciably slower, and so to lengthen the day. Recently it has been proved that the axis of the earth is subject to a very small displacement, or rather, ^'wobbling," in the course of a period of somewhat over a year. As a consequence of this, the pole shifts its place through a circle of, roughly, a few yards in width during the time in question. This movement is, perhaps, the combined result of two causes. One of these is the change of place during the year of large masses of material upon our earth ; such as occurs, for instance, when ice and snow i8o The Earth melt, or when atmospheric and ocean currents trans- port from place to place great bodies of air and water. The other cause is supposed to be the fact that the earth is not absolutely rigid, and so yields to certain strains upon it. In the course of investigation of this latter point the interesting conclusion has been reached by the famous American astronomer, Professor Simon Newcomb, that our globe as a whole is a little more rigid than steel. We will bring this chapter to a close by alluding briefly to two strange appearances which are some- times seen in our night skies. These are known re- spectively as the Zodiacal Light and the Gegenschein. The Zodiacal Light is a faint cone-shaped illu- mination which is seen to extend upwards from the w^estern horizon after evening twilight has ended, and from the eastern horizon before morning twilight has begun. It appears to rise into the sky from about the position where the sun would be at that time. The proper season of th-e year for observing it during the evening is in the spring, while in autumn it is best seen in the early morning. In our latitudes its light is not strong enough to render it visible when the moon is full, but in the tropics it is reported to be very bright, and easily seen in full moonlight. One theory regards it as the reflection of light from swarms of meteors revolving round the sun ; another supposes it to be a very rarefied extension of the corona. The Gegenschein (German for ^' counter-glow ") is a faint oval patch of light, seen in the sky exactl}^ op- posite to the place of the sun. It is usually treated of in connection with the zodiacal light, and one theory regards it similarly as of meteoric origin. i8i Astronomy of To-day Another theory, however — that of Mr. Evershed — con- siders it a sort of tail to the earth (like a comet's tail) composed of hydrogen and helium — the two lightest gases we know — driven off from our planet in the direction contrary to the sun. 182 CHAPTER XVI THE MOON What we call the moon's '^ phases " are merely the various ways in which we see the sun shining upon her surface during the course of her monthly re- volutions around the earth (see Fig. 14, p. 184). When she passes in the neighbourhood of the sun all his light falls upon that side which is turned away from us, and so the side which is turned towards us is unillumined, and therefore invisible. When in this position the moon is spoken of as new. As she continues her motion around the earth, she draws gradually to the east of the sun's place in the sky. The sunlight then comes somewhat from the side ; and so we see a small portion of the right side of the lunar disc illuminated. This is the phase known as the crescent moon. As she moves on in her orbit more and more of her illuminated surface is brought into view ; and so the crescent of light becomes broader and broader, until we get what is called half-moon, or first quarter, when we see exactly one-half of her surface lit up by the sun's rays. As she draws still further round yet more of her illuminated surface is brought into view, until three-quarters of the disc appear lighted up. She is then said to be gibbous. Eventually she moves round so that she faces the 183 Astronomy- of To-day sun completely, and the whole of her disc appears illuminated. She is then spoken of as full. When in this position it is clear that she is on the contrary Direction from which the sun's rays are coming. J V y I I c ^^ \Earth) ^^1 .7 E Various positions and illumination of the moon by the sun during her revolution around the earth. B C D E F G H «)Cooa3» _ A^ew Crescent First Qua/icr Cibdoui full Cd>6ous Lasf Quarter Crescenl Mew The corresponding positions as viewed from the earth, showing the consequent phases. Fig. 14. — Orbit and Phases of the Moon. side of the earth to the sun, and therefore rises about the same time that he is setting. She is now, in fact, at her furthest from the sun. After this, the motion of the moon in her orbit carries her on back again in the direction of the sun. 184 The Moon She thus goes through her phases as before, only these of course are in the reverse order. The full phase is seen to give place to the gibbous, and this in turn to the half-moon and to the crescent ; after which her motion carries her into the neighbourhood of the sun, and she is once more new, and lost to our sight in the solar glare. Following this she draws away to the east of the sun again, and the old order of phases repeat themselves as before. The early Babylonians imagined that the moon had a bright and a dark side, and that her phases were caused by the bright side coming more and more into view during her movement around the sky. The Greeks, notably Aristotle, set to work to examine the question from a mathematical standpoint, and came to the conclusion that the crescent and other appear- ances were such as would necessarily result if the moon were a dark body of spherical shape illumined merely by the light of the sun. Although the true explanation of the moon's phases has thus been known for centuries, it is unfortunately not unusual to see pictures — advertisement posters, for instance — in which stars appear within the horns of a crescent moon ! Can it be that there are to-day educated persons who believe that the moon is a thing which grows to a certain size and then wastes away again ; who, in fact, do not know that the entire body of the moon is there all the while ? When the moon shows a very thin crescent, we are able dimly to see her still dark portion standing out against the sky. This appearance is popularly known as the ^^old moon in the new moon's arms." The dark part of her surface must, indeed, be to some i8s Astronomy of To-day degree illumined, or we should not be able to see it at all. Whence then comes the light which illumines it, since it clearly cannot come from the sun ? The riddle is easily solved, if we consider what kind of view of our earth an observer situated on this darkened part of the moon would at that moment get. He would, as a matter of fact, just then see nearly the whole disc of the earth brightly lit up by sun- light. The lunar landscape all around would, there- fore, be bathed in what to him would be ^^ earthlight," which of course takes the place there of what we call moonlight. If, then, we recollect how much greater in size the earth is than the moon, it should not surprise us that this earthlight will be many times brighter than moonlight. It is considered, indeed, to be some twenty times brighter. It is thus not at all astonishing that we can see the dark portion of the moon illumined merely by sunlight reflected upon it from our earth. The ancients were greatly exercised in their minds to account for this '^ earthlight," or ^' earthshine," as it is also called. Posidonius (135-51 B.C.) tried to explain it by supposing that the moon was partially transparent, and that some sunlight consequently filtered through from the other side. It was not, however, until the fifteenth century that the correct solution was arrived at. Perhaps the most remarkable thing which one notices about the moon is that she always turns the same side towards us, and so we never see her other side. One might be led from this to jump to the conclusion that she does not rotate upon an axis, as do the other bodies which we see ; but, paradoxical 186 The Moon as it may appear, the fact that she turns one face always towards the earth, is actually a proof that she does rotate upon an axis. The rotation, however, takes place with such slowness, that she turns round but once during the time in which she revolves around lyv % One side of the moon only is ever presented to the earth. This side is here indicated by the letters S.F.E. (side facing earth). By placing the above positions in a row, we can see at once that the moon makes one complete rotation on her axis in exactly the same time as she revolves around the earth. Fig. 15 — The Rotation of the Moon on her Axis. the earth (see Fig. 15). In order to understand the matter clearly, let the reader place an object in the centre of a room and walk around it once, keeping his face turned towards it the ivhole time, While he is doing this, it is evident that he will face every one of the four walls of the room in succession. 187 Astronomy of To-day Now in order to face each of the four walls of a room in succession one would be obliged to turn oneself entirely round. Therefore, during the act of walking round an object with his face turned directly towards it, a person at the same time turns his body once entirely round. In the long, long past the moon must have turned round much faster than this. Her rate of rotation has no doubt been slowed down by the action of some force. It will be recollected how, in the course of the previous chapter, we found that the tides were tending, though exceedingly gradually, to slow down the rotation of the earth upon its axis. But, on account of the earth's much greater mass, the force of gravitation exercised by it upon the surface of the moon is, of course, much more powerful than that which the moon exercises upon the surface of the earth. The tendency to tidal action on the moon itself must, therefore, be much in excess of anything which we here experience. It is, in consequence, probable that such a tidal drag, extending over a very long period of time, has resulted in slowing down the moon's rotation to its present rate. The fact that we never see but one side of the moon has given rise from time to time to fantastic speculations with regard to the other side. Some, indeed, have wished to imagine that our satellite is shaped like an egg, the more pointed end being directed away from us. We are here, of course, faced with a riddle, which is all the more tantalising from its appearing for ever insoluble to men, chained as they are to the earth. However, it seems going too far to suppose that any abnormal conditions neces- i88 The Moon sarily exist at the other side of the moon. As a matter of fact, indeed, small portions of that side are brought into our view from time to time in con- sequence of slight irregularities in the moon's move- ment ; and these portions differ in no way from those which we ordinarily see. On the whole, we obtain a view of about 60 per cent, of the entire lunar surface ; that is to say, a good deal more than one-half. The actual diameter of the moon is about 2163 miles, which is somewhat more than one-quarter the diameter of the earth. For a satellite, therefore, she seems very large compared with her primary, the earth ; when we consider that Jupiter's greatest satellite, although nearly twice as broad as our moon, has a diameter only one twenty-fifth that of Jupiter. Furthermore, the moon moves around the earth comparatively slowly, making only about thirteen revolutions during the entire year. Seen from space, therefore, she would not give the impression of a circling body, as other satellites do. Her revolutions are, indeed, relatively so very slow that she would appear rather like a smaller planet accompanying the earth in its orbit. In view of all this, some as- tronomers are inclined to regard the earth and moon rather as a ''double planet" than as a system of planet and satellite. When the moon is full she attracts more attention perhaps than in any of her other phases. The moon, in order to be full, must needs be in that region of the heavens exactly opposite to the sun. The sun appears to go once entirely round the sky in the course of a year, and the moon performs the same journey in the space of about a month. The moon, 189 Astronomy of To-day when full, having got half-way round this journey, occupies, therefore, that region of the sky which the sun itself will occupy half a year later. Thus in winter the full moon will be found roughly to occupy the sun's summer position in the sky, and in summer the sun's winter position. It therefore follows that the full moon in winter time is high up in the heavens, while in summer time it is low down. We thus get the greatest amount of full moonlight when it is the most needed. The great French astronomer, Laplace, being struck by the fact that the ^Messer light" did not rule the night to anything like the same extent that the " greater light " ruled the day, set to work to examine the conditions under which it might have been made to do so. The result of his speculations showed that if the moon were removed to such a distance that she took a year instead of a month to revolve around the earth ; and if she were started off in her orbit at full moon, she would always continue to remain full — a great advantage for us. Whewell, however, pointed out that in order to get the moon to move with the requisite degree of slowness, she would have to revolve so far from the earth that she would only look one-sixteenth as large as she does at present, which rather militates against the advan- tage Laplace had in mind 1 Finally, however, it was shown by M. Liouville, in 1845, that the position of a perennial full moon, such as Laplace dreamed of, would be unstable — that is to say, the body in ques- tion could not for long remain undisturbed in the situation suggested (see Fig. 16, p. 191). There is a well-known phenomenon called harvest 190 The Moon moon, concerning the nature of which there seems to be much popular confusion. An idea in fact appears to prevail among a good many people that the moon A © Earth ^Earff, ^ San^ Earfhi B (J Earth C Various positions of Laplace's " Moon" with regard to the earth and sun during the course of a year. ■©■ isf) ^ (^o '■■■■©•-''' c The same positions of Laplace's "Moon," arranged around the earth, show that it would make only one revolution in a year. Fig, i6. — Laplace's " Perennial Full Moon." is a harvest moon when^ at rising, it looks bigger and redder than usual. Such an appearance has, how- ever, nothing at all to say to the matter ; for ^the 191 Astronomy of To-day moon always looks larger when low down in the sky, and, furthermore, it usually looks red in the later months of the year, when there is more mist and fog about than there is in summer. What astronomers actually term the harvest moon is, indeed, something entirely different from this. About the month of September the slant at which the full moon comes up from below the horizon happens to be such that, during several evenings together, she rises almost at the same hour, instead of some fifty minutes later, as is usually the case. As the harvest is being gathered in about that time, it has come to be popularly con- sidered that this is a provision of nature, according to w^hich the sunlight is, during several evenings, re- placed without delay by more or less full-m.oonlight, in order that harvesters may continue their work straight on into the night, and not be obliged to break off after sunset to wait until the moon rises. The same phenomenon is almost exactly repeated a month later, but by reason of the pursuits then carried on it is know^n as the ^^ hunter's moon." In this connection should be mentioned that curious phenomenon above alluded to, and w^hich seems to attract universal notice, namely, that the moon looks much larger when near the horizon — at its rising, for instance, than when higher up in the sky. This seeming enlargement is, however, by no means con- fined to the moon. That the sun also looks much larger when low down in the sky than when high up, seems to strike even the most casual watcher of a sunset. The same kind of effect will, indeed, be noted if close attention be paid to the stars them- selves. A constellation, for instance, appears more 192 The Moon spread out when low down in the sky than when high up. This enlargement of celestial objects when in the neighbourhood of the horizon is, how- ever, only apparent and not real. It must be entirely an illusion; for the most careful measurements of the discs of the sun and of the moon fail to show that the bodies are any larger when near the horizon than when high up in the sky. In fact, if there be any difference in measurements with regard to the moon, it will be found to be the other way round ; for her disc, when carefully measured, is actually the slightest degree greater when high in the sky, than when low down. The reason for this is that, on account of the rotundity of the earth's surface, she is a trifle nearer the observer when overhead of him. This apparent enlargement of celestial objects, when low down in the sky, is granted on all sides to be an illusion ; but although the question has been dis- cussed with animation time out of mind, none of the explanations proposed can be said to have received unreserved acceptance. The one which usually figures in text-books is that we unconsciously compare the sun and moon, when low down in the sky, with the terrestrial objects in the same field of view, and are therefore inclined to exaggerate the size of these orbs. Some persons, on the other hand, imagine the illusion to have its source in the structure of the human eye ; while others, again, put it down to the atmosphere, maintaining that the celestial objects in question loom large in the thickened air near the horizon, in the same way that they do when viewed through fog or mist. 193 N Astronomy of To-day The writer 1 ventures, however, to think that the illusion has its origin in our notion of the shape of the celestial vault. One would be inclined, indeed, to suppose that this vault ought to appear to us as the half of a hollow sphere ; but he maintains that it does not so appear, as a consequence of the manner in which the eyes of men are set quite close together in their heads. If one looks, for instance, high up in the sky, the horizon cannot come within the field of view, and so there is nothing to make one think that the expanse then gazed upon is other than quitejlat — let us say like the ceiling of a room. But, as the eyes are lowered, a portion of the encircling horizon comes gradually into the field of view, and the region of the sky then gazed upon tends in consequence to assume a hollowed-out form. From this it would seem that our idea of the shape of the celestial vault is, that it is flattened down over our heads and hollowed out all around in the neighbourhood of the horizon (see Fig. 17, p. 195). Now, as a consequence of their very great distance, all the objects in the heavens neces- sarily appear to us to move as if they were placed on the background of the vault ; the result being that the mind is obliged to conceive them as expanded or contracted, in its unconscious attempts to make them always fill their due proportion of space in the various parts of this abnormally shaped sky. From such considerations the writer concludes that the apparent enlargement in question is merely the natural consequence of the idea we have of the shape of the celestial vault — an idea gradually built up in '^Journal of the British Astronomical Association^ vol. x. (1899- 1900), Nos. I and 3. 194 The Moon childhood, to become later on what is called '^ second nature." And in support of this contention, he would point to the fact that the enlargement is not by any means confined to the sun and moon, but is every whit as marked in the case of the constellations. To one who has not noticed this before, it is really quite a revelation to compare the appearance of one of the A The observer is supposed to be standing at A. Fig, 17. — Illustrating the author's explanation of the apparent enlarge- ment of celestial objects. large constellations (Orion, for instance) when high up in the sky and when low down. The widening apart of the various stars composing the group, when in the latter position, is very noticeable indeed. Further, if a person were to stand in the centre of a large dome, he would be exactly situated as if he were beneath the vaulted heaven, and one would consequently expect him to suffer the same illusion as to the shape of the dome. Objects fixed upon its background would therefore appear to him under the 195 Astronomy of To-day same conditions as objects in the sky, and the illu- sions as to their apparent enlargement should hold good here also. Some years ago a Belgian astronomer, M. Stroobant, in an investigation of the matter at issue, chanced to make a series of experiments under the very con- ditions just detailed. To various portions of the inner surface of a large dome he attached pairs of electric lights ; and on placing himself at the centre of the building, he noticed that, in every case, those pairs which were high up appeared closer together than those which were low down ! He does not, however, seem to have sought for the cause in the vaulted expanse. On the contrary, he attributed the effect to something connected with our upright stature, to some physiological reason which regularly makes us esti- mate objects as larger when in front than when overhead. In connection with this matter, it may be noted that it always appears extremely difficult to estimate with the eye the exact height above the horizon at which any object (say a star) happens to be. Even skilled observers find themselves in error in attempt- ing to do so. This seems to bear out the writer's contention that th^ form under which the celestial vault really appears to us is a peculiar one, and tends to give rise to false judgments. Before leaving this question, it should also be men- tioned that nothing perhaps is more deceptive than the size which objects in the sky appear to present. The full moon looks so like a huge plate, that it astonishes one to find that a threepenny bit held at arm's length will a long way more than cover its disc. 196 1 r^^^^^Hbr, r if. ^ i''^ > ^h|^H| ir iT. o i < ' p^^" ,; i- t il ^: A»ii 1 1 > 1 1 1 .^mT""*^*', Plate Xlll. Thk Moon From a photograph taken at the Paris Observatorj' by M. P. Paiseux. (Page 197) The Moon The moon is just too far off to allow us to see the actual detail on her surface with the naked eye. When thus viewed she merely displays a patchy appearance,^ and the imaginary forms which her darker markings suggest to the fancy are popularly expressed by the term '' Man in the Moon." An examination of her surface with very moderate optical aid is, however, quite a revelation, and the view we then get is not easily comparable to what we see with the unaided eye. Even with an ordinary opera-glass, an observer wall be able to note a good deal of detail upon the lunar disc. If it be his first observation of the kind, he cannot fail to be struck by the fact to which we have just made allusion, namely, the great change which the moon appears to undergo when viewed with magnifying power. "Cain and his Dog," the '' Man in the Moon gathering sticks," or whatever indeed his fancy was wont to conjure up from the lights and shades upon the shining surface, have now completely disappeared ; and he sees instead a silvery globe marked here and there with extensive dark areas, and pitted all over with crater-like formations (see Plate VIII., p. 196). The dark areas retain even to the present day their ancient name of '^ seas," for Galileo and the early telescopic observers believed them to be such, and they are still catalogued under the mystic appellations given to them in the long ago ; as, for instance, '^ Sea of Showers," " Bay of Rainbows," " Lake of Dreams." ^ The improved telescopes of later ^ Certain of the ancient Greeks thought the markings on the moon to be merely the reflection of the seas and lands of our earth, as in a badly polished mirror. 2 Mare Imbrium, Sinus Iridum, Lacus Somniorum. 197 Astronomy of To-day times showed, however, that they were not really seas (there is no w^ater on the moon), but merely areas of darker material. The crater-like formations above alluded to are the "lunar mountains." A person examining the moon for the first time with telescopic aid, will perhaps not at once grasp the fact that his view of lunar mountains must needs be w^hat is called a "bird's-eye" one, namely, a view from above, like that from a balloon ; and that he cannot, of course, expect to see them from the side, as he does the mountains upon the earth. But once he has realised this novel point of view, he wdll no doubt marvel at the formations which lie scattered as it were at his feet. The type of lunar mountain is indeed in striking contrast to the terrestrial type. On our earth the range-formation is supreme ; on the moon the crater-formation is the rule, and is so-called from analogy to our volcanoes. A typical lunar crater may be described as a circular wall, en- closing a central plain, or "floor," which is often much depressed below the level of the surface out- side. These so-called "craters," or "ring-mountains," as they are also termed, are often of gigantic propor- tions. For instance, the central plain of one of them, known as Ptolemaeus,^ is about 115 miles across, while that of Plato is about 60. The walls of craters often rise to great heights ; which, in proportion to the small size of the moon, are very much in excess of our highest terrestrial elevations. Nevertheless, a person posted at the centre of one of the larger craters ^ The lunar craters have, as a rule, received their names from celebrated persons, usually men of science. This system of nomenclature was origi- nated by Riccioli, in 165 1. w r/.../...5-^C) -% MARE ,/' //)/•'/ '/>r/?/Av NeaiomontariusC^ ,.^„^ HUMORUM (9— .....,...r^ Q:!':::r ^^^s^'A Q/r/fAiucc/icr J///''//t//.S FOECUNDATIS — ^„. MARE TRANOUILLITATIS maR£\ MARE VAPORUM MARE ■ SERENITATIS \% ^^-/(e/Ue. . i^mOi PROCELLARUM CRISIUM %L raucusM IMBRiUM Q£llC/oxuf: -X ^^"r^^<^^-^- Plate IX. Map of the Moon, showing the principal "Craters, Mountain- Ranges, and "Seas" In this, as in the other plates of the Moon, the South will be found at the top of the picture ; such being the view given by the ordinary astronomical telescope, in which all objects are seen inverted. (Page 199) The Moon might be surprised to find that he could not see the encompassing crater-walls, which would in every direction be below his horizon. This would arise not alone from the great breadth of the crater itself, but also from the fact that the curving of the moon's surface is very sharp compared with that of our earth. We have mentioned Ptolemaeus as among the very large craters, or ring-mountains, on the moon. Its encompassing walls rise to nearly 13,000 feet, and it has the further distinction of being almost in the centre of the lunar disc. There are, however, several others much wider, but they are by no means in such a conspicuous position. For instance, Schickard, close to the south-eastern border, is nearly 130 miles in diameter, and its wall rises in one point to over 10,000 feet. Grimaldi, almost exactly at the east point, is nearly as large as Schickard. Another crater, Clavius, situated near the south point, is about 140 miles across ; while its neighbour Bailly — named after a famous French astronomer of the eighteenth century — is 180, and the largest of those which we can see (see Plate IX., p. 198). Many of the lunar craters encroach upon one another ; in fact there is not really room for them all upon the visible hemisphere of the moon. About 30,000 have been mapped ; but this is only a small portion, for according to the American astronomer, Professor W. H. Pickering, there are more than 200,000 in all. Notwithstanding the fact that the crater is the type of mountain associated in the mind with the moon, it must not be imagined that upon our satellite there 199 Astronomy of To-day are no mountains at all of the terrestrial type. There are indeed many isolated peaks, but strangely enough they are nearly always to be found in the centres of craters. Some of these peaks are of great altitude, that in the centre of the crater Copernicus being over 11,000 feet high. A few mountain ranges also exist ; the best known of which are styled, the Lunar Alps and Lunar Apennines (see Plate X., p. 200). Since the mass of the moon is only about one- eightieth that of the earth, it will be understood that the force of gravity which she exercises is much less. It is calculated that, at her surface, this is only about one-sixth of what we experience. A man transported to the moon would thus be able to jump six times as high as he can here. A building could therefore be six times as tall as upon our earth, without causing any more strain upon its foundations. It should not, then, be any subject for wonder, that the highest peaks in the Lunar Apennines attain to such heights as 22,000 feet. Such a height, upon a comparatively small body like the moon, for her volume is only one-fiftieth that of the earth, is rela- tively very much in excess of the 29,000 feet of Hima- layan structure. Mount Everest, the boast of our planet, 8000 miles across ! High as are the Lunar Apennines, the highest peaks on the moon are yet not found among them. There is, for instance, on the extreme southern edge of the lunar disc, a range known as the Leibnitz Mountains ; several peaks of w^hich rise to a height of nearly 30,000 feet, one peak in particular being said to attain to 36,000 feet (see Plate IX., p. 198). But the reader will surely ask the question : 200 HE Moon We have here (see " Map," Plate IX, p. 198) the mountain ranges of the Apennines, tlie Caucasus and the Alps ; also the craters Plato, Aristotle, Eudoxus, C'a-sini. Ai istillus, Autolycus, Archimedes and I,inn6. The crater Linne is the very bnuht spi.t in the dark area at the upper left liand side of the picture. From a photogr.iph taken at the Paris Observatory by M.M. Loewy and I'uiseux. (Page 2co) The Moon " How is it possible to determine the actual height of a lunar mountain, if one cannot go upon the moon to measure it ? " The answer is, that we can calculate its height from noting the length of the shadow which it casts. Any one will allow that the length of a shadow cast by the sun depends upon two things : firstly, upon the height of the object which causes the shadow, and secondly, upon the elevation of the sun at the moment in the sky. The most casual observer of nature upon our earth can scarcely have failed to notice that shadows are shortest at noon-day, when the sun is at its highest in the sky ; and that they lengthen out as the sun declines towards its setting. Here, then, we have the clue. To ascertain, therefore, the height of a lunar moun- tain, we have first to consider at what elevation the sun is at that moment above the horizon of the place where the mountain in question is situated. Then, having measured the actual length in miles of the shadow extended before us, all that is left is to ask ourselves the question : <^ What height must an object be whose shadow cast by the sun, when at that elevation in the sky, will extend to this length ? " There is no trace whatever of water upon the moon. The opinion, indeed, which seems generally held, is that water has never existed upon its surface. Erosions, sedimentary deposits, and all those marks which point to a former occupation by water are notably absent. Similarly there appears to be no atmosphere on the moon ; or, at any rate, such an excessively rare one, as to be quite inappreciable. Of this there are several proofs. For instance, in a solar eclipse the 201 Astronomy of To-day moon's disc always stands out quite clear-cut against that of the sun. Again during occultations, stars disappear behind the moon with a suddenness, which could not be the case were there any appreciable atmosphere. Lastly, we see no traces of twilight upon the lunar surface, nor any softening at the edges of shadows ; both which effects would be apparent if there were an atmosphere. The moon's surface is rough and rocky, and dis- plays no marks of the ^^ weathering " that would necessarily follow, had it possessed anything Of an atmosphere in the past. This makes us rather in- clined to doubt that it ever had one at all. Suppos- ing, however, that it did possess an atmosphere in the 1 past, it is interesting to inquire what may have become of it. In the first place it might have gradu- ally disappeared, in consequence of the gases w^hich composed it uniting chemically with the materials of which the lunar body is constructed ; or, again, its constituent gases may have escaped into space, in accordance with the principles of that kinetic theory of which w^e have already spoken. The latter solution seems, indeed, the most reasonable of the two, for the force of gravity at the lunar surface appears too weak to hold down any known gases. This argument seems also to dispose of the question of absence of water ; for Dr. George Johnstone Stoney, in a careful investigation of the subject, has shown that the liquid in question, when in the form of vapour, will escape from a planet if its mass is less than one-fourth that of our earth. And the mass of the moon is very much less than this ; indeed only the one-eightiethy as we have already stated. 202 The Moon In consequence of this lack of atmosphere, the condition of things upon the moon will be in marked contrast to what we experience upon the earth. The atmosphere here performs a double service in shield- ing us from the direct rays of the sun, and in bottling the heat as a glass-house does. On the moon, how- ever, the sun beats down in the day-time with a merciless force ; but its rays are reflected away from the surface as quickly as they are received, and so the cold of the lunar night is excessive. It has been calculated that the day temperature on the moon may, indeed, be as high as our boiling-point, while the night temperature may be more than twice as low as the greatest cold known in our arctic regions. That a certain amount of solar heat is reflected to us from the moon is shown by the sharp drop in temperature which certain heat-measuring instru- ments record when the moon becomes obscured in a lunar eclipse. The solar heat which is thus re- flected to us by the moon is, however, on the whole extremely small ; more light and heat, indeed, reach us direct from the sun in half a minute than we g^i by reflection from the moon during the entire course of the year. With regard to the origin of the lunar craters there has been much discussion. Some have considered them to be evidence of violent volcanic action in the dim past ; others, again, as the result of the impact of meteorites upon the lunar surface, when the moon was still in a plastic condition ; while a third theory holds that they were formed by the bursting of huge bubbles during the escape into space of gases from the interior. The question is, indeed, a very difficult 203 Astronomy of To-day one. Though volcanic action, such as would result in craters of the size of Ptolemaeus, is hard for us to picture, and though the lone peaks which adorn the centres of many craters have nothing reminiscent of them in our terrestrial volcanoes, nevertheless the volcanic theory seems to receive more favour than the others. In addition to the craters there are two more features which demand notice, namely, what are known as rays and rills. The rays are long, light- coloured streaks which radiate from several of the large craters, and extend to a distance of some hundreds of miles. That they are mere markings on~ the surface is proved by the fact that they cast no shadows of any kind. One theory is, that they were originally great cracks which have been filled with lighter coloured material, welling up from beneath. The rills, on the other hand, are actually fissures, about a mile or so in width and about a quarter of a mile in depth. The rays are seen to the best advantage in con- nection with the craters Tycho and Copernicus (see Plate XL, p. 204). In consequence of its fairly forward position on the lunar disc, and of the re- markable system of rays which issue from it like spokes from the axle of a wheel, Tycho commands especial attention. The late Rev. T. H. Webb, a famous observer, christened it, very happily, the ^' metropolitan crater of the moon." A great deal of attention is, and has been, paid by certain astronomers to the moon, in the hope of find- ing out if any changes are actually in progress at present upon her surface. Sir William Herschel, 204 ■ "■ il ■•,1.. -i, U^n'' ■ ^m J f ^^^^H^^^^^^Hwii tf 1 4 . I: •1 ^^^^^^^^S H 1 fl ^H^r] pBJ HI Hfl^l ^Ikfl -"^^^^^l i B^^3 ^^^^^^^^^^^^F^^^^^^l lg *% fe i^|i ^•^^^9|H El Plate XL The Moon The systems of rays from the craters Tycho, Copernicus and Kepler are well shown here. From a photograph taken at the Paris Observatory by ]\1. P. Puiseux-. (Page C04) # The Moon indeed, once thought that he saw a lunar volcano in eruption, but this proved to be merely the effect of the sunlight striking the top of the crater Aristarchus, while the region around it was still in shadow — sunrise upon Aristarchus, in fact 1 No change of any- real importance has, however, been noted, although it is suspected that some minor alterations have from time to time taken place. For instance, slight varia- tions of tint have been noticed in certain areas of the lunar surface. Professor W. H. Pickering puts forward the conjecture that these may be caused by the growth and decay of some low form of vegetation, brought into existence by vapours of water, or carbonic acid gas, making their way out from the interior through cracks near at hand. Again, during the last hundred years j one small crater known as Linnd (Linnaeus), situated in the Mare Serenitatis (Sea of Serenity), has appeared to undergo slight changes, and is even said to have been invisible for a while (see Plate X,, p. 200). It is, however, believed that the changes in question may be due to the varying angles at which the sun- light falls upon the crater ; for it is an understood fact that the irregularities of the moon's motion give us views of her surface which; always differ slightly. The suggestion has more than once been put for- ward that the surface of the moon is covered with a thick layer of ice. This is generally considered improbable, and consequently the idea has received very little support. It first originated with the late Mr. S. E. Peal, an English observer of the moon, and has recently been resuscitated by the German observer, Herr Fauth. 205 Astronomy of To-day The most unfavourable time for telescopic study of the moon is when she is full. The sunlight is then falling directly upon her visible hemisphere, and so the mountains cast no shadows. We thus do not get that impression of hill and hollow which is so very noticeable in the other phases. The first map of the moon was constructed by Galileo. Tobias Mayer published another in 1775 ; while during the nineteenth century greatly improved ones iwere made by Beer and Madler, Schmidt, Neison and others. In 1903, Professor W. H. Picker- ing brought out a complete photographic lunar atlas ; and a similar publication has recently appeared, the work of MM. Loewy and Puiseux of the Observatory of Paris. The so-called " seas " of the moon are, as we have seen, merely dark areas, and there appears to be no proof that they were ever occupied by any liquid. They are for the most part found in the northern portion of the moon ; a striking contrast to our seas and oceans, which take up so much of the southern hemisphere of the earth. There are many erroneous ideas popularly held with regard to certain influences which the moon is supposed to exercise upon the earth. For instance, a change in the weather is widely believed to depend upon a change in the moon. But the word '' change " as here used is meaningless, for the moon is con- tinually changing her phase during the whole of her monthly round. Besides, the moon is visible over a great portion of the earth at the same moment^ and certainly all the places from which it can then be seen do not get the same weather ! Further, careful observa- 206 The Moon tions, and records extending over the past one hundred years and more, fail to show any rehable connection between the phases of the moon and the condition of the weather. It has been stated, on very good authority, that no telescope ever shows the surface of the moon as clearly as we could see it with the naked eye were it only 240 miles distant from us. Supposing, then, that we were able to approach our satellite, and view it without optical aid at such com- paratively close quarters, it is interesting to consider what would be the smallest detail which our eye could take in. The question of the limit of what can be appreciated with the naked eye is somewhat uncertain, but it appears safe to say that at a distance of 240 miles the minutest speck visible would have to be at least some 60 yards across. Atmosphere and liquid both wanting, the lunar surface must be the seat of an eternal calm ; where no sound breaks the stillness and where change, as we know it, does not exist. The sun beats down upon the arid rocks, and inky shadows lie athwart the valleys. There is no mellowing of the harsh contrasts. We cannot indeed absolutely affirm that Life has no place at all upon this airless and waterless globe, since we know not under what strange conditions it may manifest its presence ; and our most powerful telescopes, besides, do not bring the lunar surface sufficiently near to us to disprove the existence there of even such large creatures as disport themselves upon our planet. Still, we find it hard to rid our- selves of the feeling that we are in the presence of a dead world. On she swings around the earth month 207 Astronomy of To-day after month, with one face ever turned towards us, leaving a certain mystery to hang around that hidden side, the greater part of which men can never hope to see. The rotation of the moon upon her axis — the lunar day — has become, as we have seen, equal to her revolution around the earth. An epoch may likewise eventually be reached in the history of our own planet, when the length of the terrestrial day has been so slowed down by tidal friction that it will be equal to the year. Then will the earth revolve around the central orb, with one side plunged in eternal night and the other in eternal sunshine. But such a vista need not immediately distress us. It is millions of years forward in time. 208 CHAPTER XVII THE SUPERIOR PLANETS ♦ Having, in a previous chapter, noted the various aspects which an inferior planet presents to our view, in consequence of its orbit being nearer to the sun than the orbit of the earth, it will be well here to consider in the same way the case of a superior planet, and to mark carefully the difference. To begin with, it should be quite evident that we cannot ever have a transit of a superior planet. The orbit of such a body being entirely outside that of the earth, the body itself can, of course, never pass between us and the sun. A superior planet will be at its greatest distance from us when on the far side of the sun. It is said then to be in conjunction. As it comes round in its orbit it eventually passes, so to speak, at the back of us. It is then at its nearest, or in opposition^ as this is technically termed, and therefore in the most favourable position for telescopic observation of its surface. Being, besides, seen by us at that time in the direction of the heavens exactly opposite to where the sun is, it will thus at midnight be high up in the south side of the sky, a further advantage to the observer. Last of all, a superior planet cannot show crescent shapes like an interior ; for whether it be on the far 209 O Astronomy of To-day side of the sun, or behind us, or again to our right or left, the sunlight must needs appear to fall more or less full upon its face. The Planetoid Eros The nearest to us of the superior planets is the tiny body, Eros, which, as has been already stated, was discovered so late as the year 1898. In point of view, however, of its small size, it can hardly be considered as a true planet, and the name '' plane- toid " seems much more appropriate to it. Eros was not discovered, like Uranus, in the course of telescopic examination of the heavens, nor yet, like Neptune, as the direct result of difficult calcula- tions, but was revealed by the impress of its light upon a photographic plate, which had been exposed for some length of time to the starry sky. Since many of the more recent additions to the asteroids have been discovered in the same manner, we shall have somewhat more to say about this special em- ployment of photography when we come to deal with those bodies later on. The path of Eros around the sun is so very ellip- tical, or, to use the exact technical term, so very *' eccentric," that the planetoid does not keep all the time entirely in the space between our orbit and that of Mars, which latter happens to be the next body in the order of planetary succession outwards. In portions of its journey Eros, indeed, actually goes outside the Martian orbit. The paths of the plane- toid and of Mars are, however, not upon the same plane, so the bodies always pass clear of each other, 210 The Superior Planets and there is thus as little chance of their dashing together as there would be of trains which run across a bridge at an upper level, colliding with those which pass beneath it at a lower level. When Eros is in opposition, it comes within about 13J million miles of our earth, and, after the moon, is therefore by a long way our nearest neighbour in space. It is, however, extremely small, not more, perhaps, than twenty miles in diameter, and is subject to marked variations in brightness, which do not appear up to the present to meet with a satisfactory explanation. But, insignificant as is this little body, it is of great importance to astronomy ; for it happens to furnish the best method known of calculating the sun's distance from our earth — a method which Galle, in 1872, and Sir David Gill, in 1877, suggested that asteroids might be employed for, and which has in consequence supplanted the old one founded upon transits of Venus. The sun's distance is now an ascertained fact to within 100,000 miles, or less than half the distance of the moon. The Planet Mars We next come to the planet Mars. This body rotates in a period of slightly more than twenty-four hours. The inclination, or slant, of its axis is about the same as that of the earth, so that, putting aside its greater distance from the sun, the variations of season which it experiences ought to be very much like ours. The first marking detected upon Mars was the notable one called the Syrtis Major, also known, on 211 Astronomy of To-day account of its shape, as the Hour-Glass Sea. This observation was made by the famous Huyghens in 1659; and, from the movement of the marking in question across the disc, he inferred that the planet rotated on its axis in a period of about twenty-four hours. There appears to be very little atmosphere upon Mars, the result being that we almost always obtain a clear view of the detail on its surface. Indeed, it is only to be expected from the kinetic theory that Mars could not retain much of an atmosphere, as the force of gravity at its surface is less than one- half of what we experience upon the earth. It should here be mentioned that recent researches with the spectroscope seem to show that, whatever atmosphere there may be upon Mars, its density at the surface of the planet cannot be more than the one-fourth part of the density of the air at the surface of the earth. Professor Lowell, indeed, thinks it may be more rarefied than that upon our highest mountain-tops. Seen with the naked eye. Mars appears of a red colour. Viewed in the telescope, its surface is found to be in general of a ruddy hue, varied here and there with darker patches of a bluish-green colour. These markings are permanent, and were supposed by the early telescopic observers to imply a distribu- tion of the planet's surface into land and water, the ruddy portions being considered as continental areas (perhaps sandy deserts), and the bluish-green as seas. The similarity to our earth thus suggested was further heightened by the fact that broad white caps, situated at the poles, were seen to vary with the planet's seasons, diminishing greatly ih extent during the 212 The Superior Planets Martian summer (the southern cap in 1894 even dis- appearing altogether), and developing again in the Martian winter.^ Readers of Oliver Wendell Holmes will no doubt recollect that poet's striking lines : — " The snows that glittered on the disc of Mars Have melted, and the planet's fiery orb Rolls in the crimson summer of its year." A state of things so strongly analogous to what we experience here, naturally fired the imaginations of men, and caused them to look on Mars as a world like ours, only upon a much smaller scale. Being smaller, it was concluded to have cooled quicker, and to be now long past its prime ; and its ^' in- habitants" were, therefore, pictured as at a later stage of development than the inhabitants of our earth. Notwithstanding the strong temptation to assume that the whiteness of the Martian polar caps is due to fallen snow, such a solution is, however, by no means so simple as it looks. The deposition of water in the form of snow, or even of hoar frost, would at least imply that the atmosphere of Mars should now and then display traces of aqueous vapour, which it does not appear to do.2 It has, indeed, been suggested that the whiteness may not after all be due to this cause, but to carbonic acid gas (carbon dioxide), which is known to freeze at a very low temperature. The suggestion is plainly ^ Sir William Herschel was the first to note these polar changes. 2 Quite recently, however, Professor Lowell has announced that his observer, Mr. E. C. Slipher, finds with the spectroscope faint traces of water vapour in the Martian atmosphere. 213 Astronomy of To-day based upon the assumption that, as Mars is so much further from the sun than we are, it would receive much less heat, and that the little thus received would be quickly radiated away into space through lack of atmosphere to bottle it in. We now come to those well-known markings, popu- larly known as the "canals" of Mars, which have been the subject of so much discussion since their discovery thirty years ago. It was, in fact, in the year 1877, when Mars was in opposition, and thus at its nearest to us, that the famous Italian astronomer, Schiaparelli, announced to the world that he had found that the ruddy areas, thought to be continents, were intersected by a net- work of straight dark lines. These lines, he reported, appeared in many cases to be of great length, so long, indeed, as several thousands of miles, and from about twenty to sixty miles in width. He christened the lines channels^ the Italian word for which, "canali," was unfortunately translated into English as " canals." The analogy, thus accidentally suggested, gave rise to the idea that they might be actual waterways.^ In the winter of 1 881-1882, when Mars was again in opposition, Schiaparelli further announced that he had found some of these lines doubled ; that is to say, certain of them were accompanied by similar lines running exactly parallel at no great distance away. There was at first a good deal of scepticism on the subject of Schiaparelli's discoveries, but gradually other observers found themselves seeing both the ^ In a somewhat similar manner the term "crater," as applied to the ring-mountain formation on the moon, has evidently given a bias in favour of the volcanic theory as an explanation of that peculiar structure. 214 The Superior Planets lines and their doublings. We have in this a good example of a curious circumstance in astronomical observation, namely, the fact that when fine detail has once been noted by a competent observer, it is not long before other observers see the same detail with ease. An immense amount of close attention has been paid to the planet Mars during recent years by the American observer, Professor Percival Lowell, at his famous observatory, 7300 feet above the sea, near the town of Flagstaff, Arizona, U.S.A. His observations have not, like those of most astronomers, been con- fined merely to *' oppositions," but he has systemati- cally kept the planet in view, so far as possible, since the year 1894. The instrumental equipment of his observatory is of the very best, and the " seeing " at Flagstaff is de- scribed as excellent. In support of the latter state- ment, Mr. Lampland, of the Lowell Observatory, maintains that the faintest stars shown on charts made at the Lick Observatory with the 36-inch telescope there, are perfectly visible with the 24-inch telescope at Flagstaff. Professor Lowell is, indeed, generally at issue with the other observers of Mars. He finds the canals extremely narrow and sharply defined, and he attri- butes the blurred and hazy appearance, which they have presented to other astronomers, to the unsfeady and imperfect atmospheric conditions in which their observations have been made. He assigns to the thinnest a width of two or three miles, and from fifteen to twenty to the larger. Relatively to their width, however, he finds their length enormous. 215 Astronomy of To-day Many of them are 2000 miles long, while one is even as much as 3540. Such lengths as these are very- great in comparison with the smallness of the planet. He considers that the canals stand in some peculiar relation to the polar cap, for they crowd together in its neighbourhood. In place, too, of ill-defined con- densations, he sees sharp black spots where the canals meet and intersect, and to these he gives the name of *' Oases." He further lays particular stress upon a dark band of a blue tint, which is always seen closely to surround the edges of the polar caps all the time that they are disappearing ; and this he takes to be a proof that the white material is something which actually melts. Of all substances which we know, water alone, he affirms, would act in such a manner. The question of melting at all may seem strange in a planet which is situated so far from the sun, and possesses such a rarefied atmosphere. But Professor Lowell considers that this very thinness of the atmos- phere allows the direct solar rays to fall with great intensity upon the planet's surface, and that this heating effect is accentuated by the great length of the Martian summer. In consequence he concludes that, although the general climate of Mars is de- cidedly cold, it is above the freezing point of water. The observations at Flagstaff appear to do away with the old idea that the darkish areas are seas, for numerous lines belonging to the so-called ^^ canal system" are seen to traverse them. Again, there is no star-like image of the sun reflected from them, as there would be, of course, from the surface of a great sheet of water. Lastly, they are observed to vary in tone and colour with the changing Martian seasons, 216 S So, 1) o o o o J3 - - u i) 7? ^nd 16^ days respectively, at distances lying roughly between a quarter of a million and one million miles. Perrine's two satellites are at a distance of about seven million miles, and take about nine months to complete their revolutions. The larger satellites, when viewed in the telescope, exhibit certain defined markings ; but the bodies are so far away from us, that only those details which are of great extent can be seen. The satellite lo, accord- ing to Professor Barnard, shows a darkish disc, with a broad white belt across its middle regions. Mr. Douglass, one of the observers at the Lowell Obser- 233 Astronomy of To-day vatory, has noted upon Ganymede a number of markings somewhat resembHng those seen on Mars, and he concludes, from their movement, that this satelHte rotates on its axis in about seven days. Pro- fessor Barnard, on the other hand, does not corro- borate this, though he claims to have discovered bright polar caps on both Ganymede and Callisto. In an earlier chapter we dealt at length with eclipses, occultations, and transits, and endeavoured to make clear the distinction between them. The system of Jupiter's satellites furnishes excellent ex- amples of all these phenomena. The planet casts a very extensive shadow, and the satellites are con- stantly undergoing obscuration by passing through it. Such occurrences are plainly comparable to our lunar eclipses. Again, the satellites may, at one time, be occulted by the huge disc of the planet, and at another time seen in transit over its face. A fourth phenomenon is what is known as an eclipse of the planet by a satellite, which is the exact equivalent of what we style on the earth an eclipse of the sun. In this last case the shadow, cast by the satellite, appears as a round black spot in movement across the planet's surface. In the passages of these attendant bodies behind the planet, into its shadow, or across its face, respec- tively, it occasionally happens that Galileo's four satellites all disappear from view, and the planet is then seen for a while in the unusual condition of being apparently without its customary attendants. An instance of this phenomenon took place on the 3rd of October 1907. On that occasion, the satellites known as I. and III. (i,e. lo and Ganymede) were 234 The Superior Planets eclipsed, that is to say, obscured by passing into the planet's shadow ; Satellite IV. (Callisto) was occulted by the planet's disc ; while Satellite II. (Europa), being at the same moment in transit across the planet's face, was invisible against that brilliant background. A number of instances of this kind of occurrence are on record. Galileo, for example, noted one on the 15th of March 161 1, while Herschel observed another on the 23rd of May 1802. It was indirectly to Jupiter's satellites that the world was first indebted for its knowledge of the velocity of light. When the periods of revolution of the satellites were originally determined, Jupiter happened, at the time, to be at his nearest to us. From the periods thus found tables were made for the pre- diction of the moments at which the eclipses and other phenomena of the satellites should take place. As Jupiter, in the course of his orbit, drew further away from the earth, it was noticed that the dis- appearances of the satellites into the shadow of the planet occurred regularly later than the time pre- dicted. In the year 1675, Roemer, a Danish astro- nomer, inferred from this, not that the predictions were faulty, but that light did not travel instantaneously. It appeared, in fact, to take longer to reach us, the greater the distance it had to traverse. Thus, when the planet was far from the earth, the last ray given out by the satellite, before its passage into the shadow, took a longer time to cross the intervening space, than when the planet was near. Modern experiments in physics have quite confirmed this, and have proved for us that light does not travel across space in the twinkling of an eye, as might hastily be supposed, 235 Astronomy of To-day but actually moves, as has been already stated, at the rate of about 186,000 miles per second. The Planet Saturn Seen in the telescope the planet Saturn is a wonder- ful and very beautiful object. It is distinguished from all the other planets, in fact from all known celestial bodies, through being girt around its equator by what looks like a broad, flat ring of exceeding thinness. This, however, upon closer examination, is found to be actually composed of three concentric rings. The outermost of these is nearly of the same brightness as the body of the planet itself. The ring which comes immediately within it is also bright, and is separated from the outer one all the way round by a relatively narrow space, known as " Cassini's division," because it was discovered by the celebrated French astro- nomer, ]. D. Cassini, in the year 1675. Inside the second ring, and merging insensibly into it, is a third one, known as the " crape ring," because it is darker in hue than the others and partly transparent, the body of Saturn being visible through it. The inner boundary of this third and last ring does not adjoin the planet, but is everywhere separated from it by a definite space. This ring was discovered inde- pendently^ in 1850 by Bond in America and Dawes in England. ^ In the history of astronomy two salient points stand out. The first of these is the number of " independent " discoveries which have taken place ; such, for instance, as in the cases of Le Verrier and Adams with regard to Neptune, and of Lockyer and Janssen in the matter of the spectroscopic method of observing solar prominences. The other is the great amount of "anticipation." Copernicus, as we 236 w u .1? tJO ■~ T3 E 0! |g: 2 o ^ lit The Superior Planets As distinguished from the crape ring, the bright rings must have a considerable closeness of texture ; for the shadow of the planet may be seen projected upon them, and their shadows in turn projected upon the surface of the planet (see Plate XV., p. 236). According to Professor Barnard, the entire breadth of the ring system, that is to say, from one side to the other of the outer ring, is 172,310 miles, or some- what more than double the planet's diameter. In the varying views which we get of Saturn, the system of the rings is presented to us at very different angles. Sometimes we are enabled to gaze upon its broad expanse ; at other times, however, its thin edge is turned exactly towards us, an occurrence which takes place after intervals of about fifteen years. When this happened in 1892 the rings are said to have disappeared entirely from view in the great Lick telescope. We thus get an idea of their small degree of thickness, which would appear to be only about 50 miles. The last time the system of rings was exactly edgewise to the earth was on the 3rd of October 1907. The question of the composition of these rings has given rise to a good deal of speculation. It was formerly supposed that they were either solid or liquid, but in 1857 it was proved by Clerk Maxwell that a structure of this kind would not be able to stand. He showed, however, that they could be fully explained by supposing them to consist of an immense number of have seen, was anticipated by the Greeks ; Kepler was not actually the first who thought of elliptic orbits ; others before Newton had imagined an attractive force. Both these points furnish much food for thought ! Astronomy of To-day separate solid particles, or, as one might otherwise put it, extremely small satellites, cirding in dense swarms around the middle portions of the planet. It is there- fore believed that we have here the materials ready for the 'formation of a satellite or satellites ; but that the powerful gravitative action, arising through the planet's being so near at hand, is too great ever to allow these materials to aggregate themselves into a solid mass. There is, as a matter of fact, a mini- mum distance from the body of any planet within which it can be shown that a satellite will be unable to form on account of gravitational stress. This is known as ^' Roche's limit," from the name of a French astronomer who specially investigated the question. There thus appears to be a certain degree of analogy between Saturn's rings and the asteroids. Empty spaces, too, exist in the asteroidal zone, the relative position of one of which bears a striking resemblance to that of ^'Cassini's division." It is suggested, indeed, that this division had its origin in gravitational disturbances produced by the attraction of the larger satellites, just as the empty spaces in the asteroidal zone are supposed to be the result of per- turbations caused by the Giant Planet hard by. It has long been understood that the system of the rings must be rotating around Saturn, for if they were not in motion his intense gravitational attraction would quickly tear them in pieces. This was at length proved to be the fact by the late Professor Keeler, Director of the Lick Observatory, who from spectroscopic observations found that those portions of the rings situated near to the planet rotated faster 238 The Superior Planets than those farther from it. This directly supports the view that the rings are composed of satellites ; for, as we have already seen, the nearer a satellite is to its primary the faster it will-revolve. On the other hand, were the rings solid, their outer portions would move the fastest; as we have seen takes place in the body of the earth, for example. The mass of the ring system, however, must be exceedingly small, for it does not appear to produce any disturbances in the movements of Saturn's satellites. From the kinetic theory, there- fore, one would not expect to find any atmosphere on the rings, and the absence of it is duly shown by spectroscopic observations. The diameter of Saturn, roughly speaking, is about one-fifth less than that of Jupiter. The planet is very flattened at the poles, this flattening being quite noticeable in a good telescope. For instance, the diameter across the equator is about 76,470 miles, while from pole to pole it is much less, namely, 69,770. The surface of Saturn bears a strong resemblance to that of Jupiter. Its markings, though not so well defined, are of the same belt-like description ; and from observation of them it appears that the planet rotates on an average in a little over ten hours. The rotation is in fact of the same peculiar kind as that of the sun and Jupiter ; but the difference of speed at which the various portions of Saturn go round are even more marked than in the case of the Giant Planet. The density of Saturn is less than that of Jupiter ; so that it must be largely in a condition of vapour, and in all probability at a still earlier stage of planetary evolution. Up to the present we know of as many as ten 239 Astronomy of To-day satellites circling around Saturn, which is more than any other planet of the solar system can lay claim to. Two of these, however, are very recent discoveries ; one, Phoebe, having been found by photography in August 1898, and the other, Themis, in 1904, also by the same means. For both of these we are in- debted to Professor W. H. Pickering. Themis is said to be the faintest object in the solar system. It cannot be seen^ even with the ^argest telescope in existence ; a fact which should hardly fail to impress upon one the great advantage the photographic plate possesses in these researches over the human eye. The most important of the whole Saturnian family of satellites are the two known as Titan and japetus. These were discovered respectively by Huyghens in 1655 and by Cassini in 1671. Japetus is about the same size as our moon ; - while the diameter of Titan, the largest of the satellites, is about half as much again. Titan takes about sixteen days to revolve around Saturn, while Japetus takes more than two months and a half. The former is about three-quarters of a million miles distant from the planet, and the latter about two and a quarter millions. To Sir William Herschel we are indebted for the discovery of two more satellites, one of which he found on the evening that he used his celebrated 40-foot telescope for the first time. The ninth satellite, Phoebe, one of the two discovered by Professor Pickering, is perhaps the most remarkable body in the solar system, for all the other known members of that system perform their revolutions in one fixed direc- tion, whereas this satellite revolves in the contrary direction. 240 The Superior Planets In consequence of the great distance of Saturn, the sun, as seen from the planet, would appear so small that it would scarcely show any disc. The planet, indeed, only receives from the sun about one-nine- tieth of the heat and light which the earth receives. Owing to this diminished intensity of illumination, the combined light reflected to Saturn by the whole of its satellites must be very small. With the sole exception of Jupiter, not one of the planets circulating nearer to the sun could be seen from Saturn, as they would be entirely lost in the solar glare. For an observer upon Saturn, Jupiter would, therefore, fill much the same position as Venus does for us, regularly displaying phases and being alternately a morning and an evening star. It is rather interesting to consider the appearances which would be produced in our skies were the earth embellished with a system of rings similar to those of Saturn, In consequence of the curving of the terrestrial surface, they would not be seen at all from within the Arctic or Antarctic circles, as they would be always below the horizon. From the equator they would be continually seen edgewise, and so would appear merely as line of light stretching right across the heaven and passing through the zenith. But the dwellers in the remaining regions would find them very objectionable, for they would cut off the light- of the sun during lengthy periods of time. Saturn was a sore puzzle to the early telescopic observers. They did not for a long time grasp the fact that it was surrounded by a ring — so slow is the human mind to seek for explanations out of the ordinary course of things. The protrusions of the 241 Q Astronomy of To-day ring on either side of the planet, at first looked to Galileo like two minor globes placed on opposite sides of it; and slightly overlapping the disc. He therefore informed Kepler that ^^ Saturn consists of three stars in contact with one another." Yet he was genuinely puzzled by the fact that the two attendant bodies (as he thought them) always retained the same position with regard to the planet's disc, and did not appear to revolve around it, nor to be in any wise shifted as a consequence of the movements of our earth. About a year and a half elapsed before he again examined Saturn ; and, if he was previously puzzled, he was now thoroughly amazed. It happened just then to be one of those periods when the ring is edgewise towards the earth, and of course he only saw a round disc like that of Jupiter. What, indeed, had become of the attendant orbs ? Was some demon mocking him ? Had Saturn devoured his own chil- dren ? He was, however, fated to be still more puzzled, for soon the minor orbs reappeared, and, becoming larger and larger as time went on, they ended by losing their globular appearance and became like two pairs of arms clasping the planet from each side ! (see Plate XVI., p. 242). Galileo went to his grave with the riddle still un- solved, and it remained for the famous Dutch astro- nomer, Huyghens, to clear up the matter. It was, however, some little time before he hit upon the real explanation. Having noticed that there were dark spaces between the strange appendages and the body of the planet, he imagined Saturn to be a globe fitted with handles at each side; "ansae" these came to be 242 :