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 OF MODERN KNOWLEDGE 
 
 ASTRONOMY 
 
 BY ARTHUR R. HINKS 
 
 LONDON 
 WILLIAMS & NORGATE 
 
 HENRY HOLT & Co., NEW YORK 
 
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 PROF. GILBERT MURRAY, D.LITT., 
 
 LL.D., F.B.A. 
 PROF. J. ARTHUR THOMSON, M.A. 
 
 PROF. WILLIAM T. BREWSTER, M.A. 
 (COLUMBIA UNIVERSITY, U.S.A.) 
 
 
 
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ASTRONOMY 
 
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 ARTHUR R. HINKS 
 
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ASTRONOMY 
 
 CHAPTER I 
 
 ASTRONOMERS AND OBSERVATORIES 
 
 IN a strictly limited work upon a quite 
 unlimited subject we are compelled to exercise 
 a strict economy. However tempting it may 
 be to enquire what knowledge of astronomy 
 existed among the ancient people of the 
 world, how it comes about that their buildings 
 are built astronomically, or why it was that 
 their considerable skill in the science was so 
 nearly lost to future generations we must 
 refuse to be drawn into the charming but 
 devious paths which lead through these parts 
 of our subject. The Chaldaean shepherd 
 had a far better knowledge of the stars and 
 their movements than the modern educated 
 dweller in a glaringly-lit town, with a cloudy 
 and smoky sky ; he had a traditional lore of 
 the stars which enabled him to regulate his 
 husbandry and to find his way; and his 
 7 
 
8 ASTRONOMY 
 
 religion, his legends, and perhaps some of his 
 early history can be traced in the names 
 which he gave to the groups of stars which 
 were familiar to him. 
 
 The very natural association of religious 
 ideas with the heavenly bodies led to 
 the building of temples with a definite 
 astronomical significance, which we can trace 
 in the Temple of the Sun at Luxor, in Stone- 
 henge, and in many other buildings of 
 antiquity. Let us be careful, however, not 
 to go too far in finding an astronomical 
 meaning for every line and for every angle 
 in such a building. At the present time 
 there is a decided tendency to go further than 
 strict prudence would permit ; to ascribe to 
 the builders of old a knowledge and an 
 accuracy which the facts do not warrant ; 
 and to pretend by measurement and cal- 
 culation to arrive at the date of the building 
 with all the authority which should attach 
 to the solution of an astronomical problem. 
 We shall do well to receive all such results 
 with caution ; they are interesting archaeology, 
 but their astronomy is questionable. 
 
 It must appear curious at first sight to 
 any reader of the history of astronomy, that 
 while many of the principal facts, the 
 
ASTRONOMERS & OBSERVATORIES 9 
 
 rotundity of the earth, the gradual change 
 in the position of the pole among the stars, 
 were known to the famous astronomers of 
 antiquity, who were Greeks, yet the influence 
 of the Greek philosopher Aristotle was for 
 many centuries sufficient to stifle any spirit 
 of enquiry into the truths of astronomy. 
 That their teaching was contrary to Aristotle 
 was enough to condemn Copernicus or Galileo. 
 Why was this particular Greek of authority 
 so much greater than Hipparchus or Ptolemy ? 
 The answer is not hard. These Greek 
 astronomers lived not in Greece, but at 
 Alexandria, whither learning migrated as the 
 famous Greece of history decayed. Their 
 science was hardly known to the Romans, 
 but such of it as survived was preserved by 
 the Arabs and came to Europe by the Moslem 
 invasion of Spain, tainted no doubt in public 
 opinion with the discredit attaching among 
 the Christians to all things Moslem. Thus 
 it affected at first but slightly the revival of 
 interest in learning and science which took 
 place in Europe in the fifteenth and sixteenth 
 centuries. The first astronomers of Europe 
 had to work against, not with the support 
 of, whatever remained in repute of the ancient 
 learning of Greece. At every turn they were 
 
10 ASTRONOMY 
 
 stopped with the objection that Aristotle 
 said so and so. 
 
 Now what Aristotle said was founded upon 
 the vaguest kind of foundation. Planets 
 must move in circles because the circle is the 
 only " perfect " figure. Seven is a perfect 
 number, and therefore if you have found 
 seven of a thing you need not waste time 
 looking for an eighth. This was the kind 
 of argument that passed for scientific in the 
 middle ages ; this was the kind of prejudice 
 which the early European astronomers had 
 to rid their own minds of first, and then to 
 banish from the minds of others. It could be 
 done in only one way, by insisting on first 
 observing what actually happened and then 
 constructing arguments to fit the facts. 
 Strange as it may appear, this very sensible 
 line of conduct was quite foreign to the ideas 
 of the time, and was received with very bad 
 grace by those in authority. 
 
 Foremost among the men who insisted 
 upon observation and experiment was the 
 Danish nobleman Tycho Brahe, who was 
 above all things an experimenter. On the 
 little island of Hven, lying off Elsinore in 
 the Sound between Denmark and Sweden, 
 Tycho built a magnificent observatory, and 
 
ASTRONOMERS & OBSERVATORIES 11 
 
 fitted it with splendid instruments all of his 
 own design and make. He founded in fact the 
 modern science of observational astronomy. 
 In the Royal Library at Copenhagen one may 
 see the great journals of observations which 
 Tyeho and his assistants made for some 
 twenty years in that island catalogues of 
 stars, places of the planets and of the Moon, 
 places of comets, and all the regular routine 
 of a modern observatory. There are two 
 things to be remembered about Tycho's 
 work ; it was immensely more accurate 
 observation than had ever been made before, 
 and it was done before the telescope was 
 invented. 
 
 One is so accustomed to think of an 
 astronomer as placed at the eye end of a 
 telescope, that it seems extraordinary that 
 much could be done without it. True, Tycho 
 was limited to the stars and the planets which 
 the unaided eye can show, but there was 
 much to be done with these which had never 
 yet been done. Instruments mounted on 
 pillars, furnished with circles or with portions 
 of circles divided into degrees and parts of 
 degrees, provided with carefully made sights, 
 were capable of considerable accuracy in the 
 hands of a man with a genius for handling 
 
12 ASTRONOMY 
 
 instruments. At least they were capable of 
 providing the mass of observations upon which 
 Kepler founded his laws of planetary motion, 
 and upon which in the end Newton rested his 
 demonstration of the principle of universal 
 gravitation. 
 
 But we are going too fast. Tycho Brahe 
 was an admirable astronomer, justly considered 
 the greatest that ever lived ; but he was 
 not quite so admirable as a neighbour. It 
 seems that a haughty spirit and a high-handed 
 manner of disregarding his public duties were 
 responsible at least in part for the catastrophe 
 which overtook him. His revenues were 
 withdrawn ; he was forced to leave Denmark ; 
 and so he came to Prague, and Kepler became 
 possessed of his observations, with the great 
 consequences to astronomy which we have 
 briefly mentioned. " Had Tycho remained 
 in his island," said a famous historian of 
 astronomy, " perhaps we should still have 
 been ignorant of the true system of the 
 
 universe." 
 
 But a few years after his death in 1604 the 
 telescope was invented in Holland. There are 
 several claimants to the invention, and their 
 respective merits are somewhat obscure. 
 Nor is it quite certain that posterity has been 
 
ASTRONOMERS & OBSERVATORIES 13 
 
 right in giving to Galileo the undivided credit 
 of first turning the telescope to the skies. 
 His famous discovery of the four satellites 
 of Jupiter led to his immediate recognition of 
 the truth they demonstrated, that the earth 
 and the planets move round the sun, as 
 Copernicus taught. The persecution which 
 he suffered, for teaching his opinions, threw 
 a romance about his life which obscured the 
 fact that the injustice of others made him 
 unjust himself. A German astronomer, Simon 
 Marius, had himself discovered the satellites 
 of Jupiter within a few hours of and probably 
 before Galileo's discovery. He had made 
 many observations of them and constructed 
 a first rough table of their motions. Galileo 
 roundly accused him of stealing his 
 observations and his credit, and the world, 
 believing Galileo, went on calling Marius an 
 impudent pretender until two or three years 
 ago. So difficult it is to be just in assigning 
 credit to rival discoverers. 
 
 The first discoveries of the telescope opened 
 a new heaven to men's eyes. There were 
 mountains in the moon, spots upon the 
 supposed unblemished surface of the sun ; 
 the planets became worlds like the earth, 
 and the Milky Way was resolved into 
 
14 ASTRONOMY 
 
 innumerable stars. Astronomers of all 
 countries set themselves to improve the 
 telescope, and up to a certain point their 
 progress was excellent ; but at that point 
 they stopped. For technical reasons, into 
 which we can hardly enter, those who aspired 
 to make their telescopes more powerful were 
 forced to make them inordinately long ; 
 and such instruments, though they were fairly 
 satisfactory for viewing the planets, were 
 very little good on stars. Hence we find in 
 the 150 years after the invention of the 
 telescope that little progress was made in 
 exploring among the stars and nebulae. 
 
 In other directions, however, immense 
 advances had been made. Newton at 
 Cambridge had made his immortal discovery 
 of the principle of universal gravitation ; 
 Halley in London had spurred him on to 
 complete his work and publish it. On the 
 continent of Europe mathematicians had 
 opened up regions of mathematical analysis, 
 in whichNewton's principle was the foundation, 
 though his methods were superseded by others 
 more tractable in ordinary hands. It is a 
 remarkable fact that no one since Newton 
 has succeeded in doing much by his methods. 
 Everywhere the telescope had been applied 
 
ASTRONOMERS & OBSERVATORIES 15 
 
 to the instrument of measurement, with great 
 gain in accuracy. Bradley had discovered 
 the important principles of the aberration 
 of light and the nutation of the axis of the 
 earth principles which at once reduced to 
 order all the questions connected with the 
 accurate determination of star places in the 
 sky. The foundations of the astronomy 
 which concerns itself with minute nicety 
 of observation, with profound depths of 
 calculation, had been well and truly laid. 
 Upon that foundation generation after 
 generation of mathematicians and of observers 
 have built ; with hardly any pause the 
 edifice of precise astronomy has arisen, 
 stone by stone, till to-day it forms the immense 
 structure into which it is hard to enter without 
 the master key of some mathematics and a 
 great deal of technical knowledge. All this 
 side of astronomy lies outside the range of a 
 small book like the present. We shall make 
 some acquaintance with its results in the 
 chapters which follow ; with its methods 
 and with its technique we cannot concern 
 ourselves further. 
 
 Upon the other side of astronomy lies the 
 realm of exploration and discovery, whose 
 frontiers receive a great extension with every 
 
16 ASTRONOMY 
 
 considerable advance in instrument building 
 or new method of observation. Such an 
 extension was made by Sir William Herschel. 
 The reflecting telescope which Newton had 
 invented made at first slow progress in size 
 or in use. It remained an interesting example 
 of optics rather than an instrument of any 
 practical importance, until Herschel found 
 himself at last in the position to carry out his 
 cherished scheme. What it was that implanted 
 in Herschel's brain the resolution to explore 
 the sky is of course unknown. All that we 
 do know is that during years of hard work 
 as a teacher and conductor of music he steadily 
 kept in sight the purpose, that he would 
 make greater telescopes than any which had 
 yet been made, and that he would thoroughly 
 search the heavens. It was natural that 
 his thoughts should turn to the reflecting 
 telescope, whose construction, though tedious 
 and delicate, is yet relatively simple and within 
 the power of a skilled amateur as has often 
 been proved since Herschel's time. Of^the 
 way in which Herschel made his telescopes 
 very little is known, or at least very little 
 has been published. The observations which 
 he made with them are the foundation and 
 to a considerable extent the structure of what 
 
ASTRONOMERS & OBSERVATORIES 17 
 
 we know to-day about the nebulae and star 
 clusters. In the chapter dealing with the 
 nebulae we shall see how the survey of the 
 sky carried from pole to pole by Sir William 
 and his son Sir John remains to this day 
 complete and unsurpassed. 
 
 The first half of the nineteenth century was 
 again a period of steady rather than of 
 startling progress. Bessel in Germany, Airy 
 in England, systematized and organized 
 observatory work so that the methods of 
 to-day bear the indelible stamp of their 
 talent. Adams in England and Leverrier in 
 France discovered Neptune by calculation 
 a feat which produced an impression more 
 profound than any other event in the history 
 of science. In Ireland Lord Rosse made in 
 his own workshops the gigantic six-foot 
 mirror which remains up to the present day 
 unsurpassed in sheer size. Then just. after 
 the middle of the century came the invention 
 of spectrum analysis by Kirchoff, and its 
 application to the stars and nebulae by 
 Huggins. 
 
 This discovery marks an epoch as distinctive 
 as, perhaps more distinctive than, the 
 discovery of the telescope, for it gave to 
 astronomers a new sense. The telescope is 
 
18 ASTRONOMY 
 
 but an eye allowing man to see farther and 
 better than he could see before. The 
 spectroscope enables him to achieve what 
 might have seemed perfectly hopeless, to 
 determine what the stars are made of by 
 analysing the light which they send us ; and 
 to determine the speed with which they are 
 approaching or receding in the line of sight. 
 
 With this new sense at its command 
 astronomy made at once a brilliant advance 
 into regions which had been closed to all 
 examination It was immediately clear that 
 the stars are suns like our sun, while many 
 of the nebulae are but clouds and drifts of 
 shining gas. It was at once possible to prove 
 that the substances, common or rare, which 
 are found in the crust of the earth are present 
 as glowing vapours in the distant stars, and 
 that from end to end of space matter is 
 essentially the same. Some light rays there 
 are, indeed, in the nebulae or in the stars 
 which have not yet been run to earth. The 
 brilliant discovery of helium by Ramsay 
 gives us reason for hoping that a knowledge 
 of the stars may in time show us how to look 
 for and to find other substances which are 
 all the while terrestrial, though we have not 
 yet recognized them. 
 
ASTRONOMERS & OBSERVATORIES 19 
 
 The application of photography to astronomy 
 followed hard upon the introduction of light 
 analysis. But for all except the brightest 
 objects the necessary exposures were so long 
 that a wet collodion plate could not deal 
 with them ; the plate dried during the 
 exposure and lost its sensitiveness. Real 
 facility in astronomical photography came 
 with the introduction of the dry gelatine 
 plate. 
 
 We may perhaps spend a few moments in 
 considering exactly how the photographic 
 plate helps the astronomer, for there is much 
 misconception on the subject. The old- 
 fashioned astronomer was pictured in 
 imagination with his eye glued to the telescope 
 the adhesive was always used waiting and 
 watching to discover something new. The 
 modern astronomer is supposed to glue 
 a photographic plate in the place of hi eye, 
 and to sleep in confidence that the plate on 
 development in the morning will show him 
 everything that he might have observed in 
 the night. But there is not much truth in 
 this picture. Respectable photographs cannot 
 be taken without attention. The astronomer 
 is differently occupied nowadays, but his lot 
 is not made much easier ; to watch a faint 
 
20 ASTRONOMY 
 
 guiding star on the cross wires of the guide 
 telescope, during an exposure of many hours, 
 is a most exhausting business. 
 
 The real advantages of the photographic 
 plate lie in two of its properties : its power of 
 storing up impressions until they become 
 developable ; and its power of recording 
 simultaneously a number of things which 
 could be observed only one by one. To these 
 we should add its power of taking in a very 
 much larger field of view than can be seen at 
 one time in the telescope. The first and 
 the last are responsible for the great success 
 with which the star camera portrays star 
 clouds and very extensive nebulae, which 
 cannot produce their proper effect when they 
 are seen piecemeal. The second and the 
 last have revolutionized many branches of 
 precise astronomy by transferring the work 
 of measurement from the micrometer on the 
 telescope to the measuring machine in which 
 the negative is placed. This has great 
 advantages ; among them are two which can 
 be appreciated at once. It economizes time, 
 because one can take advantage of an hour 
 of clear and steady sky to take a photograph 
 which will provide work for a week of cloudy 
 weather. And secondly a more technical 
 
ASTRONOMERS & OBSERVATORIES 21 
 
 reason it greatly facilitates the troublesome 
 corrections which have to be applied to the 
 measures, because they can be calculated 
 and applied all at once, instead of one by one. 
 
 The requirements of photography have 
 made new and very interesting demands upon 
 the skill of the optician and the mechanic. 
 The optician is called upon to design lenses 
 which shall give the greatest concentration 
 of light, for photographing faint objects ; 
 and the best possible definition over a large 
 field of view, for photographing extensive 
 objects like the star clouds of the Milky 
 Way. And what passes for good definition 
 in a terrestrial camera turns out to be 
 very bad definition when it is a question of 
 photographing a star far from the centre of 
 the field. One wants the minute image, 
 hardly visible on the plate without a glass, 
 to be quite round and quite free from*wings 
 and from surrounding glare. In spite of 
 long calculation and trials, it is not as yet 
 possible to obtain a satisfactory picture 
 more than about ten degrees across. 
 
 The mechanic is required to make long 
 telescopes without flexure, and to make 
 driving clockwork that shall cause the 
 telescope to follow the daily motion of the 
 
22 ASTRONOMY 
 
 stars without allowing them to wander by 
 ever so little upon the photographic plate. 
 One such device never fails to fascinate the 
 visitor to an observatory the electrically 
 controlled clockwork devised by Sir Howard 
 Grubb, of Dublin, and fitted to many modern 
 British instruments. A pendulum in a corner 
 of the telescope house has nothing to do but 
 to send a current every second to the clock. 
 This current detects whether the clock has 
 gone fast or slow, and if the error amounts to 
 one-fortieth of a second of time, it is 
 automatically corrected. The reader will 
 have no difficulty in believing that an 
 apparatus of this delicacy does not always 
 work smoothly. Like all delicate machines, 
 it wants " tuning up " from time to time ; and 
 these modern refinements and complications 
 in astronomical instruments put something 
 of a strain upon the astronomer who has 
 to add the technique of photography, of 
 electrical testing, of clock making, and of 
 silvering mirrors to his more old-fashioned 
 professional attainments. 
 
 With every increase of size in astronomical 
 instruments the question is asked, Has the 
 limit of successful construction now been 
 reached ? Is it possible or profitable to go 
 
ASTRONOMERS & OBSERVATORIES 23 
 
 farther ? Such questions cannot receive any 
 positive answer, but it may be worth while 
 to enquire for a moment what prospect 
 there may be of providing the astronomer 
 with greatly increased telescopic power. 
 
 The greatest object glass which has been 
 successful is that mounted at the Yerkes 
 Observatory of the University of Chicago. 
 Its diameter is 40 inches ; its focal length 
 is 62 feet ; and it requires for its installation 
 a dome only two or three feet smaller than 
 the inner dome of St. Paul's in London. 
 It has been in existence some sixteen years, 
 and it remains the largest lens at work. 
 What has put a stop to the series of larger 
 and larger glasses which was made during 
 the last thirty years of last century ? There 
 are several reasons. 
 
 The first of course is the expense. The cost 
 of mounting one of these big refracting 
 telescopes is immense. The telescope itself 
 increases in cost at a rate out of all proportion 
 to its aperture ; the building in which it is 
 housed, the revolving dome and the rising 
 floor, soon come to cost as much as the 
 telescope. Hence there has arrived a time 
 when the increased cost of installation is out of 
 proportion to the gain in light ; and while it is 
 
24 ASTRONOMY 
 
 impossible to say that the Yerkes telescope 
 marks the limit to construction in that 
 particular style, one can at any rate say that 
 there are no signs at present of any one 
 trying to build a larger 
 
 The second reason against attempting to 
 build larger refractors is the extraordinary 
 difficulty in getting large discs of optical 
 glass. The optician could handle a bigger 
 disc than the glass founder can supply. 
 About the business of fashioning these discs 
 there has always been something of a mystery, 
 concerning a secret strictly guarded, in the 
 possession of only two or three persons at 
 one time. Unless a firm was in possession 
 of the secret, it was hopeless to think of 
 making big discs, so the story went ; and the 
 man who knew it was paid a princely salary 
 on condition he never went outside the 
 gates with the inevitable result. Whether 
 or no there is any truth in the story as it is 
 told, it is certain that the manufacture of 
 large discs of optical glass has always been 
 difficult ; that not more than three or four 
 firms in all have succeeded ; and that at the 
 present time there is only one a French firm 
 which has any success. 
 
 The difficulty lies, of course, in getting the 
 
ASTRONOMERS & OBSERVATORIES 25 
 
 disc of glass uniform throughout ; a patch 
 of density slightly greater than the rest will 
 ruin it, unless the excess can be removed by 
 re-annealing a process which occasionally 
 succeeds in a remarkable way, which is very 
 curious when one considers that the glass is 
 not, in its last stages, ever raised to anything 
 like melting point. We have spoken, up to 
 the present, of the ordinary crown glass and 
 flint glass. With the more complex kinds of 
 glass success has been still less. It seems to 
 be impossible to produce discs of the celebrated 
 Jena glasses larger than about twelve inches 
 in diameter, and this puts a severe restriction 
 on the optician, who could do ever so much 
 more if he had a larger range of kinds of glass 
 with which to work. 
 
 A third reason why no more big refractors 
 have been built is that enterprise has been 
 turned into other channels the construction 
 of big mirrors for reflecting telescopes, and 
 horizontal or vertical telescopes for solar 
 work. The great centre of activity in such 
 work is at present the instrument shop at 
 Pasadena belonging to the Solar Observatory 
 of Mount Wilson, California. With financial 
 resources which seem illimitable, administered 
 by a director of boundless energy and an 
 
26 ASTRONOMY 
 
 engineer of consummate skill, the Mount 
 Wilson Observatory has created an equipment 
 which is extraordinarily fine. The reflector 
 of 60 inches aperture is for the moment 
 the last word in instrument construction ; 
 that it will not be so for long is clear from 
 the fact that a 100-inch mirror is already 
 under way, finally to beat the record of size 
 which Lord Rosse's reflector has held for 
 some sixty years. Into the 60-inch telescope 
 the constructor has put every refinement and 
 device that he can think of as possibly useful ; 
 the result is a formidable complication 
 somewhat discouraging to the astronomer 
 of strictly limited means, as most astronomers 
 are. 
 
 These magnificent instruments can do things 
 which no other telescopes can do, and it is 
 an immense advance that their constructors 
 have made. There is naturally a danger 
 that the owners of smaller telescopes will be 
 depressed, and will perhaps under-estimate 
 the possibilities that they have within their 
 reach if they have courage. There is, as a 
 matter of fact, still plenty of excellent work 
 for the smaller instruments to do both for 
 the modest observatory and for the still 
 more modest amateur. In various places 
 
ASTRONOMERS & OBSERVATORIES 27 
 
 as we proceed we shall consider what work 
 there is within reach of the amateur observer. 
 For the moment let us return to the public 
 observatory, and glance at its equipment, 
 We shall suppose that it is a University 
 Observatory, well but not extravagantly 
 equipped, and prepared for teaching as well 
 as for research. 
 
 First there will be a meridian circle a 
 telescope of moderate aperture, say 6 inches 
 mounted upon very solid piers so that it will 
 turn from north to south, through the pole 
 and the zenith, pointing always to the 
 meridian of the place. In the focus of this 
 telescope there is a set of wires, equally spaced 
 about a centre wire which forms the visible 
 instrumental meridian. And the telescope is 
 provided with a divided circle which serves 
 to measure how far it is turned from the 
 zenith, north or south. With this instrument 
 a double observation is made the time at 
 which a star crosses the meridian, and the 
 distance from the zenith at which it passes. 
 The purpose of the instrument is first to find 
 the time, the correction to the clock from 
 observations of known stars ; and then to 
 observe the places of stars which are not so 
 well known, for making star catalogues ; or 
 
28 ASTRONOMY 
 
 to observe the places of the moon and the 
 planets, for improving the theory of their 
 motions. All this kind of work is called 
 astronomy of position, and belongs to the 
 department of meridian observat on. 
 
 Secondly, there will be the department of 
 the equatorial, a telescope mounted and 
 driven by clockwork so that the effects of the 
 rotation of the earth are annulled. In the 
 equatorial the star can be kept in the field of 
 view for as long as may be desired ; it does 
 not rush across the field of view and vanish, 
 as it does in the meridian circle. With the 
 equatorial one studies and makes drawings of 
 the planets, measures the relative positions 
 of the components of double stars, or searches 
 for new comets and nebulae. 
 
 Then, more likely than not, there will be 
 another equatorial telescope, with its lens 
 corrected for the photographic instead of for 
 the visual rays of light. Such a telescope 
 is in reality a camera, though different 
 in proportions from the ordinary landscape 
 camera of commerce. The lens w r ill be perhaps 
 12 inches in diameter ; the tube about ten 
 times this diameter in length. There will be 
 no bellows, because the camera is in this case 
 used at a fixed focus, the focus for infinity, 
 
ASTRONOMERS & OBSERVATORIES 29 
 
 and the small range of focusing required to 
 bring the plate into exact adjustment can 
 be provided in other ways ; and bellows 
 would be very much out of place in an 
 instrument which requires to be rigid in a 
 high degree. Lastly comes the carrier for 
 the photographic plate, which is perhaps 
 six inches square, not nearly so large as 
 the objective. It is this curious proportion 
 between the lens and the plate which makes the 
 photographic telescope look unlike a camera. 
 One is so much accustomed to seeing an inch 
 of objective covering a photographic plate 
 8J X 6| inches, that it seems at first sight 
 absurd to require 12 inches of objective to 
 cover six inches of plate. But we must 
 remember that we want a large objective 
 because we are going to photograph very 
 faint objects ; and we use a small plate 
 because we are content only with the centre 
 of the field, where the definition is of the 
 very best. 
 
 The photographs taken with this instrument 
 will be intended for accurate measurement. 
 Hence a measuring machine equipped with a 
 microscope, with accurate divided scales, and 
 with micrometer screws, will form part of 
 the equipment of the photographic telescope. 
 
30 ASTRONOMY 
 
 With such an outfit the observatory will be 
 able to undertake a share in the work of 
 charting and cataloguing the stars by 
 photography, which is now being carried 
 out by an international co-operation of 
 observatories. 
 
 The photographs taken with this 
 instrument, being intended for accurate 
 measurement, will be on a large scale, with 
 the stars widely separated. They are 
 excellent for their purpose, but they make 
 very poor pictures. The magnificent, densely 
 crowded pictures of the Milky Way which 
 make a splendid show on the screen at a 
 lecture, or in a work on astronomy which 
 can afford illustrations, are made with an 
 instrument differently proportioned, with a 
 focal length only four and a half or five times 
 the aperture, and with an objective of four 
 lenses arranged in pairs, very much like the 
 ordinary high-class camera lens. Then, with 
 an exposure of several hours and a relatively 
 much larger plate, one can concentrate 
 many square degrees of sky into one picture, 
 which looks magnificent, and is very useful 
 for study of the broad features of the sky, 
 but is not good for measurements of precision. 
 
 Our observatory will also have a department 
 
ASTRONOMERS & OBSERVATORIES 31 
 
 devoted to astrophysics, that is to say, to 
 the study of stars and nebulae with the 
 spectroscope. One of the equatorials will 
 be equipped with the apparatus of slit, prism, 
 and camera which photographs the result 
 of the light analysis, the spectrum with its 
 bright lines coming from glowing gas, or its 
 dark lines telling of absorption in the stellar 
 atmospheres. When the spectrum is obtained 
 it requires interpretation ; the lines in the 
 spectrum, for example, may correspond in 
 position exactly with the lines which are 
 produced by the vapour of iron, but some 
 may be abnormally strong and others almost 
 invisible. It is then the task of the astro- 
 physicist to discover what this difference 
 means. He turns the observatory into a 
 physical laboratory, equipped with dynamos, 
 electric arcs, furnaces, pressure pumps, and 
 vacuum pumps, and he tries to reproduce 
 upon earth the conditions, indicated by the 
 abnormal spectrum, which are present in 
 the stars. 
 
 Again, it may be that a department of our 
 observatory is devoted to solar physics, to 
 the study of those details in our own particular 
 star which cannot be observed in the others 
 because they are so far away. The solar 
 
82 ASTRONOMY 
 
 \ 
 
 physicist will have quite a different equipment. 
 
 He has plenty of light available, so that his 
 instruments need not be very great in aperture, 
 and may be very long, to give him pictures 
 upon a large scale. So he will probably 
 have a very long fixed horizontal telescope 
 and spectroscope, into which the sunlight 
 will be reflected by a slowly rotating mirror, 
 the coelostat. Or if he is quite up to date 
 he will decide that a horizontal telescope is 
 too much affected by the radiation from the 
 heated ground, and he will substitute a 
 vertical telescope, with the coelostat and 
 objective mounted on a high tower, and the 
 spectroscope at the bottom of a deep well 
 below. But he will not be able to do this 
 unless he is very rich. 
 
 Lastly, our observatory will be equipped 
 with a library and with a staff of calculators, 
 called computers. The library will contain 
 the publications of every other observatory 
 all of them presented to it ; the journals 
 of the astronomical and kindred societies ; and 
 the four or five technical journals published 
 by subscription. The bulk of astronomical 
 literature is becoming enormous. There are 
 some hundred and fifty observatories that 
 publish observations more or less regularly ; 
 
ASTRONOMERS & OBSERVATORIES 33 
 
 the journals amount in the total to several 
 thousand pages annually, and it is quite 
 impossible for the astronomer to read every- 
 thing that appears. Hence there has arisen 
 in astronomy, as in other sciences, the 
 need for carefully prepared and arranged 
 abstracts of each year's work, and several 
 volumes are now devoted annually to this 
 purpose. From this mass of raw material the 
 evidence on any point is gradually extracted 
 and worked up into a case ; the case is 
 examined and tried by discussion and con- 
 troversy, at meetings of the astronomical 
 societies or in the pages of the journals ; and 
 in the end years of observation, hundreds 
 of pages of publication, months of calculation 
 and discussion, produce the brief conclusion, 
 for example, that the gaseous nebulae are 
 concentrated upon the Milky Way. 
 
 But we must not overlook the computer, 
 who plays an indispensable part in the 
 observatory. Hardly a measure which is 
 made at the telescope is of any use until it 
 has passed through a process of calculation 
 and correction more or less prolonged. The 
 continued variation in the state of adjustment 
 of the instruments, the refraction of the air, 
 and the changes day by day and year by 
 
34 ASTRONOMY 
 
 year in the position of the celestial equator 
 have all to be reckoned with by calculation. 
 Three places, for example, of the same star 
 on different days will all be different, and 
 each one must be reduced to the place which 
 will ultimately appear in the star catalogue. 
 A great deal of this work is quite simple 
 arithmetic, proceeding according to instruction 
 on well known lines. Under proper direction 
 it is done by boys and young ladies, who in 
 the different observatories of the world play 
 a part in scientific work which is often little 
 realized by those who hear only of the final 
 conclusions. Their labour is often greatly 
 lightened by the calculating machine, which 
 performs long multiplications and divisions 
 at a speed almost incredible to the uninitiated. 
 We have referred already to the astronomer's 
 dependence upon the engineer, the optician, 
 and the glass maker. We must not omit 
 from our list the photographic plate maker. 
 The question is often asked at an observatory, 
 whether the astronomer uses specially 
 sensitive plates. The answer is that it is 
 not possible to obtain better emulsions 
 than those which are prepared by the leading 
 makers of photographic plates. Not that they 
 are perfect ; they are far from being perfect, 
 
ASTRONOMERS & OBSERVATORIES 35 
 
 and it is particularly desirable that some 
 means should be found of getting rid of the 
 coarseness of grain at present universal in 
 rapid plates. For the time being, however, 
 it seems that not much progress is being 
 made to this desirable end. In one way only 
 do astronomical plates differ from those in 
 ordinary use. They are coated upon thin 
 plate glass, which is nearly flat, instead of 
 upon the ordinary glass, which is generally 
 twisted and billowy. 
 
 In this necessarily rapid sketch of the 
 evolution of the modern observatory, we have 
 been compelled to leave large gaps which 
 could not be filled in the space at our disposal. 
 We have now to deal in successive chapters 
 with some of the leading results of modern 
 astronomy. We shall try as ar as possible 
 to present the subject free from the compli- 
 cations which beset it in actual practice. 
 
 In concluding a chapter on Astronomers 
 and Telescopes, we may remark that the 
 application of photography has made a 
 difference which is hardly yet realized. 
 Since photographs can be reproduced and 
 put into the hands of all, it follows that any 
 student of astronomy is the potential possessor 
 of the greatest telescopes in the world. By 
 
 B 2 
 
36 ASTRONOMY 
 
 this we mean that astronomical work of real 
 importance not merely intellectual recrea- 
 tion, but study which may make valuable 
 contributions to knowledge can be done 
 upon published photographs. The output 
 of pictures is enormous, and many of them 
 remain mere pictures for lack of students to 
 treat them seriously. If you would be an 
 astronomer, think twice before buying a 
 telescope. A measuring machine, to measure 
 photographs taken by other people, is a far 
 better investment. 
 
CHAPTER II 
 
 THE SUN AND MOON 
 
 WHEN we put the Sun and the Moon together 
 in one chapter of our study of astronomy, 
 we do not attempt to be logical. The sun, 
 the overpowering source of light and heat 
 and energy which makes life upon the earth 
 and all our activities possible, and the moon, 
 the dead lifeless companion of the earth, 
 whose most obvious use is to act as a pale 
 reflector of sunlight to mitigate the darkness 
 of our nights, are at opposite ends of the 
 series in which we may arrange the components 
 of our solar system. Nevertheless, we. will 
 study them together, and we shall find in 
 so doing that a disregard of logic is sometimes 
 as advantageous in the study of astronomy 
 as it is in the conduct of life. 
 
 The study of the sun is essentially modern, 
 
 because it depends very little on the formal 
 
 laws of geometry, and requires above 
 
 everything some elementary ideas upon the 
 
 37 
 
38 ASTRONOMY 
 
 properties of matter, which are of quite 
 modern growth. Little more than a hundred 
 years ago, for example, it was possible for 
 scientific men to discuss quite gravely, 
 without ridicule, whether or not the sun 
 might be inhabited ; not on his surface, 
 indeed, which was undeniably too hot, but 
 perhaps somewhere in the darker layers which 
 we seem to see below the brilliant surface, 
 when we look down into the depths of a 
 sunspot. To-day, such an idea has only to 
 be mentioned to appear ridiculous. We can 
 no longer be content with the plain fact 
 that the surface of the sun is pouring out 
 immense supplies of heat and light ; we ask 
 ourselves at once, whence come those supplies ; 
 we realize that they must be brought up to 
 the surface from within ; and we see that 
 it must be even hotter inside than it is on 
 the surface. There is no more a question of 
 cool, habitable depths below. Nor is it 
 probable that the sun is anywhere solid, in 
 the ordinary sense of the term. 
 
 With a diameter one hundred times that of 
 the earth, the average density of the sun is no 
 more than a quarter the average density of the 
 earth, and not more than one-half the density 
 of the earth's rocky surface layers. One can 
 
THE SUN AND MOON 39 
 
 account for this combination of great size and 
 little density only by supposing that the sun is 
 is in a state of what must be called gas, since 
 it is neither solid nor liquid, but which is 
 gas in a state which we cannot reproduce 
 experimentally, a kind of thick treacly fluid, 
 denser than water, yet with the elastic proper- 
 ties of a gas. Between the interior and the 
 surface there must be a continual rush of 
 hot currents bringing up from inside the 
 heat which is poured out at the surface. 
 Beyond that bare fact we still know 
 comparatively little of what goes on inside 
 the sun. 
 
 The first question that must be examined 
 is this : Whence comes the heat ? It is 
 brought up by currents from below ; but 
 how is the supply below maintained ? The 
 sun is not burning, in the sense in which a 
 coal fire is burning and producing hea by 
 combustion in the oxygen of the air. Its 
 heat is more like the heat of a white-hot poker, 
 which radiates away, and the poker becomes 
 cold, not having the means of renewing its 
 heat as the sun has. The way in which a 
 gaseous body may radiate heat, and yet 
 remain hot or even become hotter, was 
 explained by Helmholtz in a simple manner. 
 
40 ASTRONOMY 
 
 The sun maintains his heat because he is 
 contracting. 
 
 When a stone is dropped upon a pavement, 
 it acquires a store of energy during the fall 
 which is suddenly dissipated when the motion 
 is stopped. It is not lost, however. No 
 energy can be lost. It can only be transformed 
 from one kind to another ; and in the case 
 of the stone suddenly stopped, the energy 
 which it had acquired in falling is converted 
 into heat, which is radiated away. But the 
 stoppage need not be sudden. The same 
 amount of heat would be produced if the 
 stoppage were ever so gradual. Suppose then 
 that the sun, under the influence of its own 
 strong gravitation, contracted in diameter 
 a thousand feet. Every particle at the 
 surface would have fallen a thousand feet 
 and have been stopped; all the matter 
 inside would have fallen in proportion, and 
 the amount of heat generated would have 
 been enormous. It is not hard to calculate 
 that a shrinkage of a thousand feet would 
 supply all the heat which the sun radiates 
 in five years. At this rate the sun would 
 decrease in diameter about 40 miles in a 
 thousand years ; and since his diameter is 
 800,000 miles, it is quite clear that this 
 
THE SUN AND MOON 41 
 
 principle of Helmholtz is able to explain 
 how the sun's heat might be maintained for 
 a hundred thousand years without any 
 appreciable decrease in his apparent size. 
 During hundreds of thousands, or millions 
 of years, the sun might be pouring out his 
 enormous supply of heat, year by year, and 
 might all the while be actually growing 
 hotter. He will continually grow hotter 
 so long as his material obeys the laws of 
 gases. 
 
 Gravitation, therefore, is a cause sufficient 
 to maintain the sun's heat for ages. Yet 
 it has been the opinion of the most eminent 
 mathematicians that we cannot in this way 
 provide for the maintenance of the earth 
 as a planet deriving its heat from the sun, 
 for a period extending into the past more 
 than twenty million years ; while geologists 
 have demanded a hundred million for* the 
 evolution of the earth into its present 
 condition. How far this difficulty was ever 
 serious it is hard to say, for it cannot be 
 denied that both estimates rested upon 
 assumptions a little difficult to verify. Hap- 
 pily it would seem that there is no longer 
 any need to be concerned at the difference 
 between mathematicians and geologists. 
 
42 ASTRONOMY 
 
 Within the past few years all our notions 
 of the constitution of matter have been 
 revolutionized. We have become familiar 
 with radium and with all the radio-active 
 substances. From these we have learned that 
 atoms of matter contain stores of energy which 
 are ordinarily locked up, but which in certain 
 bodies can gradually escape, and produce 
 heat and many kinds of electrical action. 
 These radio-active bodies exist in the crust 
 of the earth in most minute quantities as 
 is proved by the enormous price of radium 
 yet it has been calculated by Strutt that the 
 earth would be far hotter than it is, were the 
 interior as rich in radium as are the surface 
 layers, and were the radium able to produce 
 heat in the interior as it can at the surface. 
 Moreover, it by no means follows that the 
 energy of the atom, which is ordinarily 
 imprisoned in terrestrial matter, remains 
 imprisoned under all conditions under such 
 conditions, for example, as prevail in the 
 interior of the sun. Hence it would seem 
 that by the discovery of the energy within 
 the atom, all our difficulties are inverted. 
 It was a question how to account for a 
 sufficient supply of heat ; in future the 
 physicists may be embarrassed by the 
 
THE SUN AND MOON 43 
 
 problem, how to explain that the sun's supply 
 of heat is no greater than it is. 
 
 There is another source, not yet considered, 
 from which the sun might derive heat by 
 the fall of material upon his surface from 
 what we are accustomed to regard as empty 
 space. This differs from the first way we 
 considered, in that it involves a continual 
 increase in the mass of the sun, which would 
 shorten the year for dynamical reasons 
 quite simple, into which we need not enter 
 for the moment. The question then is, 
 how much matter could the sun absorb, 
 without causing a difference in the length 
 of the year which we could detect. An 
 interesting calculation which gives the answer 
 has been published lately by Newall. He 
 shows that four thousand million tons of 
 matter might fall on the sun every second, 
 without diminishing the length of the; year 
 by more than a thousandth of a second 
 per year. " It sounds," he adds, " a large 
 quantity. But if we regard our earth as the 
 quarry from which the material had to be 
 supplied, we should have a source that 
 would last for fifty thousand years." The 
 combined effects of shrinkage, radio-activity, 
 and fall of matter from without, are amply 
 
44 ASTRONOMY 
 
 sufficient to maintain the sun's heat for a 
 time to which we can set no limit. 
 
 It is an interesting question, why does the 
 sun, though it is but a sphere of gas, have 
 a visible and fairly definite surface ? The 
 telescope shows that the surface is granular, 
 formed of small bright cloudlets in rapid 
 motion, changing in pattern from minute to 
 minute, so that on photographs taken ten 
 minutes apart it is impossible to identify 
 the individual cloudlets, so rapidly have they 
 moved. In this shining surface, the photo- 
 sphere, are the rents or vortices from which 
 the cloudlets have been sucked away the 
 sunspots which were discovered immediately 
 the first telescope was turned upon the sun, 
 and which are still most fascinating objects 
 for study in small telescopes. In spite of 
 the prolonged and minute attention which 
 they have received for many years, and in 
 spite of the refined and ingenious modern 
 processes which are now employed in their 
 examination, their nature remains very much 
 of a mystery. They are evidently holes in 
 the photosphere from which the bright 
 cloudlets have been temporarily removed. 
 One may watch the edges of a spot as it were 
 melting away and falling in ; one may see, 
 
THE SUN AND MOON 45 
 
 in the spectro-heliograph pictures which we 
 shall presently describe, how the masses of 
 hydrogen and calcium vapours which lie above 
 and around them, are whirled in vortices. But 
 whether the sunspot represents an uprush of 
 heated matter or a downrush of cooled gas 
 from without remains still uncertain. 
 
 Whatever may be the actual physical 
 structure of sunspots, it is evident that they 
 are an index to an activity which is not 
 merely superficial, but affects the whole 
 structure and functions of the sun. For 
 many years the ever varying number of spots 
 had been observed and recorded without any 
 suspicion that they were governed by a set 
 of complicated and far-reaching laws. But 
 about the middle of the nineteenth century 
 it~was^emarked by Schwabe that the number 
 ofspot^ increasedTand diminished in aj^riod 
 which is roughly eleven years ; and ver.y soon 
 afterwards it was found that other pheno- 
 mena, with no apparent connection, obeyed 
 the same law. Displays of the aurora are 
 frequent in high latitudes, and most frequent 
 when the number of spots is great. 
 Occasionally the aurora is seen in temperate 
 latitudes, and a remarkable display is 
 generally associated with an outbreak of 
 
46 ASTRONOMY 
 
 unusually large spots upon the sun. Again, 
 the magnetic compass is subject to a small 
 daily swing backwards and forwards, which 
 is greater at some times than at others ; the 
 amount of the daily swing is closely associated 
 with the number of sunspots, and upon 
 some days when sunspots are unusually active 
 we have what is called a magnetic storm, 
 when the compass needle is violently disturbed 
 and telegraphic communication is interrupted 
 by currents of electricity in the earth. 
 
 These widespread effects can hardly be 
 ascribed directly to the spots upon the sun's 
 surface. Rather should we consider that 
 all three, sunspots, aurorse, and magnetic 
 disturbances, are manifestations of some 
 deep-lying cause in the sun, of whose nature 
 we are at present ignorant. The whole 
 phenomenon is indeed extremely complicated. 
 At the beginning of each new period the 
 spots break out in middle latitudes upon the 
 sun. They break out rather suddenly, and 
 in a year or two the number and area of the 
 spots becomes a maximum. Then they 
 gradually decline, and, as they do so, they 
 approach more and more nearly to the sun's 
 equator. Their period of eleven years is 
 fax from regular ; at different latitudes they 
 
THE SUN AND MOON 47 
 
 cross the sun's disc in different times, making 
 it appear that the sun does not rotate as a 
 whole ; at one time a great sunspot will be 
 associated with aurorae and magnetic storms ; 
 another equally great will not ; and sometimes 
 one will have the disturbances upon earth 
 without a visible disturbance on the sun, 
 in which case it is of course permissible to 
 suppose that the spot is on the far side of 
 the sun. 
 
 Such complex facts necessarily require 
 long continuous observation before their laws 
 can be elucidated. For many years there 
 has been an organization at work in the 
 observatories of Greenwich, South Kensington, 
 Dehra Dun, and Mauritius, by which the 
 sun has been photographed every day, and 
 the number and area of the spots has been 
 recorded. More recently new methods of 
 investigation have been developed. . The 
 Carnegie Institution of Washington maintains 
 a magnificent solar observatory upon Mount 
 Wilson, in California ; the Government of 
 India has established an observatory of the 
 first class 7,000 feet above sea at Kodaikanal, 
 in Madras. South Kensington is the English 
 station specially equipped for this work ; and 
 there are hopes that before long a fourth solar 
 
48 ASTRONOMY 
 
 observatory, in Australia, may complete the 
 chain and provide for the almost continuous 
 study of all the phenomena of the sun's 
 surface. There appears to be some reason 
 to think that in course of time it may be 
 possible to discover the connection, which 
 probably exists, between the activity of the 
 sun and the larger features of terrestrial 
 weather, more particularly the conditions 
 which produce drought and famine by 
 failure of the Indian monsoon. 
 
 Above the photosphere of the sun lie the 
 regions which become visible to the eye only 
 when, at a total eclipse, the moon intervenes 
 exactly between the sun and the earth. 
 So nearly of the same apparent diameter are 
 the sun and moon that it depends upon the 
 position of the moon in her not quite circular 
 orbit, whether she shall appear at the moment 
 of eclipse slightly larger or slightly smaller 
 than the sun. If she is smaller thgjgclipse is 
 annular, that is to say, a narrow ring of the 
 sun is visible all round the moon, and the 
 phenomenon is of no particular importance. 
 But if the moon happens to be in a part of 
 her orbit nearer the earth, and the sun looks 
 a little smaller than the moon, then as the 
 last segment of the sun's photosphere is 
 
THE SUN AND MOON 49 
 
 covered by the moon, there appears a narrow 
 belt of bright crimson with prominences 
 rising torn it, and the most beautiful halo of 
 pale wiite light, which is the sun's corona. 
 
 It is great good fortune that we can see the 
 corona at all. Were the moon ten per cent 
 less in diameter eclipses would always be 
 annular, and we should be unaware to this 
 day of the very existence of the sun's most 
 beautiful member. As it is, the moon is 
 just large enough, when circumstances are 
 favourable, to cast a narrow shadow, perhaps 
 seventy or eighty miles across, on the earth. 
 
 It is the object of eclipse observers to get 
 within this shadow, and with a battery of 
 instruments to make the most of the precious 
 few minutes while the shadow is sweeping 
 across them ; the nearer they are to the 
 central line, the longer does the shadow take 
 to cross them ; but in no case can the duration 
 of total eclipse exceed about seven minutes, 
 and the average is more nearly three or four. 
 The path of an eclipse passes indiscriminately 
 over land and sea, habitable country and 
 desert ; so that eclipse observers find plenty 
 of variety in their journeys, and travel to 
 out-of-the-way places. Within the last fifteen 
 years astronomical expeditions have gone to 
 
50 ASTRONOMY 
 
 the North Cape, Novaya Zemlya, and Japan 
 in 1896 ; to India and Mauritius in 7898 ; 
 Florida, Spain, and Algiers in 1900 ; Sumatra 
 in 1901 ; Labrador, Spain, and Tunis ia 1905 ; 
 Flint Island in the South Pacific in 1908 ; 
 Tasmania in 1910 ; and at the time ol writing 
 several expeditions are on their way back 
 from Vavau, near Tonga, again in the South 
 Pacific. In 1912 they will have to go to 
 Rio, and in 1922, after a lapse of nearly 200 
 years the shadow track of a total eclipse 
 will cross the British Isles. 
 
 There are few quests of a more sporting 
 nature than the pursuit of the solar corona. 
 An eclipse expedition involves months of 
 preparation at home ; the construction and 
 testing of special instruments ; the building 
 of portable huts and the provision of all kinds 
 of stores both scientific and domestic ; then 
 the long voyage and sometimes the perilous 
 landing in the surf of a coral island ; the 
 anxious weeks of building the observing huts 
 and setting up the instruments ; the long 
 practice and drills, so that each member of 
 the party may do exactly the right thing at 
 the right instant in operat ng a battery of 
 cameras and spectroscopes during the precious 
 seconds of totality. Then if the sky has 
 
THE SUN AND MOON 51 
 
 been clear comes the anxious, trying work 
 of developing photographs in a temporary 
 darkroom, with the air much too dusty or 
 water much too hot and dirty to make 
 photography easy. And finally, after a safe 
 return home, there are more months to be 
 spent in measuring and discussing the photo- 
 graphs obtained, and in publishing the new 
 conclusions which result from their study. 
 
 The writer's personal experience has been 
 confined to one eclipse, which was completely 
 cloudy ; the kind of eclipse which scoffers 
 say is the most satisfactory, because there 
 is no responsibility for any failure, and nothing 
 to do afterwards. A cloudy eclipse does 
 show one part of the phenomenon particularly 
 well the shadow which comes sweeping across 
 the country at twenty miles a minute. 
 From the hill above Vadso on the Varanger 
 Fiord in 1896, one had a long view up the 
 fiord to the west. The party had left their 
 ship at midnight in the rain ; the eclipse was 
 to be at five in the morning, and in those 
 high latitudes in August the sun scarcely 
 set. Breaks in the cloud at intervals gave 
 rise to hopes which were as quickly dashed. 
 For a few moments during the partial phase 
 of the eclipse the sun came out ; then 
 
52 ASTRONOMY 
 
 the clouds settled down and took on a 
 gloomy purple as the light failed and totality 
 approached. Then the shadow, plainly visible 
 upon the sea, came tearing down the fiord 
 from the west, and in the semi-darkness, 
 with no trace visible of the spectacle which 
 was passing behind the clouds, the party, as 
 is always ordered, performed the solemn 
 farce of exposing the plates and operating all 
 the instruments as if everything were well. 
 
 Such an experience is by no means unusual. 
 In the same eclipse the parties in Japan had 
 fog. In Spain, in Sumatra, in Labrador, 
 in Tasmania, and lately at Vavau, the 
 observations have been more or less completely 
 spoiled by cloud. An astronomer plays for 
 high stakes when he devotes the best part 
 of a year and a good deal of money, not 
 always his own, to an enterprise whose 
 success depends absolutely on clear sky at 
 the critical moment. 
 
 When he wins, the stakes are worth having. 
 He no longer occupies himself with the 
 prominences, for he has found a w^ay of 
 observing them without an eclipse. But in 
 spite of many attempts, no one has yet 
 succeeded in observing the corona without 
 the friendly aid of the moon, to cut off the 
 
THE SUN AND MOON 53 
 
 sun's overpowering light. All that we know 
 of the corona has been gathered in the 
 couple of hours or less obtained by adding 
 up the scattered minutes of totality, during 
 the sixty years or so in which eclipses have 
 been seriously observed. The main facts 
 about the corona are relatively simple. Its 
 light is largely light reflected from the sun, 
 as if by small particles of dust ; but there 
 is also present a gas which emits a green line 
 belonging to no known element. Its structure 
 depends upon the sunspots. When the spots 
 are many the corona is condensed, with 
 short rays somewhat evenly distributed. When 
 the spots are few the corona has long curving 
 extensions like wings in the plane of the sun's 
 equator, with a brushlike structure of short 
 rays proceeding from the poles. The form 
 of the corona is, then, one of the inter-related 
 effects of that prime cause, whatever it 
 may be, which is responsible for the sunspot, 
 the aurora, and the magnetic storm. How 
 the effect is produced remains nearly as much 
 a mystery as it was when the corona was first 
 photographed, 
 
 We have, however, obtained an idea of the 
 manner in which the streamers of the corona 
 are probably formed. Within the last few 
 
54 ASTRONOMY 
 
 years it has been realized that light can 
 exercise a very effective pressure upon 
 particles which are within quite narrow 
 limits of the right size, about a third of the 
 wave length of the light. The flood of light 
 poured out by the sun must be capable of 
 sorting out the dust particles in surrounding 
 space, and of sending flying those which happen 
 to be of the right size. Very probably the 
 corona is formed in great part of dust streams 
 driven outwards by this pressure of light ; 
 but the mechanism by which precise shape 
 is given to the streamers, and the reason why 
 this shape depends upon the sunspot period, 
 remain quite obscure. 
 
 Apart from these rather complicated 
 questions the corona has an interest all its 
 own as the most splendid feature of a rare and 
 intensely impressive event. So long as the 
 slightest segment of sun remains, no corona 
 is to be seen ; nothing is ever seen of it in 
 the narrowest annular eclipse. So long as even 
 a single bead of bright light on the moon's 
 limb marks where the sun is still shining 
 through the notch of a lunar valley, the 
 corona remains invisible ; as the moment of 
 complete totality arrives it flashes out clear 
 and pale on a deep blue or purple sky ; 
 
THE SUN AND MOON 55 
 
 and it vanishes again at the first instant 
 of returning sunlight. 
 
 Leaving aside the many other very technical 
 investigations which are made during an 
 eclipse, let us glance for a moment at the 
 prominences, which before the eclipse of 1868 
 were visible only during totality, but since 
 then have been visible on any day to any 
 one who can follow the path which Lockyer 
 and Janssen indicated. The prominences 
 are eruptions of glowing vapours, principally 
 hydrogen and calcium, which are thrown up 
 from the sun's surface, and appear during 
 eclipses as the red tongues of flame round the 
 dark edge of the moon. Since they are gas, 
 they emit rays of a few definite wave lengths 
 only ; the spectroscope is able to spread the 
 general white light of the sun into a band as 
 long and consequently as diluted as one may 
 please ; but it can do nothing to diluie the 
 pure bright images of the prominences in the 
 lines characteristic, let us say, of hydrogen. 
 Hence with a good spectroscope the prom- 
 inences may be seen and photographed 
 at any time, without waiting for an eclipse, 
 and in fact the record of prominences is 
 included in the daily routine of a solar 
 observatory. 
 
56 ASTRONOMY 
 
 This brings us back to the new methods of 
 invest 'gating how the vapours of hydrogen, 
 calcium, iron, and other metals are disposed 
 in the atmosphere of the sun. The inst ument 
 employed is the spectro-heliograph, developed 
 by Hale at Chicago and by Deslandres at 
 Meudon. We can scarcely, without pre- 
 supposing some knowledge of instrument 
 construction, explain the principle of this 
 excellent invention. Let it suffice to say that 
 by means of two slits moving in unison it is 
 possible to separate out the light coming from 
 the vapour of a given element in a given layer 
 of the solar atmosphere, and to produce, for 
 example, a picture of the sun in hydrogen 
 light from the upper layers or in calcium 
 light from the denser lower layers just above 
 the photosphere. The spectro-heliograph has 
 opened out a wholly new line of possibilities, 
 which must have in the future a profound 
 influence on our knowledge of what goes on 
 in the solar atmosphere. But for the time 
 being the profusion of results, outrunning 
 the powers of interpretation in many cases, 
 makes it impossible to sum up in a few words 
 the clear gain which has resulted. 
 
 In considering these new methods of solar 
 research we must not overlook the fact that 
 
THE SUN AND MOON 57 
 
 the results achieved may have a double 
 significance and value. We study the sun 
 because we are interested in all knowledge of 
 the great power-house, so to speak, of the 
 solar system. But we study him also because 
 the sun is in a sense a gigantic laboratory, 
 where processes can be observed at tempera- 
 tures, at pressures, and on a scale which 
 cannot be imitated in terrestrial laboratories. 
 In the sun may be found the answer to 
 questions which can hardly be investigated 
 otherwise, questions of the highest importance 
 in physics and chemistry. Are all the 
 chemical elements permanent, however high 
 the temperature ? Or are some of them 
 capable of resolution into simpler elements ? 
 How shall we account for the fact that some 
 elements hydrogen for example give two 
 spectra completely different, one of which 
 can be produced easily enough in* the 
 laboratory, while the other cannot be produced 
 at all, and is indeed recognized as belonging 
 to hydrogen only by some curious arith- 
 metical relations between the lines in one 
 spectrum and the lines in another ? All 
 questions such as these may be solved some 
 day by experiment in the solar atmosphere, 
 conducted at a distance of more than ninety 
 
58 ASTRONOMY 
 
 million miles from our solar observatories 
 upon earth. 
 
 Moreover, there is yet another side to the 
 question. The sun is a star; and the only 
 star which we can study in detail at close 
 quarters. A thorough understanding of what 
 is going on in the sun's atmosphere will lead 
 us gradually, no doubt, to an interpretation 
 of the many problems presented by stars of 
 varying types. 
 
 As we pass from the sun to the moon let 
 us glance for a moment at the tides, in the 
 production of which the moon and sun play 
 similar but somewhat unequal parts the 
 proximity of the moon considerably more 
 than compensating for the much greater mass 
 of the sun. Every one knows in a general 
 way that the tides are due to the moon 
 though it is on record that one observer 
 became doubtful because he had sometimes 
 noticed that there is a tide when there is not 
 any moon ! But the subject is complex 
 and difficult ; and the well-known figure, of 
 high water under the moon, where the moon 
 drags the water away from the earth, and an- 
 other high water on the opposite side, " where 
 the moon drags the earth away from the water," 
 is responsible for not a little misconception. 
 
THE SUN AND MOON 59 
 
 The crest of the principal tide wave 
 produced by the moon is not necessarily 
 under the moon, but its position depends 
 upon a number of conditions. Perhaps the 
 best way to think of the tides is to imagine 
 in the first place a free ocean, of uniform 
 depth, unobstructed by land. Then the 
 actions of the sun and moon, in their various 
 configurations, combined with the rotation 
 of the earth, would cause a number of tidal 
 waves of various periods and different heights, 
 and the effect at any moment is due to the 
 combination of these waves. Their tendency 
 to run round the earth is prof pundly_modjfied 
 by the configurations of the land, which 
 check the^waves^jajid deflect them, and con- 
 centrate them in certain places, so that in 
 different parts of the world the phenomena 
 oflbhe tides are extremely various. 
 
 Afoun3 the coasts of the British Isles the 
 tides are somewhat exceptional, in that the 
 two tides each day are approximately of the 
 same height. In consequence the tides are 
 not very difficult to predict with fair accuracy 
 by the use of rather rough and ready methods. 
 In other parts of the world India, for 
 example the two successive tides are very 
 unequal, and they vary apparently very 
 
60 ASTRONOMY 
 
 irregularly. Far more scientific procedure 
 is then required, to predict the tides with 
 the accuracy demanded by navigators in the 
 Eastern seas. First, the tides, for a series 
 of years, must be observed at various ports. 
 Then the observed tidal curve must be 
 analysed into its separate components by 
 the method known as harmonic analysis. 
 When that is done, the tides for future 
 years can be predicted by the beautiful 
 machine, the property of the Government of 
 India, which used to stand in the Science 
 Museum at South Kensington, and is now 
 in the National Physical Laboratory at 
 Bushy House. Every year a series of pulley 
 wheels in this machine are set to execute the 
 oscillations prescribed for them by the results 
 of the analysis A cord passes over the whole 
 series and carries a pen at the end. The 
 machine is set in motion, and the pen, 
 actuated by the combined motion of all the 
 pulleys, draws in a few hours the predicted 
 curves for the year, one after the other, for 
 the principal Indian ports. 
 
 For its practical importance the study of 
 the tides ranks with the most indispensable 
 applications of scientific enquiry. Success in 
 predicting the apparently haphazard, the 
 
THE SUN AND MOON 61 
 
 really complex but orderly tides of a port 
 such as Aden, is justly regarded as one of 
 the triumphs of the theory of gravitation. 
 But the most fascinating chapter in the theory 
 of tides is the chapter in which Sir George 
 Darwin has traced the past and predicted 
 the future history of the moon. The friction 
 of the tides retards the rotation of the earth, 
 so that the day is becoming longer by very 
 slow degrees. By a process of reaction, 
 which we cannot attempt to follow here in 
 detail, the moon is by equal necessity driven 
 away very slowly from the earth, so that the 
 month becomes longer also. Carrying the pro- 
 cess back, Sir George Darwin has pictured for 
 us a time when the earth and the moon were 
 almost in contact, and the day and the month 
 were equal, probably between three and five 
 hours long. With the two bodies so close 
 together the tidal forces were much more 
 powerful and efficacious- then than they are 
 now ; the moon was driven away, and the 
 month was lengthened ; the day was lengthened 
 also, but not so fast ; by slow degrees we 
 arrived at the present condition, with a lunar 
 month 27.3 days. Looking forward, we 
 are told that the day will grow longer more 
 rapidly than the month; that in the far 
 
62 ASTRONOMY 
 
 distant future the day and the month will be 
 again equal, at about fifty-five of our present 
 days; after which the moon will begin to 
 approach gradually nearer and nearer to the 
 earth until, if the heavens survive, her ultimate 
 fate is to return to the earth from which 
 perhaps she was formed in the beginnings of 
 our system. 
 
 The most familiar result of these intricate 
 tidal effects is that the moon turns the same 
 face constantly towards the earth not 
 absolutely, for her varying motion in an 
 elliptical path allows us to see occasionally a 
 few degrees one side or the other beyond the 
 usual limit. But, speaking roughly, we see 
 always the same face of the moon, and are 
 condemned to remain in ignorance of what 
 may be on the hidden side. Let us, however, 
 say at once that there is no probability that 
 the side we cannot see is anywise different 
 from the side which we can see. The 
 speculations which have given the far side 
 of the moon an atmosphere, water, vegetation, 
 and inhabitants, have no justification 
 whatever. When, after the lapse of millions 
 of years, the effects of tidal friction allow 
 future astronomers to examine at last the side 
 which is hidden from us, we may feel sure 
 
THE SUN AND MOON 63 
 
 that they will be disappointed if they expect 
 to see anything more than the volcanic 
 wilderness which the moon's near face presents 
 to us. No trace of atmosphere survives upon 
 the moon ; a star passing behind the moon 
 moves steadily up to the moon's limb without 
 dimming or refraction, and at the limb, 
 especially if it be the dark limb, the star 
 goes out with a suddenness which never fails 
 to surprise the observer. No trace of cloud, 
 no trace of the action of water in any form, 
 can be descried now upon the moon's surface. 
 No twilight mitigates the hardness of the 
 black shadows cast in the intense sunlight, 
 and it is not perfectly certain that any change 
 has ever been observed in the exquisite 
 detail w r hich even a small telescope will show. 
 In the opinion of one who has spent many 
 Saturday evenings in showing celestial objects 
 to members of a University, the moon is* the 
 one object in the sky which may be shown 
 to anybody without fear of disappointment. 
 The exaggerated ideas which are current 
 as to the power of a big telescope ; the 
 expectations that any casual observer should 
 see as much as is depicted in the finest astro- 
 nomical drawings or photographs, make it a 
 matter of some uncertainty whether any 
 
64 ASTRONOMY 
 
 other object will please or disappoint. The 
 moon alone among telescopic objects looks 
 immeasurably better in the telescope than 
 any published representation ; her beautiful 
 silvery colour, the exquisite sharpness of the 
 shadows on plain or crater floor, the brilliance 
 of the mountain peaks as they catch the first 
 rays of the rising or the last of the setting 
 sun, cannot be reproduced upon paper. 
 
 The many excellent published photographs 
 of the moon give, however, a perfect idea of 
 the moon's features, if not of her beauty. 
 Those features are in great part volcanic 
 or so we are prone to believe. The volcanic 
 features are indeed upon a scale which is 
 immense compared with the size of the moon, 
 and they differ in some important respects 
 from the generality of volcanic craters upon 
 the earth. The perfect development of the 
 central cone, the clean cut which one crater 
 will make into the wall of a neighbour, and 
 the immensely long, bright, straight streaks 
 which radiate from some of the principal 
 craters, are features to which the earth's 
 volcanic regions afford no very perfect 
 counterpart. In considering how far such 
 differences are significant we must remember 
 the lesser power of gravity upon the moon, 
 
THE SUN AND MOON 65 
 
 the absence of atmospheric denudation, 
 perhaps also the absence of the steam which 
 plays such an important part in volcanic 
 action upon earth. Interesting and suggestive 
 as are the theories which would explain lunar 
 craters as due to the splash of meteorites, or 
 the bursting of bubbles, it does not appear 
 that there is much justification for refusing 
 to believe our eyes, when they tell us so 
 plainly that the moon is volcanic but that 
 the volcanoes have long been dead and cold. 
 For the remarkable difference between 
 the earth and the moon, or, so far as we 
 can tell, between the moon and other 
 satellites, no reasonable explanation has been 
 given. The problem is a curious one when 
 we regard the moon as a satellite of our 
 planet earth. It is no less curious if we 
 regard the earth and moon as twin planets 
 moving together about the sun. One analogy 
 may perhaps be mentioned here, though 
 we must be careful not to press it far. Of 
 late years we have become familiar with 
 the fact that quite a large proportion of the 
 stars are twin stars, and that the two members 
 of the pair are often surprisingly different 
 in their physical state. Is it perhaps possible 
 that when one body separates into two 
 
66 ASTRONOMY 
 
 as we suspect has happened with the earth- 
 moon system, and in the binary stars there 
 is a considerable sorting out of materials, so 
 that the two bodies which are produced 
 differ from one another very much in 
 composition and in character ? It may be 
 so, though we cannot pretend to under- 
 stand the mechanism of the process. And 
 unfortunately nothing is happening now 
 upon the moon's surface, to give us any clue 
 as to the processes which have gone on in 
 the past 
 
 To revert for a moment to our consideration 
 of the sun : we were able to regard him as 
 a laboratory in which we could study, though 
 at a distance, the performance of experiments 
 in physics upon a vast scale. The moon we 
 must liken rather to a closed museum case 
 in which the past is so to speak crystallized, 
 hard, brilliant, and exquisite, but unchang- 
 ing, and lacking, therefore, the fascination 
 that belongs to continual change and 
 reconstruction. 
 
CHAPTER III 
 
 THE PLANETS AND THEIR SATELLITES 
 
 FOR many people there is just one question 
 in astronomy : Is there life upon Mars ? 
 The possibility that astronomers may find 
 in some other planet traces of intelligent life 
 has exercised a fascination that inspires the 
 novelist, the dramatist, and above all the 
 journalist So the world has become quite 
 familiar with the idea that something or other 
 has been found upon Mars which seems to 
 prove the presence of intelligent inhabitants. 
 For Mars, at any rate, the question is con- 
 sidered almost settled : why else did a 
 generous benefactor leave to the 'Paris 
 Academy of Sciences a magnificent prize, to be 
 given to the man who first succeeds in 
 establishing communication with a planet 
 other than Mar*. 
 
 A rapid survey of the planets will show us 
 
 why it is always Mars that is chosen for the 
 
 abode of life. Mercury is very near the sun ; 
 
 Mercury, if he has an atmosphere, which 
 
 C2 67 
 
68 ASTRONOMY 
 
 seems doubtful, must be very hot ; and if 
 there is no atmosphere can hardly be reckoned 
 habitable. Moreover, Mercury is small and 
 never gets very far away from the sun, and 
 so is difficult to observe ; and it is not certain 
 that any definite markings have ever been 
 seen upon his surface. Now, for any 
 proof of habitation, definite markings 
 are indispensable. Mere habitability as an 
 abstract quality has little attraction for those 
 whose interest lies in a search for evidence that 
 life actually exists. And if we ask ourselves 
 what kind of evidence would prove, or at any 
 rate strongly suggest actual habitation, the 
 answer seems to be that we must see something 
 that looks unnatural, but can be interpreted 
 with some reason as artificial. We must look, 
 in fact, for evidences of intelligent engineering 
 upon a gigantic scale. 
 
 From this point of view Mercury is hopeless ; 
 and the case is no better with Venus, though 
 Venus is nearly the same size as the earth, 
 is not impossibly near the sun, and has a 
 large cloud-laden atmosphere that might 
 very well make life as we know it tolerable 
 there. Unfortunately it is just this atmos- 
 phere that makes it impossible for us ever to 
 see down to the surface of the planet. None 
 
THE PLANETS 69 
 
 but the most vague and indefinite markings 
 are visible upon the disc of Venus : so vague 
 indeed are they, that it is not yet quite 
 certain what is the period of rotation of the 
 planet, though curiously enough the doubt 
 lies between 24 hours and 227 days. So 
 long a day as 227 of ours might well seem to 
 make the planet undesirable, even though 
 other conditions were not very dissimilar to 
 those upon the earth. But in any case we 
 are debarred from a sight of the surface, and 
 from any possibility of detecting what may 
 lie upon it. Hence, in our search for 
 inhabitants, Venus must be passed by. 
 
 Proceeding outwards beyond the orbit of 
 the earth we come to Mars, and then to the 
 zone of the minor planets. These small 
 fragments of worlds have an interest all 
 their own, but it lies remote from possibilities 
 of habitation. Making up in number 'what 
 they lack in size, the minor planets are at 
 once an interest and burden objects of 
 that sporting instinct which prefers always 
 to discover some body unimportant but new, 
 rather than to work soberly at the known 
 bodies which are from their great number an 
 incubus to those whose duty it is to keep 
 track of them, and to predict their places 
 
70 ASTRONOMY 
 
 so that they may be observed. The time 
 has long gone by for gratitude at the discovery 
 of a new minor planet. Very early in their 
 history they broke down the rule that every 
 planet must have a symbol of its own or, 
 at any rate, they reduced the symbol to the 
 simple form of a number enclosed in a circle ; 
 and the day is not far distant when those 
 numbers will run into four figures, and the 
 circles will have to be elongated to hold them. 
 Next, the increase in their numbers made it 
 very difficult to maintain the rule that 
 every minor planet should have a female 
 name drawn from classical mythology. The 
 selection of the name Victoria led to a heated 
 argument with staunch republicans, till a 
 learned editor pointed out that Victoria 
 was a daughter of Pallas, adding the delightful 
 note : " Pallas, a giant not the goddess, 
 who is believed to have left no children." 
 Later, the classical dictionaries became 
 exhausted ; then came the names of 
 sweethearts and girl babies ; afterwards 
 Alleghennia, Pittsburghia, and Chicago ; and 
 finally, the naming became so burdensome 
 that discoverers neglected it altogether, and 
 the German commission who look after such 
 matters made the rash threat that if a 
 
THE PLANETS 71 
 
 discoverer failed to name his discovery within 
 a given time, they would do it for him. 
 
 There are many fascinating problems 
 connected with the minor planets, but they 
 lie on the mathematical side ; for these 
 planets are all so small that only three or 
 four have measurable discs. The biggest 
 are only a few hundred miles in diameter, 
 and if we look for places wliere we may hope 
 to find intelligent life, they are negligible. 
 Their mathematical interest may be indicated 
 very briefly. First, there was the question 
 whether they were the result of the explosion 
 of a large planet ; that seems to have been 
 settled definitely in the negative. Then 
 came the question, why are they arranged in 
 zones, with considerable gaps between the 
 zones, and the answer to that is, that the 
 gaps have been formed by the disturbing 
 action of Jupiter and Saturn. Finally, of 
 late years it has been found that some of 
 these small bodies are so arranged that 
 they provide beautiful examples of certain 
 celebrated theorems in the problem of the 
 motion of three bodies under their mutua 
 attractions ; but it would take us too far to 
 cast even a glimpse into this highly specialized 
 field of enquiry. 
 
72 ASTRONOMY 
 
 One of them, the planet Eros, we must 
 mention for its great importance as an aid in 
 determining the distance of the sun, as we 
 shall see in Chapter VI. But Eros is an 
 exceptional body, with an orbit considerably 
 inclined to the ecliptic, of decidedly elliptical 
 form, interlacing the orbit of Mars, and on 
 occasion approaching the earth within fifteen 
 million miles. Perhaps we should not think of 
 Eros as a minor planet at all, but as a small 
 planet, the only one of its class at present 
 known, which, like the new satellites of 
 Jupiter and Saturn, seems to have been 
 reserved for the purpose of upsetting pre- 
 conceived notions at the beginning of the 
 twentieth century. 
 
 Passing on now to Jupiter, the largest of our 
 sun's family, we perceive at once that as we 
 cross the zone of the minor planets, we cross 
 into a region wiiere the planets are no longer 
 terrestrial bodies, in the sense that they are 
 something like our own Earth, but are 
 much larger, much lighter, almost certainly 
 much hotter, and seem to belong altogether to 
 a more primitive age, such as that which the 
 smaller planets doubtless passed through 
 long ago The surface of the planet is 
 obscured by strongly marked belts of cloud, 
 
THE PLANETS 73 
 
 flecked with bright and dark patches which 
 change from month to month ; and from 
 year to year the belts change also, a single 
 belt becoming double, or a pair coalescing to 
 form one. At first sight it may be thought 
 that this appearance is very much like what 
 the earth must present from without, with 
 its equatorial belt of clouds and its trade 
 wind zones. But the cloud belts upon the 
 earth, and consequently its appearance from 
 without, vary with the seasons and repeat 
 themselves pretty exactly year after year ; 
 while upon Jupiter there is little appearance 
 of season, and the changes do not appear to 
 repeat themselves after the lapse of Jupiter's 
 year. We are forced to conclude that Jupiter 
 is in a condition very different from that of 
 planets which we may regard as habitable. 
 If Jupiter has a solid surface, which is not 
 altogether probable, it must be sunk so deep 
 in the cloud envelope that the rays of the 
 sun can never penetrate there, and we cannot 
 regard it as fitted at present for life anything 
 like our own. 
 
 As an abode of life the planet Saturn 
 seems to be in worse case than Jupiter. The 
 mean density of the visible globe of Saturn is 
 less than that of water, from which we may, 
 
74 ASTRONOMY 
 
 of course, conclude with certainty that we 
 see nothing of any solid nucleus, which, if it 
 exists, must lie far within the cloud envelope 
 and can hardly be reached by the rays of the 
 sun. It is idle, therefore, to speculate how the 
 rings which encircle the planet, and the nine 
 or ten satellites which accompany it, may 
 ook to the inhabitants on the planet's 
 surface. These splendid appurtenances are 
 thrown away upon an uninhabitable world. 
 
 Proceeding still farther outwards w r e come 
 to Uranus, and then to Neptune, of which, 
 as planets, there is little more to say than 
 that they are large, each about 30,000 miles 
 in diameter ; that they are light in substance 
 like Jupiter and Saturn ; and that they 
 show no detail upon their tiny discs. Neptune 
 in a large telescope is of a fine green colour, 
 which makes him readily distinguishable 
 from the surrounding stars, and the spectro- 
 scope shows that both he and Uranus have 
 large and dense atmospheres, that absorb 
 some part of the sunlight falling upon them, 
 before it is reflected back to the earth. More 
 than that we cannot say of these planets on 
 the boundary of our solar system. 
 
 The result of our review is, then, that no 
 planet excepting Mars affords us any oppor- 
 
THE PLANETS 75 
 
 tunity of scrutinizing its surface so closely 
 that we may hope to find there any evidences 
 of intelligent life. We may consider it 
 very fortunate, therefore, that Mars whose 
 position next outside the earth's orbit allows 
 us to study his surface to greatest advantage 
 is the one planet whose real surface seems 
 to be open to our examination. 
 
 At first sight Mars is by no means unlike 
 what we may believe the earth would look 
 like if we could study her from Venus. Mars 
 is smaller, indeed, than the earth, with a 
 diameter not much more than half as great ; 
 but still Mars is amply large enough to be a 
 comfortably habitable globe. And though 
 the force of gravitation would be less there 
 than upon the earth, and a man suddenly 
 transport d there would feel that his size 
 and his strength were out of proportion to his 
 weight, nevertheless we need not imagine 
 that he would require any immense structural 
 alteration to be adapted to the new conditions. 
 Mars has an atmosphere not, indeed, so 
 dense as that of the earth, nor so laden with 
 clouds and moisture but still an atmosphere 
 which can carry some amount of cloud, and 
 which is not altogether too thin, we may 
 believe, to allow of the existence of beings 
 
76 ASTRONOMY 
 
 something like ourselves. In estimating such 
 probabilities we must always remember how 
 extraordinarily adaptable our own organisms 
 are. To a native of the Congo the life which 
 the Esquimaux 4ead might very well seem 
 impossible ; still more is it remarkable how 
 on the expedition to I^hasa the British and 
 Indian troops were able to pass almost with 
 impunity from the heat of India through the 
 terrible cold of the passes into Thibet. 
 
 Mars has polar caps not indeed so large as 
 the polar caps of the earth, nor so permanent 
 but still caps of brilliant white which form 
 in the winter and melt in the spring very 
 much as if they were formed of snow or ice. 
 Their existence shows that a circulation 
 takes place in the Martian atmosphere as in 
 our own, and we shall not be straining 
 probability too much if we assume that they 
 really are composed of water frozen into 
 ice or snow. In saying this we do not over- 
 look the arguments which have been put 
 forward to prove that water vapour cannot 
 exist in the Martian atmosphere ; and the 
 suggestion that the polar caps of Mars may 
 be made of frozen carbonic acid gas. The 
 possibility of the escape of gases at the limits 
 of a planet's atmosphere has been held, in 
 
THE PLANETS 77 
 
 recent years, to account for the absence 
 from our atmosphere of the rare gas helium, 
 which is given off by certain minerals and 
 mineral springs. While no one J doubts that 
 a certain quantity of gas may thus escape, it 
 is by no means certain whether the process 
 is effectively rapid or ineffectively slow. 
 And we may very well hold it as still uncertain 
 whether water vapour, which cannot escape 
 at all from our own atmosphere, can escape 
 so effectively, and in such quantity from the 
 atmosphere of Mars, that that atmosphere is 
 practically drained of moisture. 
 
 Until something more than thirty years 
 ago the drawings of Mars which were made 
 at the telescope showed a distribution of 
 light reddish areas, taken for land, and of 
 greenish areas, which were supposed to be 
 seas ; and though the relative distribution 
 of land and sea was a good deal different in 
 its character from ours, there was no difficulty 
 in accepting it as a reasonable arrangement 
 which might serve very well the parts of 
 water and land upon the earth. At that 
 time no one had heard of the canals. 
 
 The canals of Mars were discovered by the 
 Italian astronomer Schiaparelli at the very 
 favourable opposition of the planet in 1877 
 
78 ASTRONOMY 
 
 when its two satellites were also found. Their 
 discoverer was struck by their narrow 
 straightness, quite unlike any features that 
 had been seen before upon the planet, and he 
 almost at once considered the possibility that 
 these features might be artificial an hypothe- 
 sis, he affirmed, which is in no way impossible. 
 For many years, however, they remained 
 invisible to all but Schiaparelli ; then gradually, 
 with increasing skill, perhaps with increasing 
 desire to see them, other observers succeeded ; 
 and now it is only occasionally that a 
 drawing of Mars is published which is not 
 covered with a network of fine black lines, 
 almost as conspicuous as the hands of a watch 
 upon the dial. No one looking at a modern 
 drawing of Mars, and comparing it with an 
 old one, would be disposed at first to believe 
 that the two drawings could possibly represent 
 the same object. And it is this striking 
 dissimilarity between the old drawings and 
 the new which leads sceptics to deny even 
 now the existence of the canals, and to affirm 
 that they are visible only in virtue of a 
 contagious enthusiasm and almost hypnotic 
 suggestion. If that explanation fails, then 
 they are ascribed to an alleged natural tend- 
 ency for an observer to represent in the form 
 
THE PLANETS 79 
 
 of hard dark lines any fortuitous arrange- 
 ment of minute and intricate detail very 
 imperfectly seen. 
 
 We may say at once that it seems impossible 
 to maintain this line of argument. The canals 
 have been observed by many independent 
 observers, who are positive that they are not 
 deceived ; they admit the difficulty of the 
 observation, but maintain that at the rare 
 moments when the air is really steady the 
 canals come out clean and sharp. 
 
 They say, and apparently with justice, that 
 critics who have never undergone the long 
 training in observation, and especially critics 
 who have never spent weeks at the telescope 
 waiting for the occasional moment when a 
 really fine climate gives really good seeing 
 those critics have no right to say what can 
 or cannot be seen upon the disc of Mars. 
 The casual visitor who asks for a glimpse of 
 Mars is invariably disappointed. He expects 
 to see the planet looking at least a little like 
 the spider webs of canals which figure so 
 conspicuously in the published drawings. 
 He is disappointed, and he thinks poorly of the 
 telescope. The wise astronomer does not, 
 if he can help it, allow visitors to look at Mars 
 at all. For the casual visitor never realizes 
 
80 ASTRONOMY 
 
 that the published drawing represents the 
 very most that can be seen at the most 
 exceptional moment, and that the observer 
 himself, with all his training, cannot see five 
 per cent, of the whole for ninety-nine per cent, 
 of the time. 
 
 We do not propose, then, to enter upon 
 the thorny question, whether the published 
 drawings of Mars are representations of genuine 
 detail or are untrustworthy guesses at 
 something which really lies beyond the range 
 of accurate visibility. It is far better to 
 admit the whole of the evidence, and to see 
 whether with that the advocates for life on 
 Mars can prove their case. 
 
 To do so they must establish two propo- 
 sitions : 
 
 1. That the canals are not natural. 
 
 2. That if they are artificial they can be 
 supposed to serve a useful purpose, a specific 
 purpose, which we can believe is valuable or 
 necessary to intelligent inhabitants. 
 
 Now it is impossible to lay down hard and 
 fast rules for distinguishing the artificial 
 from the natural. A graceful piece of 
 engineering which fits its surroundings, such 
 as the railway bridge below the Victoria Falls, 
 looks almost natural ; a natural crystal 
 
THE PLANETS 81 
 
 may look exceedingly artificial. Hence to 
 establish the first proposition must involve 
 what is really an appeal to reasoned faith. 
 If a man will not believe that a certain 
 pattern is artificial, it is hard to make him 
 do so ; the most one can do is to point out 
 that if it is artificial, it has a very natural 
 interpretation. 
 
 The arguments, as they now stand, for 
 asking the world to believe that Mars is 
 inhabited, must always be associated with 
 the name of Professor Percival Lowell. He 
 built his observatory in the hope of proving 
 his point, and on the observations which pro- 
 ceeded from it he has constructed an argu- 
 ment of high interest. His chief observations 
 are these : The polar caps melt in the spring 
 and a broad belt of dark colour surrounds the 
 place where they have been. At that time 
 the canals are invisible, but they soonbegin 
 to appear, and they appear in order, first those 
 near the poles, then those in temperate, then 
 those in equatorial regions. As Lowell puts it, 
 with advancing spring in either hemisphere a 
 wave of seasonal change sweeps down from the 
 pole to the equator. 
 
 The second fact is that there are canals 
 in the dark areas which were formerly supposed 
 
82 ASTRONOMY 
 
 to be seas. In the summer they are not 
 visible ; but in autumn the dark greenish 
 areas begin to turn brown perhaps to dry 
 up and then it is seen that they are crossed 
 by narrow dark lanes which remain dark when 
 the regions they traverse have turned brown. 
 
 Further, it is assumed by Lowell, as a 
 general principle, that planets tend to lose 
 their water and to dry up. We do not think 
 that this principle is accepted by geologists, 
 nor does the evidence which Lowell brings 
 forward convince us that he is right. The 
 fact that certain districts of the earth, once 
 well watered, are now dry desert, proves 
 nothing. England in Silurian times was a 
 dry desert swept by sandstorms, as the 
 polished granite boulders testify, and since 
 then England has been submerged a few times 
 and is now pleasantly watered. However, 
 not to raise objections on the threshold, we 
 will admit that Mars may be drying up, losing 
 water at the top of his atmosphere or absorb- 
 ing it into his interior ; and we will see how 
 the explanation proceeds. 
 
 Mars, we are asked to believe, is now 
 badly watered, and the inhabitants are put 
 to great straits to live. Their atmosphere 
 is thin ; they have little or no cloud or rain ; 
 
THE PLANETS 83 
 
 they have only one source of water supply 
 which can be utilized, and that is the flood 
 of water released by the melting of the 
 polar cap. The way in which the canals 
 appear successively from the polar regions 
 towards the equator is then quite simple. 
 No one supposes that we see actually the 
 canals themselves ; but we are invited to 
 believe that we see the effect of the flood, the 
 burst of vegetation which turns the dusty 
 fields to a deep green colour as the water is 
 led over them. Such an effect would no 
 doubt be visible to the inhabitants of Venus, 
 if they could watch the Nile flood spread 
 over the plains of lower Egypt. 
 
 The supposed proof of the existence of 
 Martian inhabitants depends wholly upon this 
 theory that the canals form a gigantic irriga- 
 tion system, preserving the people for a time 
 from the fate which threatened them when 
 their crops began to fail for want of water, 
 To the working out of this part of the theory 
 we must devote a little space ; and this is 
 the more necessary since Professor Lowell 
 has nowhere explained how his irrigation 
 system would work. Yet there are several 
 points which are, to say the least of it, 
 somewhat obscure. 
 
84 ASTRONOMY 
 
 In the first place, if water is carried every 
 spring from the pole to the equator s by the 
 canals, it must return in the winter by the 
 atmosphere ; yet there is no evidence of 
 sufficient cloud upon the planet to produce 
 a heavy snowfall. On the contrary, the detail 
 of the planet is but little obscured. Moreover, 
 the polar cap forms so late in the winter and 
 melts so early in the spring that it is 
 impossible to imagine it of any considerable 
 thickness, and very hard to believe that by its 
 melting it can supply water for half a world. 
 
 In the second place, it is very difficult to 
 believe in the possibility of carrying water 
 in canals three or four thousand miles long. 
 The waste by leakage and by evaporation 
 would be enormous. Or if one replies that 
 the water is carried in leak-proof covered 
 channels, the same difficulty remains when 
 the water is spread over the land. By hypo- 
 thesis the planet is becoming drier, and 
 has lost the greater part of its water by 
 absorption or escape. In the absence of 
 any evidence to the contrary, one must believe 
 that those processes are active still. How 
 then has the scanty supply which remains 
 been saved from the fate of the rest, during 
 the immense time which would have been 
 
THE PLANETS 85 
 
 occupied in constructing an irrigation system 
 over the whole Martian world ? No answer 
 to this important question has ever been 
 suggested. 
 
 Again, there is a difficulty with those dark 
 areas which were called seas, until Lowell 
 found that they also have their canals ; 
 these remain dark in the autumn, thin black 
 lines across the areas which, from being dark 
 and green, are becoming light and brown. 
 Why waste precious water in preserving 
 verdure at a time when things do not want 
 watering, but only heat and drought to ripen 
 them ? It seemed to the author that this was 
 a serious difficulty, until he realized that the 
 purpose of irrigation in Egypt is not what he 
 had imagined. The aim of the irrigation 
 engineer is to supply water in spring, when 
 neither river nor clouds give any, and by 
 this means make it possible to grow the-very 
 valuable cotton, for which the natural water 
 supply of Egypt is quite ill-arranged. There 
 is, then, one way out of the difficulty. If it 
 is alleged that the Martians have given up 
 growing rice and have taken to growing 
 cotton, the objector is silent. 
 
 He may, however, be quite sceptical of 
 the engineering merit of the canals, which do 
 
86 ASTRONOMY 
 
 not run down the meridians from the poles, 
 but in many conspicuous cases prefer to go 
 at a long slant towards the equator. Then 
 comes the question of the dark spots at the 
 junctions, which give the system an appearance 
 like the conventional railway map. What 
 are these round dark areas at the junctions ? 
 We are invited to believe that they are the 
 naturally fertile regions whither people are 
 attracted, and where they make their homes. 
 Why then should they lie ten or a dozen in a 
 direct line ? 
 
 In fact, when one begins to examine the 
 canal system as a working irrigation scheme 
 one finds all kinds of features which are not 
 to be explained as reasonably artificial. The 
 works of man upon the earth are so restricted 
 by the conditions which nature imposes 
 that they must follow natural lines, and from 
 a little distance they generally lose their 
 crudely artificial look. But we are asked 
 to believe that on Mars no natural 
 obstacles have stood in the way of a per- 
 fectly geometrical design. Artificiality has 
 grown up unchecked by natural restrictions. 
 
 To cut short an argument which may 
 proceed too long, we must conclude that we 
 do not, with the best intentions in the world, 
 
THE PLANETS 87 
 
 understand how the irrigation scheme would 
 work " Very well," say its advocates, 
 " that proves only that the Martians are more 
 intelligent than you are ! " But we are loth 
 to admit that, as the final conclusion of the 
 whole matter, and so we can say only that 
 there is as yet no proof at all of the actual 
 existence of intelligent life on any world 
 but ours. 
 
 To turn now to another part of our subject 
 we must say a few words on the possibility 
 whether great planets may exist, beyond 
 the orbit of Neptune, but undiscovered by 
 reason of their distance and faintness. It is 
 not surprising that the romantic story of 
 the discovery of Neptune should have fired 
 other astronomers to emulate the achievement 
 of Leverrier and Adams, who calculated the 
 position in which Neptune was to be found? 
 from the existence of unexplained irregularities 
 in the motion of Uranus. Naturally enough 
 the motion of Neptune has been scrutinized, 
 in order to see whether there are traces in it 
 of disturbance by some planet yet further 
 away, and more than one search has been 
 instituted, on more or less plausible grounds. 
 Up to the present these attempts have met 
 with no success, and it seems likely that they 
 
88 ASTRONOMY 
 
 are premature ; for Neptune has not yet 
 completed half a revolution in his orbit, since 
 his discovery in 1846. Another, and appar- 
 ently for the time being more promising 
 chance, is to look for further irregularities 
 in the motion of Uranus, left unexplained 
 after a careful examination of all that Neptune 
 can do ; but this, like the other, has led as 
 yet to no result. 
 
 If, however, an extra-Neptunian planet 
 exists, it is highly probable that it will be 
 discovered some day, not by any kind of 
 mathematical prediction, but by direct com- 
 parison of two photographs. The chart of 
 the heavens which is now under construction 
 by a co-operation of observatories is supposed 
 to show all stars down to the 14th magnitude, 
 and the plates overlap so that every region of 
 the sky is shown twice. When adjacent 
 overlapping plates are compared, it is not 
 infrequent to find an object shown upon one 
 which is not upon the other. Generally it is a 
 variable star ; sometimes it is one of the minor 
 planets, and if the plates have been taken 
 at only a short interval of time, the planet 
 which has gone from one place can be found 
 in another on the same plate. More generally 
 the interval is too long, it has travelled too 
 
THE PLANETS 89 
 
 far, and wants a deal of finding. The motion 
 of an extra-Neptunian planet must be, how- 
 ever, so slow that it might not be so hard to 
 recover as a minor planet is. 
 
 The search has, so far, been conducted in 
 and around the ecliptic ; but recent discoveries 
 of new satellites to Jupiter and Saturn suggest 
 very forcibly that the outer bodies belonging 
 to a system may deviate very greatly from 
 the rules which govern the inner. Quite the 
 most unexpected discovery of recent years 
 has been the finding of very distant faint 
 satellites whose paths are very excentric, 
 very much inclined to the plane of the planet's 
 equator and of the inner satellites, and in 
 two cases are pursued in a retrograde direction. 
 That all the planets revolved in the same 
 direction round the sun, and each family of 
 satellites in the same direction round their 
 respective primaries, was supposed, until a 
 few years ago, to be one of the canons 
 of astronomy, inevitable according to the 
 nebular hypothesis, and necessary to the 
 stability of the solar system. In the year 
 1905 the discovery that a new distant satellite 
 of Saturn, named Phoebe, moving in a direc- 
 tion opposite to that of the others, excited 
 great enthusiasm. The satellite had been 
 
90 ASTRONOMY 
 
 announced by Professor W. H. Pickering, of 
 Harvard, some years before. Nothing further 
 had been heard of it, and its very existence 
 became doubted, when an English astronomer 
 visiting Harvard brought back the news that 
 a great surprise was in store. It came very 
 quickly in the announcement that Phoebe's 
 motion was retrograde. About the same time 
 Dr. Perrine, at the Lick Observatory, dis- 
 covered first one, and then another, new 
 satellite of Jupiter, lying in paths far outside 
 the older ones, highly inclined, nearly equal, 
 and interlaced. They were put under 
 observation at Greenwich, and in February, 
 1908, Mr. Melotte, of the Royal Observatory 
 staff, discovered another, which, like Phoebe, 
 moved in a retrograde orbit. 
 
 These capital discoveries are due to 
 photography. The small bodies discovered 
 are barely visible in the largest telescopes, 
 and they lie so far from their planets that 
 they are outside the range of ordinary 
 observation. It is highly improbable that 
 they would ever have been found visually, 
 while to obtain photographs of them is 
 arduous in the extreme. But it is imperative 
 that every effort should be made to get 
 observed places of Jupiter's eighth satellite, 
 
THE PLANETS 91 
 
 for its motion offers the most curious and 
 fascinating problems. Of any other satellite 
 one may say that it moves in an ellipse, and 
 continues to revolve nearly in the same orbit 
 for ever. Small modifications will be caused 
 by the influence of the sun and the other 
 planets, and the other satellites, but these 
 changes are not large, and they balance one 
 another after a time ; the motion is said to be 
 stable, which means in effect that the planet 
 has an absolutely controlling influence over 
 its satellite. The eighth satellite of Jupiter 
 is so far away from him that this can scarcely 
 be true for it. We may say that the satellite 
 is always trying to get away from Jupiter 
 and move round the sun as a planet which is 
 true of all satellites but in this one case it 
 comes pretty near to succeeding. The result 
 of this is that the satellite describes a highly 
 irregular curve which does not repeat itself 
 in the least, and which defies all the usual 
 methods of calculation. Very fortunately 
 the remarkable methods which Cowell and 
 Crommelin had applied with so much success 
 to the motion of Halley's comet were all in 
 working order ready to tackle the new satellite, 
 which they have done with complete success. 
 Having spent so much of a limited space 
 
92 ASTRONOMY 
 
 on the new and doubly fascinating because 
 quite unexpected discoveries of these distant 
 satellites, we have but a few lines to give to the 
 old. The theory of their motions, like that 
 of our own satellite, is exquisitely complicated, 
 and for that reason hardly to be touched 
 upon here. As to their physical appearance, 
 there is not very much to be said. Only the 
 four satellites of Jupiter discovered by Galileo 
 are large enough to show any visible disc. 
 From the little that can be seen, it appears 
 that they are belted, and that they rotate 
 on their axes in the same time that they 
 revolve about Jupiter. Whether, like our 
 moon, they are barren and airless, has yet to 
 be determined. As objects for the smallest 
 telescope, these four satellites have an 
 unequalled interest. Their eclipses by the 
 shadow, their passages before and behind the 
 planet, their ever varying groupings, provide 
 for us, as they did for Galileo, an object- 
 lesson in celestial motions, which all may see 
 and which none should miss. There are 
 few to-day who would miss them deliberately, 
 lest they should be compelled to believe what 
 they are anxious not to believe. One can but 
 hope with Galileo that his obstinate opponents 
 saw them on their way to Heaven. 
 
CHAPTER IV 
 
 COMETS AND METEORS 
 
 A COMET, and an expected shower of meteors 
 have been responsible in recent years for 
 two great public disappointments. The 
 famous Leonid meteors, which in 1799, in 
 1833, and in 1866 provided displays of a 
 grandeur never forgotten by those privileged 
 to see them, were due to return in 1899 or 
 1900, and they failed to appear in any con- 
 siderable numbers. Halley's comet famous 
 in the history of the world, and not less 
 renowned in the particular history of astron- 
 omy was awaited with a degree of interest 
 without parallel in our recollection. In 
 Britain and in northern latitudes generally, it 
 proved a miserable object, and it was hard 
 to credit the accounts cabled from the south, 
 of the tail of the comet stretching from 
 horizon to zenith like a gigantic searchlight. 
 The great public interest which Halley's 
 comet aroused, the extraordinary precision 
 93 
 
94 ASTRONOMY 
 
 of the prediction of its return, and the 
 remarkable work done by Cowell and 
 Crommelin in tracing its past history, render 
 it inevitable that we should make this comet 
 our leading theme in a chapter written before 
 it has yet passed completely from our sight, 
 Halley's comet is unique. There are many 
 comets which have made a splendid appear- 
 ance, and have passed never to return, or to 
 return after so many thousand years that we 
 may as well say never. There are many 
 comets which return regularly to the sun at 
 intervals of comparatively few years ; but 
 they are faint and undistinguished things, 
 hardly, if at all, visible to the unaided eye, 
 and never likely to cause public excitement 
 or to be taken as heralds of war and the death 
 of kings. Halley's is the one comet which is 
 periodic, coming back again and again, and is 
 at the same time a magnificent object, a 
 comet of the first class, worthy to play the 
 spectacular part which it has in fact played 
 for more than two thousand years of history. 
 Halley, by whose name the comet is 
 known, was a man who, more than most, 
 deserves a biography which has not yet been 
 written. The scattered references to him 
 in the early history of the Royal Society 
 
COMETS AND METEORS 95 
 
 picture for us a man of excellent capacity, 
 energy, and common sense not a profound 
 philosopher, but one of those equally valuable 
 people who have the faculty of undertaking 
 the right thing and making a success of it. 
 At an early age he was sent in command of one 
 of his Majesty's ships on a long voyage to 
 study the deviation of the compass in different 
 parts of the w^orld. On his return he was 
 employed by the Royal Society to measure 
 an arc of meridian, for determining the size 
 of the earth. Later we find him Secretary of 
 the Royal Society, active in promoting enquiry 
 of all kinds, and particularly interested in 
 the problem which was then unsolved, what 
 causes a planet to describe an ellipse about 
 the sun. Halley made a journey to Cambridge 
 in the confidence that if anyone could throw 
 light upon the matter it would be Newton ; 
 to his delight he found upon arrival that 
 Newton was already in possession of the 
 theory. But that eminent philosopher does 
 not seem to have had any ambition to publish 
 his great discovery to the world. It was 
 Halley who urged upon him the importance of 
 writing the book which became the immortal 
 " Principia." It was Halley who persuaded 
 the Royal Society to publish the book, 
 
96 ASTRONOMY 
 
 Halley who paid for it out of his own pocket, 
 and Halley who occupied himself with 
 applying the new theory of gravitation to 
 calculating the orbits of comets, while Newton 
 had become Master of the Mint and was 
 refusing to "be dunned and teased by 
 foreigners about mathematical things, or to 
 be thought by our own people to be trifling 
 away my time about them, when I should 
 be about the King's business." 
 
 It must be remembered that when Halley 
 began his work, no one had the least idea that 
 a comet could reappear after a period of years. 
 Newton himself had actually scouted the 
 suggestion that a comet seen on one side oJ 
 the sun might be identical with another seen 
 some weeks later on the other side. Halley 
 calculated the paths of twenty-four comets 
 all that he could find sufficiently observed 
 and of these three moved in orbits which were 
 very nearly the same. Was it possible that 
 they were the same comet returning again 
 and again to the sun ? There was a grave 
 difficulty in believing it possible. The inter- 
 vals between the successive returns were not 
 equal. Between 1682, September 4, and 
 1607, October 16, was a period of 74 years 
 11 months : between the latter date and 1531, 
 
COMETS AND METEORS 97 
 
 August 24, was 76 years and 2 months. 
 Now, if a comet returned again and again 
 to the sun, it must be to all intents and 
 purposes a planet ; and if so, why did it 
 not perform its revolution in a regular time ? 
 In seeing the answer to this difficulty Halley 
 showed how keen was his intelligence. 
 Jupiter and Saturn disturb the revolutions of 
 one another to the extent of a few days only, 
 for they are massive bodies to perturb. A 
 comet is lighter and sensitive to disturbance, 
 easily captured and easily turned adrift ; 
 it might be the attractions of the planets 
 which caused the irregularities in its re- 
 appearance. Satisfied with this solution of 
 the difficulty, Halley staked his reputation 
 upon a prediction that the comet would 
 return again in 1758. He knew that he could 
 never live to see the truth of his prediction ; 
 but in a fine sentence, often quoted, he 
 expressed himself content with the faith that 
 impartial posterity would not refuse to 
 admit that this was first discovered by an 
 Englishman. 
 
 Two circumstances of the last return are 
 noteworthy. So accurate was the prediction 
 of Cowell and Crommelin, that the comet was 
 found within two seconds of arc of its predicted 
 
98 ASTRONOMY 
 
 place, on the first photograph which showed 
 it in 1909, August. It had made the most 
 rapid revolution on record. Between 1759, 
 March, and 1835, November, was 76 years 
 8 months. If it had traversed exactly the 
 same path again, it would not have been due 
 at perihelion until 1912, July ; whereas it 
 actually reached perihelion in 1910, April. 
 This acceleration of 27 months was explained 
 as the effect of a near approach to Jupiter in 
 1835. The disturbance then impressed upon 
 the path of the comet had little time in which 
 to affect the date of the 1835 return, but its 
 effect was felt throughout the succeeding 
 revolution, with the result that the comet 
 made its record passage, as we have said. 
 Its actual return to perihelion, as observed in 
 1910, differed by 2.7 days from the prediction 
 an insignificant error, one might suppose, 
 in so immense a prediction. But the authors 
 were not satisfied. They repeatedly checked 
 their work, and they are able to affirm that 
 if the theory of gravitation is rigorously true, 
 and no forces except gravitation act upon the 
 motion of the comet, then not more than a 
 fraction of a day can be attributed to defects 
 of calculation. The balance of some 2| days 
 can be explained only by the existence of 
 
COMETS AND METEORS 99 
 
 forces which are not pure gravitation. It is 
 not the first time, as we have seen, that a 
 reputation has been staked upon the comet. 
 We cannot but hope that the faith of Messrs. 
 Cowell and Crommelin will be justified as 
 fully as was the confidence of Halley in his 
 prediction. 
 
 The second circumstance which made the 
 last return of Halley 's comet noteworthy 
 was the passage of the comet across the plane 
 of the earth's orbit at a point exactly between 
 the earth and the sun. If the comet's head 
 were large and solid, we should have seen it 
 projected as a dark body against the sun ; 
 but nothing was seen ; nothing was photo- 
 graphed. If the comet's tail had been straight, 
 the earth would have passed through it, and 
 this probability gave rise to a quite remarkable 
 excitement. People who should have known 
 better predicted that we should be asphyxiated 
 by poisonous gases. Some cautious people 
 laid in bottles of oxygen to sustain life during 
 the fatal night. One or two committed 
 suicide to escape death from the comet. 
 And after all, nothing very much happened to 
 the earth, perhaps because the tail was not 
 straight. 
 
 There were in fact at least two tails. As 
 
 D 2 
 
100 ASTRONOMY 
 
 the time for the passage across the sun 
 approached, the comet in the morning sky, 
 before sunrise, became each day a more 
 wonderful object to those observers, south of 
 the Mediterranean, who had the good fortune 
 to see it in a clear dark sky. As the comet 
 closed up to the sun its tail grew longer till 
 it reached the zenith. The day of the transit 
 passed, and next evening the tail was seen, 
 as was expected, in the west. But the 
 following morning, quite unexpectedly, it 
 was still visible in the east. There must have 
 been two tails, and that night the earth was 
 between the two. 
 
 Before we turn to the discussion of other 
 comets, let us glance for a few moments at 
 some features of interest in the work of 
 tracing back into the remote past the history 
 of Halley's. We have seen already that the 
 comet returns after periods which vary by a 
 couple of years on each side of seventy- six 
 years. That makes it impossible to go back 
 by a simple series of subtractions, to say that 
 Halley's comet should have appeared in such 
 and such a year, and to search the chronicles 
 for records of a comet that will more or less 
 fit the date. We have, in fact, to perform 
 two distinct operations. First, from the 
 
COMETS AND METEORS 1G3 
 
 recorded path of a comet among the stars, 
 we must calculate its orbit and see if its 
 position can be reconciled with the path of 
 Halley's. Next, having made a preliminary 
 identification, we must examine whether the 
 actions of the planets can account for a return 
 at that particular date. For the first step 
 the European chronicles are useless. They 
 tell that a comet appeared, and they usually 
 attribute to it whatever disaster, pestilence, 
 or illustrious death took place about that 
 time. But they tell nothing of how the 
 comet moved among the stars ; they give us 
 no details by which we can calculate even 
 roughly what orbit the comet pursued. To 
 gain the information we want for this pur- 
 pose we must go to the Chinese annals, the 
 dynastic histories and the great encyclopaedias 
 
 of ancient wisdom. From these records Hind 
 
 
 
 was able to trace the appearances of comets 
 which might very well have been Halley's, 
 as far back as 11 B.C. It remained for Cowell 
 and Crommelin to examine whether these 
 dates were consistent with the calculated dura- 
 tion of each revolution, when perturbations 
 by the planets are taken into account. This 
 formidable task they completed with success, 
 showing that Hind's identifications were nearly 
 
102 ASTRONOMY 
 
 all correct that it was Halley's comet which 
 is represented in the famous panel of the 
 Bayeux Tapestry ; that it was Halley's comet 
 which is pictured in the Nuremberg Chronicle 
 for the year A.D. 684 ; that it appeared 
 about the time of the battle where Attila 
 the Hun was defeated by the Romans and the 
 Goths. Halley's comet was the sign in the 
 shape of a sword which hung over Jerusalem 
 before the destruction of the city by Titus ; 
 it hung over Rome in 11 B.C. before the death 
 of Agrippa. 
 
 Thus are comets remembered in history by 
 the disasters which are ascribed to them, 
 and the terror which they caused, when it 
 was supposed that they swept through the 
 upper air, almost within reach. By proving 
 that at any rate they are far beyond the moon. 
 Tycho Brahe rendered the world a service 
 for which mankind should be grateful those 
 who have not still a lurking dread of what 
 they fear a comet may bring. These were 
 the reflections proper to the year 1909, when 
 Halley's comet was approaching a quiet 
 satisfaction that we lived in enlightened 
 days. The year 1910 came ; in January the 
 people of Paris watched the so-called " Day- 
 light comet " reflected in the flood which 
 
COMETS AND METEORS 103 
 
 threatened to destroy their city ; in May, 
 while the people of London watched for 
 Halley's comet, the body of King Edward 
 the Seventh lay in state at Westminster. 
 What wonder that the imagination seizes 
 upon these deplorable coincidences, and the 
 fear of comets dies hard among us. 
 
 Leaving now the historical side, let us see 
 how far it is possible to understand what a 
 comet is, and how its extraordinary pheno- 
 mena are produced. The formation of the 
 tail was well shown in the comet of 1882, 
 which passed so close to the sun that it nearly 
 grazed the solar surface. In three and a half 
 hours the comet described 180 degrees of its 
 orbit. One morning the tail stretched a 
 hundred million miles in the east ; the same 
 evening it was a hundred million miles long 
 in the opposite direction. Now it is impossible 
 to suppose that the head of the comet swung 
 that tail round through J.80 in a few hours ; 
 it is certain that the comet discarded its old 
 tail and made a new one. With the advent 
 of photography it became possible to study 
 the tail in greater detail and more completely. 
 Bright patches in the tail, photographed on 
 one night, were replaced by other patches 
 on the following night, but it was impossible 
 
104 ASTRONOMY 
 
 to trace any detailed connection between one 
 series and the other. Very often a comet is 
 so near the sun that not more than one picture 
 can be obtained each night. But occasionally 
 a comet may be photographed several times 
 in one night, and then it becomes possible to 
 see what is really happening in the tail. The 
 bright patches are travelling very fast along 
 the tail, in the direction away from the sun. 
 They tell us that the whole tail is being renewed 
 continually by some influence proceeding 
 from the sun, and that every comet's tail is 
 practically new each day. 
 
 To explain such a surprising fact is not 
 easy, but we have now at least two possible 
 explanations, whereas a few years ago one 
 could say only vaguely " electric repulsion," 
 which did not work out very well. Nowadays 
 one can speak with some confidence of the 
 part which the pressure of light may play 
 in the formation of comets' tails, for did not 
 an American professor come to Cambridge 
 with a comet's tail sealed up in a glass tube ? 
 By a beautiful piece of experimenting Messrs. 
 Nichols and Hull were able to show in actual 
 operation the process that Maxwell had seen 
 with the mathematical eye, thirty or forty 
 years before. The experiment was this : the 
 
COMETS AND METEORS 105 
 
 finest sand, and a very fine light vegetable 
 powder, lycopodium, were mixed, and put 
 into the top compartment of a tube like an 
 hour-glass, which was then exhausted of air. 
 Through a small aperture the stream of 
 powder could trickle into the lower 
 compartment of the glass, across which a 
 concentrated beam of light was passed. 
 The dense, comparatively large grains of sand 
 dropped through without deflection, but the 
 smaller and lighter lycopodium was swept 
 against the side of the tube by the pressure 
 of the light falling on it. 
 
 Now it may be that a comet's tail is formed 
 in this way. Light pressure would account 
 for the fact that the comet's tail is always 
 turned away from the sun, the powerful 
 source of light. As the comet approaches, the 
 tail streams behind ; as the comet recedes, 
 the tail goes before ; so that evidently the 
 source of repulsion lies in the sun. But light 
 pressure does not explain how the comet's 
 nucleus produces the fine dust, in the first 
 instance a part of the question to which 
 perhaps insufficient attention has been given. 
 In the second place, light pressure does not 
 explain why or how the dust can remain 
 luminous during the hours in which it is 
 
106 ASTRONOMY 
 
 being rushed through space literally on the 
 wings of light. And thirdly, light pressure 
 does not fit in particularly well with the 
 measures which have been made of the actual 
 accelerations of bright patches along the 
 tail. In fact, the explanation by light pressure 
 does not work out in detail quite so well as 
 had been hoped. 
 
 The rival theory, which has some brilliant 
 possibilities, supposes that the comet's tail 
 does not really belong to the comet, but is 
 a local lighting up of matter which is 
 continually streaming out in all directions 
 from the sun it may be dust, or it may be 
 in the form of one of those subdivisions of 
 the formerly indivisible atom, of which 
 astronomers are as yet justly a little shy. 
 If it be dust, there is no difficulty about the 
 formation of the dust, for the neighbourhood 
 of the sun must be all smothered with 
 impalpable powder, the condensation of the 
 metallic vapours so frequently ejected. The 
 dust which is about the right size one-third 
 of the wave length of the light producing the 
 pressure must be sorted out and driven away. 
 Very probably at the start of its journey it 
 has something to do with the long streamers 
 of the corona. Then it ceases to be visible 
 
COMETS AND METEORS 107 
 
 unless something excites it, and here we 
 have the explanation of comets' tails the 
 exciting cause may be the passage through 
 the shoal of meteoric stones which is supposed 
 to constitute the comet's head. This theory 
 explains some things rather well but it 
 would seem to require that all bodies in the 
 solar system should have tails. In that 
 case, we are answered, no doubt the " Gegen- 
 schein " the faint patch of light opposite the 
 sun is the earth's tail, which we can faintly 
 see because we are looking along its length. 
 A weak part of both explanations is that 
 they do not help very well to explain the 
 fountain-like envelopes that are seen, in the 
 telescope, surrounding the bright nucleus 
 of a good comet. These envelopes were 
 particularly remarkable in the head of the 
 great comet of January, 1910. At first 
 sight they suggested very strongly that 
 fountains of brilliant particles were ejected 
 towards the sun from the nucleus, and 
 meeting the pressure of light were deflected 
 and turned back into the comet's tail. A very 
 complete study of this action has been made 
 by Eddington from the wonderful series of 
 photographs of Comet Morehouse (1908) 
 made at Greenwich with the thirty-inch 
 
108 ASTRONOMY 
 
 reflector. His conclusion is that the 
 explanation is certainly incomplete. Calcula- 
 tion shows that very variable and very large 
 light pressures are required, while we find no 
 sufficient explanation of the multiple envelopes. 
 In fact, with our present limited knowledge it 
 is difficult to make the theory work ; while 
 it is a serious embarrassment to remember 
 that light pressure has no effect upon gas, 
 but only upon solid particles ; for the evidence 
 of the spectroscope certainly seems to show 
 that the tails are gaseous. 
 
 Now let us turn to the composition of 
 comets. It is generally supposed that the 
 nucleus of a comet very much resembles a 
 swarm of meteorites. The connection between 
 comets and meteor streams is certain. Several 
 well known streams of meteors travel in orbits 
 which are identical with the orbits of known 
 comets. In one celebrated case a comet 
 has been seen to break up and disappear, and 
 afterwards a bright shower of meteors came 
 from the path in which the comet had moved. 
 Yet it may be putting the connection too 
 positively to state, as it has been stated, that 
 " Biela's comet was shedding over us the 
 products of its disintegration." There does 
 not seem to be any definite evidence that the 
 
COMETS AND METEORS 109 
 
 material which makes the comet's nucleus 
 is the same as the material which flashes into 
 the atmosphere of the earth and blazes for a 
 moment as a shooting star. Indeed the 
 spectroscope shows that the light proceeding 
 from the nucleus is generally, or perhaps 
 always, the light emitted by the vapours of 
 hydro-carbons ; while Fowler, in a most 
 successful series of experiments made recently 
 at South Kensington, has been able to produce 
 in the laboratory a spectrum exactly corre- 
 sponding to the spectra exhibited by many 
 comets' tails, and has been able to show that 
 it belongs in all probability to an oxide of 
 carbon. In brief, then, the light of comets 
 proceeds chiefly from certain compounds of 
 carbon ; and it cannot be said that we have as 
 yet any precise idea how these compounds are 
 carried by meteors in the comet's head, nor 
 yet why light pressure or any other form of 
 radiation from the sun should be capable of 
 lighting up the tail in so magnificent a way. 
 When a comet goes very near the sun, we can 
 imagine all kinds of violent actions upon it. 
 But many comets Halley's for example 
 do not approach the sun very much nearer 
 than does the earth. How then, by what 
 mechanism, by what internal energy, or by 
 
110 ASTRONOMY 
 
 what method of extracting energy from solar 
 radiation of any kind, does a comet shine 
 with so brilliant a light, when planets may 
 pass through such radiation without experi- 
 encing any very noticeable effects ? In 
 spite of all the work which has been done upon 
 comets in the last few years, it does not seem 
 that we are really much nearer to a solution 
 of this mystery. We want a brilliant comet 
 that shall be visible all night for several 
 months, so that there shall be some 
 opportunity of continuous work. It happens 
 too often that an astronomer gets an hour's 
 observation, under rather poor conditions, 
 with the comet low down in the vapours after 
 sunset or before sunrise. That hour's work 
 shows him what he should do next, suggests 
 the questions demanding solution. Then 
 perhaps there is a week of cloudy weather, 
 and the comet is gone. Comet interests are 
 put aside, other work is taken up, and 
 suddenly after ten years another comet 
 demands instant attention with everything 
 in complete order for its observation. Under 
 such circumstances progress is necessarily 
 slow. 
 
 We are speaking of brilliant comets, in 
 which it is possible to see some structure, and 
 
COMETS AND METEORS 111 
 
 which give light enough for analysis. The 
 ordinary small telescopic comet, of which 
 we have four or five a year, has little interest 
 except the interest of calculating its path, 
 finding whether it belongs to the solar system 
 or is a casual visitor, and watching to see 
 that it strictly obeys the law of gravitation. 
 That is to say, its interest is not physical, 
 but almost entirely geometrical. It may be 
 of interest to describe the history of a typical 
 telescopic comet, so far as we are concerned 
 with it. 
 
 A few astronomers devote clear nights 
 when the moon is absent to comet- seeking. 
 The more fortunate have an instrument 
 specially designed for the purpose, which 
 allows an observer to sweep broad areas of 
 sky without changing his position. He sweeps 
 and picks up nebulae, which must be identified 
 from the catalogues or observed until it is 
 certain that they are not moving among the 
 stars. After wasting very many hours with 
 nebulae, a fortunate observer discovers a 
 faint object, almost invisible, which is not 
 catalogued as a nebula, and which in the 
 course of an hour has moved perceptibly 
 among the stars. It is a comet. The 
 observations made at the beginning and end 
 
112 ASTRONOMY 
 
 of the hour serve to give the position of the 
 comet at the instant of the first observation, 
 and a first rough estimate of the daily motion. 
 A hasty search through the astronomical 
 journals shows that it is not a comet already 
 announced, or one whose return is expected. 
 It may be new, and it may have been 
 discovered by another observer the night 
 before. 
 
 The observer's duty is to communicate his 
 discovery to other astronomers as soon as 
 possible, that there may be no delay in getting 
 to work. A telegram is sent to Kiel, where 
 there is an organization for circulating 
 astronomical news without delay, to all 
 subscribers, in a special code designed for the 
 purpose. Should the discovery have been 
 made in the early hours of the night it is 
 actually possible to send the news to Kiel, 
 get out the code cable to America, and have 
 the comet observed that same evening at 
 Lick. Within a few hours, then, other 
 observations of the comet may be secured, 
 and cabled to Kiel, which sends out again 
 the earliest second observation received ; and 
 all the subscribers have now two places of the 
 comet available. Directly a third is observed 
 the comet's orbit can be calculated ; so there is 
 
COMETS AND METEORS 113 
 
 some competition to get the third observation. 
 When it is obtained the fortunate observer 
 should be prepared to call up all available 
 assistance and to proceed straightway with 
 the calculation of a parabolic orbit. Eight 
 or ten hours' continuous work will finish it, 
 and perhaps provide a little ephemeris or 
 prediction for the next few days. Then for 
 the first time it is possible to say what will 
 happen whether the comet shows any 
 promise of becoming brilliant, whether it 
 will be visible for long, and what constellations 
 it will traverse. The elements that is to 
 say the numbers which define its orbit and 
 the ephemeris the numbers which define its 
 apparent path are telegraphed to Kiel, and 
 sent out this time upon a " jellygraphed " 
 post-card. 
 
 Time passes ; the comet is passing beyond 
 our range of observation ; and there is 'some 
 competition to secure the last observation. 
 In a few months all the observations obtained 
 are published in the various astronomical 
 journals, and it is time for some one to 
 announce that he proposes to undertake a 
 definitive determination of the orbit a 
 determination, that is to say, which shall 
 take account of all the niceties which were 
 
114 ASTRONOMY 
 
 neglected in the first hurried calculations. 
 The definitive orbit tells us whether the comet 
 belongs to the family which have been 
 captured for the sun by Jupiter, and it 
 provides the means of rediscovering the 
 comet at its next return, perhaps some 
 five or six years afterwards. Before the next 
 return a " search ephemeris " is calculated 
 and published as a guide to rediscovery. 
 Again there is some emulation, to be the 
 first to pick up the faint object as it approaches 
 the sun and grows gradually brighter. 
 
 The interest in these faint comets is thus 
 almost entirely mathematical. They fur- 
 nish beautiful exercises in calculation and 
 admirable tests of the completeness of theory. 
 
 Once in a while a comet will be lost it 
 has made a near approach to Jupiter or 
 Saturn, and has been deflected violently from 
 its path : sent out of the system by a reversal 
 of the process which originally captured it 
 from outer space, or turned into a new path 
 having so little affinity with the old that only 
 a complicated calculation could show the 
 connection. Thus these faint comets afford 
 puzzles and problems which are some of the 
 most elegant in astronomy. In particular 
 they become beautifully intricate in such a 
 
COMETS AND METEORS 115 
 
 case as that of the celebrated comet Encke, 
 which approaches very close to the planet 
 Mercury, and suffers besides from an 
 acceleration to its motion, exceedingly diffi- 
 cult to explain especially since it has been 
 shown by the elaborate investigations of 
 Backlund that the disturbance has changed 
 its value suddenly three times in the last 
 century. But we have no space to deal with 
 the problem of Encke's comet. We must 
 pass to the meteor showers, with which, as 
 we have seen, comets are closely allied. 
 
 The point which strikes us first as we 
 consider the question of meteors, is that we see 
 only the final catastrophe, and know nothing 
 except by inference of the events that lead 
 up to it. A bright streak of light across the 
 sky, perhaps if the meteor is large a train of 
 sparks which persists for a few seconds, and 
 the career of the meteor is ended. It volatilizes 
 with the heat engendered by its rush into the 
 air, and eventually settles down upon earth 
 as the fine meteoric dust which has been 
 recognized in samples from the deep sea 
 bottom. One might think it hard to discover 
 very much from an observation so momen- 
 tary ; but by patient observation and 
 comparison of results it is possible to deduce 
 
116 ASTRONOMY 
 
 a good deal. The important fact to be 
 observed is the direction of flight, which must 
 be plotted on a star map or globe. As 
 tracks accumulate it is seen that they are 
 not at random. A good proportion of 
 those observed on any one night, or on 
 corresponding nights in different years will be 
 found to radiate from a single point or small 
 area. From this we draw the conclusion 
 that the meteors, before they collided with the 
 earth, had been moving in parallel paths, 
 coming towards us from that point of the 
 sky in which the radiant is situated. We 
 have now information as to the orbit of the 
 meteors : it passes through the point which 
 the earth occupied at the time of observation 
 which can be taken out of the tables 
 and it passes through that point in a certain 
 direction, given by our observations. With- 
 out going further into the details, we may 
 say that in some cases, though not in all, 
 it is now possible to calculate the actual 
 orbit of the meteors. Thus, Newton (of 
 New Haven, Conn., U.S.A.), in the middle of 
 last century, found that magnificent showers 
 of meteors were recorded in history at intervals 
 of 33 years, identified them with the shower of 
 Leonid meteors that is, meteors radiating 
 
COMETS AND METEORS 117 
 
 from the constellation Leo and predicted 
 their return in 1866. This prediction was a 
 brilliant success. An immense shower of 
 meteors was seen in November of that year, 
 and a scarcely inferior display in the following 
 November. Newton showed that there were 
 five possible orbits in which they might be 
 travelling ; Adams proved that the largest 
 orbit was the true one ; and Leverrier found 
 that this path took them so close to the 
 planet Uranus in the year A.D. 126, that it is 
 very probable that the whole shower was 
 captured by Uranus and introduced to the 
 solar system at that date. 
 
 Everything being thus completely worked 
 out, one expected with some confidence a 
 return of the great shower in 1899 or 1900. 
 As the time approached, however, some 
 calculations published by Johnstone-Stoney 
 and Downing suggested the probability that 
 the meteor stream might have been perturbed 
 so much by the action of Jupiter that its 
 path would no longer intersect the path of the 
 earth, and thus we should have no display. 
 It was impossible to be certain, for there was 
 no means of estimating the size of the swarm, 
 and the prediction of failure did not attract 
 as much attention as did the general expecta- 
 
118 ASTRONOMY 
 
 tion of a grand display. Hence, when the 
 shower provided no more than the usual 
 scattering of meteors seen every November, 
 there was great disappointment, and some 
 mockery of the science which claims to be 
 the most exact of all sciences. As to the 
 mockery, it is often forgotten that many 
 astronomical events cannot be predicted 
 with the certainty of an eclipse or an 
 occultation. They are expected, in the sense 
 that it advisable to look out for them ; 
 the expectation is, or should be, accompanied 
 by a clear statement of the certainty or the 
 probability of failure involved. Doubtless 
 the warning will be neglected ; the news- 
 papers will magnify the expectation, and 
 there will be disappointment, as there was 
 with Halley's comet and with the meteors. 
 All that the astronomical prophet can do 
 is to be moderate in his promises, and 
 cautious how he excites public interest. 
 Whether in these two cases he was not 
 indiscreetly confident is perhaps arguable. 
 
 In comet-seeking and in meteor-observing 
 there is a wide field for the enthusiastic 
 non-professional astronomer. Now that there 
 is no longer a substantial prize in money for 
 each comet discovered, it is not possible to 
 
COMETS AND METEORS 119 
 
 subsist upon comet discovery, as a very 
 distinguished astronomer did for some 
 time, in his amateur youth. But the 
 possibility of attaching one's name for ever 
 to a well remembered comet must stimulate 
 enthusiasm, and is in truth a very real 
 though unsubstantial reward. How much 
 was the fame of Donati increased by his 
 discovery of the comet which at first promised 
 badly, but manifested itself in the end as the 
 most beautiful of its kind. In a less ambitious 
 way, but one which offers more continuous 
 employment, the amateur observer, with no 
 equipment but a star map, can do very 
 valuable work in the observation of meteors. 
 There are still important matters to be settled, 
 and some doubts to be resolved. Take, for 
 example, the question of the " stationary " 
 radiant points. If a shower of meteors is 
 active on several successive nights, * the 
 radiant point should, in theory, shift a little 
 each night. The August meteors from Perseus 
 do show this shift. It is alleged by the best 
 observers that other showers do not, but that 
 their radiant points, in defiance of theory, 
 remain stationary. Now this is a question 
 which must be settled ; one of the principal 
 difficulties in so doing is to decide what 
 
120 ASTRONOMY 
 
 constitutes a radiant point. Two meteors 
 cannot, for their paths produced backwards 
 must meet in some point. A third passing 
 through the same point, suggests a real 
 radiant, but it is much more difficult to decide 
 if the third passes, say, two degrees away 
 from the intersection. How much may we 
 allow for errors of observation, and how 
 much for a possibly real diffuseness of the 
 radiation ? That involves questions which 
 are not easy to answer, and it is certain that 
 there is great scope for the patient observer 
 of shooting-stars, in elucidating this curious 
 difficulty. 
 
 One word more, as to the nature of meteors, 
 or shooting-stars. We have heard of the 
 novelist who made her hero notice the absence 
 of a star, and remember that he had seen one 
 fall the night before ! In Cambridge a 
 college porter, asked to be on the look-out in 
 1899, reported to a festive party of watchers 
 indoors that " they had none of them shot 
 yet, but some of them looked as if they were 
 just going to." We may laugh at this 
 confusion, and at the same time be surprised 
 at the minute size of the bodies which can 
 produce for the moment so brilliant an effect. 
 The ordinary shooting- star weighs not more 
 
COMETS AND METEORS 121 
 
 than a grain, but it enters the atmosphere 
 with a planetary velocity, and in a brief 
 instant its energy of motion is all changed by 
 friction of the air into energy of light and 
 heat. And bearing this in mind we may see 
 in each meteor an illustration of the principle 
 which we invoked to explain the maintenance 
 of the sun's heat, that arrest of motion means 
 the production of heat. It may produce 
 brilliant incandescence in a meteor if the 
 motion is swift and the stoppage sudden ; 
 and the effect is none the less sure in a body 
 contracting under the influence of its gravi- 
 tation, though the motion is slow and the 
 stoppage gradual. 
 
 Occasionally a meteor of large size, entering 
 the atmosphere slowly, because it is overtaking 
 the earth, will escape vaporization, and will 
 fall to the ground. It can then be examined 
 and analyzed, and in a great number of cases 
 this has been done. In no case has there 
 been any evidence of the presence of an 
 element not found in the crust of the earth. 
 Space then, if filled with flying fragments of 
 matter, is filled with quite ordinary materials. 
 Whether they have been thrown out in 
 primitive days from the volcanoes of the 
 moon, whether they come from the sun or 
 
122 ASTRONOMY 
 
 from the distant stars, or whether they are 
 merely fragments of stuff which had escaped 
 absorption into a larger body, and had 
 wandered indefinitely in space, they tell us 
 at any rate that we have in the earth most 
 if not all of the elements which occur through- 
 out the universe. 
 
CHAPTER V 
 
 MOVEMENT UNDER THE LAWS OF GRAVITY 
 
 THE claim of astronomy to be considered the 
 most exact of all the sciences rests, in popular 
 acceptation, upon the seemingly miraculous 
 way in which the astronomer makes 
 predictions which come true to the second. 
 At the moment of writing several parties of 
 astronomers are on their way to a small island 
 in the Pacific Ocean, to observe a total eclipse 
 of the sun. They know the moment when 
 the shadow of the moon will come sweeping 
 across the sea and strike them, and they know 
 for how many seconds it will envelop tliem. 
 If the eclipse were due a hundred years hence 
 instead of within a few weeks of the time of 
 writing, the prediction could be made with 
 almost equal accuracy ; while if it should by 
 chance be a matter of historical interest to 
 determine whether an eclipse of the sun were 
 visible in that island two thousand years ago 
 it could be done. 
 
 123 
 
124 ASTRONOMY 
 
 Now let us consider for a moment what this 
 involves. The moon is moving round the 
 earth, and both together are moving round 
 the sun. At each instant the motion of the 
 moon, which were it not for the sun would be 
 very nearly a simple elliptic motion round the 
 earth, or were it not for the earth would be 
 very nearly a simple elliptic motion round the 
 sun, is distracted by the competing attractions 
 of both, and by the feebler but by no means 
 negligible disturbances produced by the 
 attractions of the planets. The task of the 
 mathematical astronomer has been this, that 
 starting with the simple Newtonian law of 
 gravitation every particle in the universe 
 other particle with a force 
 
 depending directly upon thejproduct o the 
 masses of those particles, 
 square of the distance between them he has 
 to disentangle this infinite number of actions, 
 to calculate the path which each body will 
 describe, and the way in which it will be 
 gradually modified with the lapse of time. He 
 must weigh each body by studying the 
 disturbance it produces in the motion of other 
 bodies ; he must reduce the whole infinitely 
 complex and laborious calculation to such a 
 shape that it can be performed by companies 
 
THE LAWS OF GRAVITY 125 
 
 of calculators who do not understand the 
 theory of what they are doing, but perform 
 an array of specified operations according to 
 fixed rules. Finally, he must organize this 
 arithmetical machine so that it can produce 
 year by year, three or four years in advance, 
 the numerical boiling down of all this mathe- 
 matics and arithmetic in the form of a 
 Nautical Almanac or as it is more properly 
 termed, an Astronomical Ephemeris or 
 prediction. There is a forcible passage in 
 Newcomb's Reminiscences of an Astronomer 
 which we may quote : "A more hopeless 
 problem than this could not be presented 
 to the ordinary human intellect. There are 
 tens of thousands of men who could be 
 successful in all the ordinary walks of life, 
 hundreds who could wield empires, thousands 
 who could gain wealth, for one who could 
 take up this astronomical problem with* any 
 hope of success. The men who have done it 
 are therefore in intellect the select few of the 
 human race an aristocracy ranking above 
 all others in the scale of being The 
 astronomical ephemeris is the last practical 
 outcome of their productive genius." 
 
 In its most general form the " problem of 
 three bodies " is insoluble by any mathematical 
 
126 ASTRONOMY 
 
 processes that have yet been invented ; in 
 other words, if only the three bodies, the sun, 
 the moon, and the earth existed, it would 
 still be impossible to obtain a solution of the 
 problem what will be the motions of these 
 three bodies under the influence of their 
 mutual attractions. Still more, then, is the 
 problem insoluble when, instead of three, 
 we have a great number of bodies involved. 
 There are, however, in the solar system certain 
 conditions which greatly modify the impractic- 
 able nature of the general problem, and as 
 very often happens, it becomes possible by 
 special devices to obtain the solution of 
 particular cases of the problem, even though 
 we recognize that its most general case is too 
 complex to deal with. For example, the sun 
 is very much more massive than any of his 
 planets ; the planets are at great distances 
 from the sun ; they move in paths which are 
 approximately though not accurately circles ; 
 and they all move round the sun in the same 
 direction. All these special conditions simplify 
 the problem so immensely that it is found 
 possible to construct a theory which can deal 
 in a laborious but sufficiently accurate and 
 thorough manner with the actual problems 
 that are presented to us in the solar system. 
 
THE LAWS OF GRAVITY 127 
 
 The principal solutions which have been 
 obtained are respectively the Lunar Theory, 
 dealing with the motion of the moon, and 
 the Planetary Theory, which can be applied 
 separately, with suitable modifications, to the 
 motions of the planets. 
 
 Without the special symbolical language of 
 mathematics one can hardly hope to indicate 
 in a general kind of way what is the nature of 
 these theories. To say that they are all 
 developments in infinite series does not 
 perhaps take us very far, yet it is not difficult 
 to understand the meaning of this brief 
 explanation. Let us suppose that we are 
 dealing with the longitude of the moon ; we 
 wish to construct a mathematical expression 
 which shall so represent this quantity that we 
 can calculate it for any given instant of time, 
 and obtain our result, the longitude of the 
 moon for that instant, expressed in degrees, 
 minutes, and seconds of arc. Now if the 
 problem were really completely solved, we 
 should have only a limited number of terms in 
 the expression which we are going to calculate. 
 The problem is really insoluble, and we cannot 
 set down that limited number of terms, 
 but several different ways have been found of 
 putting the solution into an infinite number of 
 
128 ASTRONOMY 
 
 terms. Everything then depends on the con- 
 sideration : Do these terms, as we take them 
 one by one in the series, get rapidly smaller and 
 smaller, so that although their number is 
 really infinite, we can stop at a certain point 
 in the confidence that no terms which come 
 afterwards are large enough to be appreciable ? 
 
 We shall readily believe that the decision, 
 whether we may or may not stop at a certain 
 point, involves questions of the most delicate 
 nature. Who is to say that, although the 
 second thousand terms may be negligible, 
 there is not one among the third thousand 
 that will be quite large enough to matter ? 
 And again, though all the terms after a certain 
 point may be each extremely small, their 
 number is infinite ; and how shall one be sure 
 that the sum of an infinitely large number of 
 very small quantities may not be really quite 
 large ? 
 
 It must be sufficient to say here that within 
 the last few years, an apparently satisfactory 
 answer to these questions has been found. 
 From the enormous amount of work done on 
 the problem during the nineteenth century 
 three separate and considerably different 
 ways of dealing with the Lunar Theory 
 emerged ; they are known by the names of 
 
THE LAWS OF GRAVITY 129 
 
 Hansen, Delaunay, and Hill. The two former 
 had been worked out in complete arithmetical 
 shape for many years ; and after long trial 
 they had been found insufficient. It remained 
 to see what could be done with the later and 
 considerably different way of dealing with 
 the problem devised by the American 
 mathematician, G. W. Hill. The enormous 
 work of putting his mathematics into 
 arithmetical form was undertaken by E. W. 
 Brown, a Cambridge mathematician who be- 
 came Professor of Mathematics at Haverford, 
 U.S.A., and is now at Yale. After fifteen 
 years of unremitting labour his task is now 
 complete. Beyond question the complete 
 solution of the motion of the moon under 
 gravitation has been obtained ; and the 
 suggestion has been made recently on the 
 highest authority that it will be unnecessary 
 for Greenwich Observatory to spend in the 
 future so much energy on observing the moon 
 at all hours, convenient and inconvenient ; 
 for it may be supposed that we know more of 
 the minute irregularities of her motion, from 
 the Hill-Brown theory, than we could possibly 
 observe. 
 
 Apart from its importance in providing 
 secure data for the Nautical Almanacs, a 
 
130 ASTRONOMY 
 
 complete Lunar Theory enables one to examine 
 one of the most fascinating of all scientific 
 questions. Is Newton's law of gravitation 
 rigorously true ? In the absence of any 
 sufficient physical explanation of the cause 
 of gravitational attraction which remains 
 almost completely unknown there is no 
 convincing reason to be given, why the 
 attraction should vary exactly as the square 
 of the distance between the attracting bodies. 
 Instead of having distance to the power of 
 two, might not the true law be distance to 
 the power of (two -f a very small quantity). 
 This curious possibility has been examined 
 several times, and always with a negative 
 result. The last examination is due to Brown, 
 who showed that we cannot substitute for the 
 simple 2 a quantity greater than 2-00000004, 
 without getting into more difficulties on one 
 side than we escape on the other. And as 
 it is a good principle to accept the simplest 
 explanation which is consistent with known 
 facts, there seems to be every reason to 
 believe that the simple square law is exact. 
 
 But on the other hand there is no longer 
 any reason to doubt that gravitation alone 
 is incompetent to explain completely the 
 observed motion of the moon. 
 
THE LAWS OF GRAVITY 131 
 
 Some thirty years ago the laborious re- 
 searches of Newcomb made it clear that the 
 moon never had, to that day, conformed to 
 the programme devised for her by the lunar 
 theorists ; but then we had no solid assurance 
 that the theory was without doubt complete. 
 Now we have that assurance, and still our 
 wayward satellite refuses to conform. At the 
 end of the eighteenth century the moon 
 was fourteen seconds of arc ahead ; by the 
 end of the nineteenth she was ten seconds 
 behind her calculated place. " I regard these 
 fluctuations," says Newcomb in his last paper 
 on the subject, " as the most enigmatical phe- 
 nomenon presented by the celestial motions, 
 being so difficult to account for by the action 
 of any known causes, that we cannot but 
 suspect them to arise from some action in 
 nature hitherto unknown." Several fascinating 
 possibilities are examined, only to be rejected. 
 Is the fluctuation only apparent, not due to 
 the moon at all, but caused by a trifling 
 fluctuation in the speed of rotation of the 
 earth, so that our measure of time is now 
 about 9 s. fast for that is all that is required ? 
 If so, the transit of Mercury across the sun's 
 disc in 1907 should have shown it ; but 
 Newcomb has worked up the observations of 
 
 82 
 
132 ASTRONOMY 
 
 that transit, and finds that if anything they 
 require that our time should have been slow, 
 not fast. Is it possible to suppose that the 
 force of gravitation between the moon and 
 earth is itself not constant, but subject to 
 slight fluctuations, dependent perhaps on the 
 varying magnetic activity of the sun ? That, 
 says Newcomb, would be the simplest sort of 
 action to explain all the phenomena, but he 
 cannot find any justification for it. "At 
 present," he concludes, "I see nothing more 
 to do than to invite the attention of investi- 
 gators to this most curious subject." 
 
 The motion of the moon or the accuracy 
 of our knowledge of it may be tested in 
 another way which is full of interest, and which 
 has of late years received much attention. 
 Ancient historians and poets have recorded 
 the occurrence of total eclipses of the sun 
 events so striking and awe-inspiring that they 
 naturally made a profound effect on spectators 
 and were inevitably associated with whatever 
 important business might have been on hand at 
 the time. Indeed it is not a little remarkable 
 how many decisive events seem to have been 
 accompanied by eclipses of the sun, and this 
 has even led to the suspicion that historians, 
 writing long after the event, were tempted 
 
THE LAWS OF GRAVITY 133 
 
 to embroider their narratives with any 
 picturesque details that seemed appropriate. 
 Such a suspicion does but add zest to an 
 enquiry which has been pursued with great 
 industry and remarkable success. History 
 records that more than 800 years before Christ 
 the tyrant Agathocles left Syracuse by sea, 
 and on the second day of his voyage an 
 eclipse of the sun enabled him to escape his 
 enemies. Whether he sailed round Sicily to 
 the north or the south is not recorded; but 
 it is not improbable that astronomers will one 
 day settle the question decisively. For the 
 present, however, the interest lies in the test 
 which ancient eclipses afford of the uniform 
 movement of the earth and the moon through- 
 out historic time. Is it possible for example 
 by calculating back to explain a Babylonian 
 inscription which was translated at the British 
 Museum in 1905 ? " On the 26th day 'of the 
 month Sivan, in the 7th year, the day was 
 turned into night, and lire in the midst of 
 heaven." That is to say, there was a total 
 eclipse, and the fire was, of course, the corona, 
 observed in the year 1062 B.C. A remarkable 
 controversy has raged lately over these ancient 
 eclipses a controversy which has shown 
 that mathematicians may be learned 
 
134 ASTRONOMY 
 
 historians, and historians may develop 
 unexpected mathematical talent. The work 
 of Cowell, Nevill, and Fotheringham has 
 ranged over Babylonian, Assyrian, Greek, 
 Egyptian, and Chinese records, but the 
 complexity of the puzzles which these records 
 have presented has not altogether defied 
 solution, though the solution is of an 
 unexpected kind. It is found by Cowell that 
 the tangle can be straightened out if we 
 suppose that the length of the day is increasing 
 at the rate of one-two-hundredth of a second 
 per century ! Nor is such a change insigni- 
 ficant ; on the contrary it is ten times as 
 large as what had before been considered 
 possible, and its effect, gradually accumulating 
 through centuries, is sufficient to bring into 
 line with modern theory a whole series of 
 eclipses of two thousand years ago. And it 
 is needless to point out how much depends 
 upon this question. The ultimate test of 
 the truth of our theories of the celestial 
 motions is the power to predict the future 
 and to satisfy the records of the past. When 
 we are concerned with an exceedingly slow 
 change, which takes many hundreds of years 
 to develop its effects, we cannot afford to 
 wait for the future developments, but must 
 
THE LAWS OF GRAVITY 135 
 
 be content to do our best with the past ; 
 and it is all important that we should be certain 
 of our measure of time. Otherwise we shall 
 find it very difficult to decide whether a slow 
 divergence from theory is to be attributed to 
 defects in our theories or to our having no 
 constant standard of time by which to measure. 
 But how, it will be asked, should the length 
 of the day increase, and why should the earth 
 turn more slowly than it did ? There are, 
 in fact, several minute causes at work to affect 
 the length of the day ; but they do not all 
 operate in the same direction. The continual 
 deposit of meteoric dust must lengthen it ; 
 so must the friction of the tides. Shrinking 
 due to cooling will shorten it, as will the 
 gradual wearing down of the land by the 
 action of ice and rain. But on the other 
 hand the uplifting of the whole mass of the 
 Himalayas within times geologically 'recent 
 must have lengthened it, and to a less degree 
 the building of skyscrapers in New York is 
 doing the same. On what side the balance 
 of these opposing tendencies will lie cannot 
 be predicted. But it can be discovered by 
 researches such as those on the ancient eclipses, 
 and as we have seen, there is evidence that 
 the balance lies on the side of lengthening 
 
136 ASTRONOMY 
 
 the day, by a very small but not altogether 
 inconsiderable amount. 
 
 There are several other matters we must 
 take into account, before we can say with 
 certainty whether or no the moon and the 
 planets move precisely according to the simple 
 law of gravity. We must inquire, it is 
 evident, whether any other forces than 
 gravity can act upon those bodies in their 
 orbits. What about friction ? It is 
 undoubtedly true that space is so very nearly 
 empty as to offer scarcely any resistance at all 
 to motion through it ; but it is still uncertain 
 whether there may not be some very slight 
 effect of the kind. The instance which has 
 been brought forward time and again is the 
 retardation in the motion of the small comet 
 known by the name of Encke not because he 
 discovered it, but because he investigated its 
 motion, and found that its revolution round 
 the sun was quickening a paradoxical effect 
 of resistance, one is tempted to say, but one 
 which is easily demonstrated with a little 
 mathematical reasoning, which we shall not, 
 however, attempt here. Comets, whatever 
 may be their nature, are such light and fragile 
 things that they may be very easily disturbed 
 by forces which the heavy planets would not 
 
THE LAWS OF GRAVITY 137 
 
 feel ; yet it has always been felt that one would 
 like to know why one comet only should show 
 the effect. The long researches of Backlund 
 have thrown much light upon the problem, 
 while showing at the same time that it is 
 much more complex than had been supposed. 
 By immense calculations, extending over many 
 revolutions of the comet, he has been able to 
 show that the peculiar effect in the motion of 
 this comet is not regular, but has been decreas- 
 ing by fits and starts. For twenty years or so 
 it will remain the same ; then suddenly it will 
 lose a quarter of its power ; and again, after 
 a lapse of years, another sudden change will 
 take place. Most curious of all, these changes 
 take place about the time when the sun is 
 most affected with spots, and his general 
 activities are most pronounced. It is early 
 as yet to predict whither this remarkable result 
 will lead us. But it is not the only indication 
 we have obtained that the sun may exercise 
 powers other than those belonging merely to 
 his mass. 
 
 The question had often been raised, whether 
 light falling upon a body exerted any pressure 
 upon it. Within the last few years the 
 question has been answered in the affirmative, 
 both by calculation and by experiment. It 
 
138 ASTRONOMY 
 
 has been shown mathematically that light 
 does exercise pressure, but that it is effective 
 only on very minute bodies comparable in 
 size with the length of a light wave. Experi- 
 mentally the effect has been shown in the way 
 which has been described in our chapter upon 
 comets. There seems to be no reason to 
 suppose that the pressure of light can be 
 called to account for irregularities in the 
 motion of the moon or the planets. 
 
 To sum up a difficult but fascinating enquiry 
 one which has made great progress of late, 
 yet is far from being worked out we may say 
 that the planets and the moon obey the strict 
 laws of gravitation with remarkable, but not 
 perhaps with absolute fidelity. Here and 
 there one finds a small discordance which 
 defies adjustment a wandering of the moon 
 from her predicted path, a trifling shift in the 
 orbit of a planet, a sudden effect upon the 
 motion of a comet, all of which drive us to 
 the conclusion that when the effects of gravi- 
 tation have been calculated to the most minute 
 perfection, there will still be something left 
 over for explanation. 
 
 It is always so, and herein lies the charm of 
 our subject. It is never worked out. 
 
CHAPTER VI 
 
 CELESTIAL MEASUREMENTS 
 
 A VERY important matter in any science is the 
 numerical determination of the fundamental 
 constants. A chemist is concerned for the 
 accuracy of his atomic weights and specific 
 heats, a physicist for his units of mass, of 
 electricity, of light, and what not. In the 
 same way the astronomer has to face 
 continually the necessity for greater refinement 
 in his determinations of the distance of the 
 sun, of the masses of the planets and of the 
 moon, of the aberration of light, the amounts 
 of the various motions of the axis of the earth, 
 and the velocity of the solar system in space. 
 As an example of such investigations we will 
 take the determination of the distance of the 
 sun from the earth, or its equivalent, the 
 quantity known as the solar parallax ; and 
 we will select this for three reasons. It is the 
 fundamental distance in terms of which all 
 others may be expressed ; its determination 
 involves processes which are thoroughly 
 
 139 
 
140 ASTRONOMY 
 
 characteristic of those operations of astronomy 
 related to measurement ; and thirdly, a 
 personal reason, the author has himself spent 
 ten pleasant years upon the problem, and feels 
 for it a lively affection. 
 
 There are many ways of attack some 
 direct, others leading through a maze of 
 by-paths in which one may always suspect 
 ambush and surprise. We will deal first with 
 the direct frontal attack, which is in principle 
 simply a case of surveying upon a gigantic 
 scale. The sun is an inaccessible point, but 
 that is no fatal obstacle to finding its distance. 
 The summit of Mount Everest is inaccessible, 
 yet its distance from Darjeeling can be found 
 with great accuracy, provided that there is at 
 the summit some definite point on which to 
 make the observations. Given that, the 
 accuracy of the determination depends prin- 
 cipally upon the length of the base which can 
 be employed at Darjeeling ; for the operation 
 consists of course in observing from each end 
 of a base, whose length is known, the angle 
 between the distant point and the other end 
 of the base. 
 
 Every surveyor knows that, if he wishes to 
 get accurate results, his first care must be to 
 lay out what are called " well conditioned " 
 
CELESTIAL MEASUREMENTS 141 
 
 triangles, that is, triangles of which all the 
 angles are moderate, neither too large nor 
 too small ; and of course the further away an 
 object is, and the smaller by comparison is 
 the available base, the more difficult and 
 uncertain is the resulting measurement. Now 
 as our earth is but 8,000 miles in diameter, 
 we cannot possibly have our base, from which 
 to measure the sun's distance, more than 8,000 
 miles long, and practical considerations limit 
 the distance to about 7,000. But the sun is 
 more than 13,000 times this distance from 
 the earth ; and even if we were able to select 
 stations 7,000 miles apart, and observe 
 simultaneously at each the angle between the 
 sun and the other, our triangle would be 
 hopelessly long and narrow, with the angle at 
 the sun less than 15 seconds of arc. Moreover, 
 the two ends of our base will be nearly on 
 opposite sides of the earth, altogether invisible 
 one from the other, and we must get over this 
 difficulty by a roundabout process that greatly 
 complicates the work, and makes it much less 
 accurate. As a problem of pure surveying, 
 the measurement of the sun's distance by 
 direct settings upon the sun himself is quite 
 hopeless, unless we are content with a probable 
 error of two or three per cent, in our result. 
 
142 ASTRONOMY 
 
 The problem would be much simpler if we 
 could see stars close to the sun. In that case 
 it would not be necessary for us to be able to 
 see from one end of the base to the other. 
 Instead, we could make simultaneous observa- 
 tions of the position of the sun among the 
 stars, which may be considered infinitely far 
 away in comparison with the sun. When 
 viewed from two different points, on nearly 
 opposite sides of the earth, the sun would 
 appear differently placed among the stars, 
 by that small amount of fifteen seconds of 
 arc, invisible to the unaided sight though 
 measurable with telescopes. This involves 
 the true principle of all astronomical deter- 
 minations of distance, that a near body is 
 displaced with reference to more distant 
 bodies, when observed from different points of 
 view. In astronomical language, the nearer 
 body has a parallax (Greek for a shift) relative 
 to the distant bodies, and all such determina- 
 tions are called determinations of parallax. 
 
 Though we cannot see stars close to the sun, 
 we can, on rare occasions, see the planet 
 Venus pass across the face of the sun as a 
 small dark body ; on which occasions Venus, 
 being only at about one-third the distance of 
 the sun, has a parallax relative to him. The 
 
CELESTIAL MEASUREMENTS 148 
 
 consequence of this parallax is then that the 
 time at which Venus appears to enter upon and 
 to leave the sun, and the duration of the 
 transit, depend upon the place on the earth 
 which is chosen as a station from which to 
 view the event. According as we select our 
 station we may find an acceleration or 
 retardation of about sixteen minutes in the 
 moment when we observe a particular phase 
 of the transit. 
 
 Historically, the transits of Venus have 
 supplied the most famous occasions for the 
 solution of our problem. The necessity of 
 travelling to remote places invests the 
 undertaking with an air of importance and 
 adventure. It must be something worth 
 winning, to induce Governments to send large 
 and well-found expeditions all over the earth, 
 at a total cost of many hundreds of thousands 
 of pounds. Thus the solar parallax has'among 
 astronomical problems a dignity all its own, 
 associated in public repute with the famous 
 voyage of Captain Cook to the South Seas, 
 and with the tremendous efforts put forth 
 within recent memory in 1874 and 1882. 
 Unhappily, one must admit that the money 
 spent in sending expeditions to Siberia, to the 
 Pacific, to Kerguelen Island in the Antarctic 
 
144 ASTRONOMY 
 
 Ocean, was practically thrown away, through 
 no fault of the astronomers, unless indeed 
 they can be blamed for failure to foresee the 
 difficulty which wrecked the whole enterprise. 
 They had gone to determine the exact instant 
 at which Venus appeared in contact with the 
 edge of the sun's disc. They expected to have 
 a perfectly clear cut view of this contact, and 
 they did not at all anticipate what actually 
 happened, that the atmosphere round the 
 planet, filled with sunlight, blurred the 
 sharpness of the phenomenon and left it 
 uncertain within a large number of seconds at 
 what instant the critical phase took place. 
 
 In transits of Venus we observe, not the 
 parallax of the sun directly, but the difference 
 of parallax between Venus and the sun ; so 
 that to complete our problem it is necessary 
 to know what proportion the distance of Venus 
 bears to the distance of the sun. But this 
 involves no special difficulty From Kepler's 
 Third Law the squares of the periodic times 
 are proportioned to the cubes of the major 
 axis of the planetary orbits we know the 
 relative sizes of all the orbits with great 
 accuracy. Find one, or the difference between 
 two, in miles, and the rest is simple. 
 
 This principle gives us the key to all modern 
 
CELESTIAL MEASUREMENTS 145 
 
 work on the solar parallax. Choose an exterior 
 planet as the stepping-stone, and you have at 
 once a great number of advantages. The 
 planet is nearest and its parallax greatest, 
 when it is in opposition to the sun, visible 
 all night, and observable consequently after 
 sunset and before sunrise. The planet can 
 be compared with the stars about it, so that 
 the whole parallax, and not the difference 
 between two, can be measured. And greatest 
 advantage of all, the observations can be 
 continued over weeks or months ; failure at 
 one moment can be repaired by success at 
 others, and the astronomer is relieved of the 
 disquieting anxiety lest a temporary break- 
 down in instrument or in weather should 
 ruin everything at a critical moment. 
 
 The first modern application of this method 
 was made by Sir David Gill, who had observed 
 at Mauritius the Transit of Venus in 1874, and 
 had made up his mind very definitely that 
 little good would come of the transit of 1882. 
 So in 1877, at a very favourable opposition 
 and close approach of the planet Mars, he 
 borrowed Lord Lindsay's heliometer and 
 established himself on the island of Ascension, 
 near the equator. Every night the observing 
 station in Mars Bay was carried some seven 
 
146 ASTRONOMY 
 
 thousand miles by the rotation of the earth, 
 and the planet thereby displaced among the 
 stars by about forty seconds of arc. Several 
 months of successful work on the island, 
 and a couple of years of severe calculation 
 afterwards, produced the result of 93,080,000 
 miles for the mean distance of the sun. Then, 
 as so often happens in scientific enterprises, 
 when it was hoped that all was complete, 
 some one suggested a possible flaw the red 
 colour of the planet might have produced a 
 systematic error in the observations. 
 
 The next step was to try again, this time 
 with three minor planets so small that they 
 were indistinguishable in appearance from 
 stars, and presumably free from the possibly 
 damaging effect of that difference of colour 
 which rendered the observation of Mars a little 
 uncertain. The same courageous observer, 
 by that time Her Majesty's Astronomer at 
 the Cape, organized a co-operation between 
 his observatory in the south and four in the 
 northern hemisphere, to observe the small 
 planets Victoria, Iris, and Sappho at their 
 oppositions in 1888 and 1889. The labour was 
 immense, the observations proving so accu- 
 rate that they demanded the use in a great 
 part of the work of eight-figure logarithms ; 
 
CELESTIAL MEASUREMENTS 147 
 
 but when the results were eventually published 
 in two enormous red volumes of Annals of the 
 Cape Observatory, it might well have seemed 
 that observation had now done its uttermost 
 to solve the problem. Nevertheless, within 
 a few years the whole question was revived on 
 a more extensive scale than ever. 
 
 To understand how this came about, and 
 why it was necessary, we must look at some 
 of the indirect ways of solving our problem. 
 It will be worth our while to do so, even at the 
 expense of a little difficulty, because they 
 illustrate in a peculiarly interesting way how 
 widely different branches of our science have 
 delicate relationships with one another. There 
 is an irregularity in the motion of the moon 
 which depends upon the distance of the sun, 
 though in a complicated way which we shall 
 not attempt to follow. If this can by observa- 
 tion and calculation be disentangled from the 
 many other irregularities which affect the 
 motion of our satellite, then its value will 
 serve to determine the solar parallax. Or 
 again, the mass of the earth affects by its 
 attraction the motion of the other planets, 
 and they all affect one another by amounts 
 depending upon the distances of the planets 
 apart. If the action of the earth can be 
 
148 ASTRONOMY 
 
 disentangled from the others, the mass of the 
 earth can be found, and thence, by a process 
 much too difficult to describe here, we have 
 another value for the distance of the Sun. 
 Or again, the motion of the earth about the 
 sun, combined with the velocity of light, 
 produces the well known effect of the Aberra- 
 tion of Light, which causes all the stars 
 to move apparently in little ellipses each year. 
 The size of the effect can be measured ; the 
 velocity of light can be measured independently 
 in the laboratory ; and from the two results 
 we may deduce the velocity of the earth in 
 its orbit. Then, knowing the length of the 
 year, it becomes a simple matter to find 
 the size of the orbit, and the distance of 
 the sun. 
 
 These three are typical, and the most 
 important, of the indirect methods of finding 
 the quantity of which we are in search. 
 Opinions will always be divided upon the 
 question, how far any of the indirect results 
 should be preferred to the direct results of 
 measurement ; and it is probable that those 
 who have made the direct measurements will 
 be the least willing to attach an exaggerated 
 value to the indirect. But it cannot be 
 denied that when the results fail to agree it 
 
CELESTIAL MEASUREMENTS 149 
 
 produces an awkward feeling that all is not 
 well, which is intensified when all the indirect 
 methods appear to conspire against the result 
 given by the direct. Such a crisis occurred 
 about the year 1898. The indirect results 
 combined at that time to make the distance 
 of the sun several hundred thousand mile^s 
 greater than direct measurement would allow, 
 from which arose a renewal of interest and a 
 general anxiety about the great problem. 
 
 At this moment a happy accident occurred. 
 In Berlin there is a society which maintains 
 a small observatory where non-professional 
 astronomers may find an equipment beyond 
 their private means. With the photographic 
 telescope of this observatory Dr. Gustav Witt 
 was engaged in a search for certain minor 
 planets which had been neglected and had 
 become lost. With the good luck which often 
 rewards good work, he found a prize far 
 exceeding in value that at which he aimed 
 a minor planet indeed, but moving in a path 
 remarkably situated, very eccentric, and 
 bringing the planet on rare occasions within 
 fifteen million miles of the earth. So soon as 
 a first rough approximation to the path of 
 the new planet had been calculated, the 
 great value of the discovery was apparent : 
 
150 ASTRONOMY 
 
 it provided an opportunity of determining the 
 distance of the sun with an accuracy much 
 greater than had been possible before. The 
 opportunity, it is true, would occur but rarely, 
 and the most favourable possible occasion 
 had just been missed, having occurred four 
 years before the discovery. But in the 
 autumn and winter of 1900-01 a quite satis- 
 factory opposition of the planet was to occur, 
 and it became immediately a pressing matter 
 to ensure the necessary co-operation between 
 astronomers. Here again fortune was kind. 
 A meeting of the large permanent international 
 committee which controls the photographic 
 chart of the heavens had been called for July, 
 1900, in Paris, and the accompanying congress 
 brought together a very large and representa- 
 tive body of astronomers. It was thus easy 
 to arrange a joint programme of work for the 
 ensuing winter, to ensure that every possible 
 advantage should be taken of the new 
 opportunity of making a fundamental 
 improvement in celestial measures. 
 
 Throughout Europe the skies of that winter 
 were very unfavourable. At Cambridge the 
 author had the pleasure of being on duty with 
 the photographic telescope from dusk till 
 dawn for nearly three months, during which 
 
CELESTIAL MEASUREMENTS 151 
 
 time only five or six nights were more or less 
 clear throughout. Had visual observations 
 alone been possible, one could scarcely, in 
 those brief and broken intervals of clear sky, 
 have obtained a sufficiency of measures. 
 But the photographic plate, in work of this 
 kind, is so much more expeditious that it was 
 possible to obtain at Cambridge alone about 
 five hundred photographs, on which the posi- 
 tion of the planet among the stars could be 
 measured with great precision. Many other 
 observatories obtained as much material; 
 a few, happily situated in fine climates, 
 obtained much more ; while the visual 
 observers had been equally active and had 
 accumulated an enormous mass of measures, 
 referring the planet to great numbers of faint 
 stars whose positions at the time were entirely 
 unknown. 
 
 Here, then, was an overwhelming richness 
 of raw material for the solution of the problem. 
 How to deal with* it economically and 
 effectually, and how to promote the necessary 
 and laborious supplementary investigations, 
 the observation of seven or eight hundred 
 stars with meridian circles, or the calculation 
 of the path of Eros with the most refined care 
 became a question of the highest interest. 
 
152 ASTRONOMY 
 
 In this work the Paris Observatory took a 
 leading part, under the direction of the late 
 Monsieur Loewy. His aim was to bring 
 together the material produced at each 
 co-operating observatory into a systematic 
 form. It would then be readily available to 
 the hand of whoever might feel drawn to 
 undertake the not inconsiderable labour of 
 extracting the ultimate four or five figures 
 from the existing millions. By a preliminary 
 treatment of a small part of the whole the 
 author had been able to discover the 
 appropriate methods of dealing with the 
 problem and to assure himself that the 
 enterprise was going to be a success. He will 
 always be grateful for the happy combination 
 of circumstances which resulted in the whole 
 final discussion being placed in his hands, 
 with the means necessary to secure help in 
 the calculations. 
 
 In an enterprise of this kind, which must 
 necessarily take some years to complete, the 
 unkindest thing that can happen is for all to 
 go smoothly ; for calculation is apt to be 
 tedious, and requires enlivenment. The 
 fascination of the work lies in the solution of 
 the puzzles which hardly ever fail to present 
 themselves. On the occasion of which we 
 
CELESTIAL MEASUREMENTS 153 
 
 speak there was no lack of this sporting 
 element. It was the first time there had 
 been the opportunity on a grand scale of 
 comparing minutely the results turned out 
 by different photographic telescopes, and by 
 different ways of measuring the plates and 
 reducing the measures to a common form. 
 Naturally enough, when one searched deep one 
 found discordances and disagreements of 
 small amount, but of amount which could not 
 for a moment be neglected. One had to try 
 all kinds of experiments to discover what was 
 wrong with photographic object glasses ; 
 to find out why one calculated prediction of 
 the planet's path did not accord with another, 
 though both were built upon the same 
 foundations. One had to maintain corre- 
 spondence with observatories all over the 
 world, leading to the formation of many 
 valued friendships ; and one had continually 
 tc steer in a narrow channel between dangers 
 on either hand, each offering the chance of 
 shipwreck. On the one hand there was the 
 danger of neglecting some minute precaution 
 against error which might affect the whole 
 result. On the other hand there was the 
 danger that pursuit of the tempting enquiries 
 presenting themselves at every turn might 
 
154 ASTRONOMY 
 
 so prolong the work that it would never get 
 finished. 
 
 Some one has laid down the principle that 
 it is a mistake to plan work which will take 
 longer than ten years to complete. Judged by 
 their standard the solar parallax determination 
 from the planet Eros just escapes censure. 
 It was begun in the Paris Congress of 1900, 
 and it was practically finished when the final 
 results were communicated in brief to the 
 Academic des Sciences in 1910. Whatever 
 judgment may be passed in the end upon the 
 results of that enterprise, they present at the 
 moment two features which are eminently 
 satisfactory ; the photographic and the visual 
 observations give the same result ; and that 
 result is the same as the previous result made 
 by direct measurement. 
 
 Moreover, within the last two years a 
 beautiful piece of work has come from the 
 Cape Observatory a determination of the 
 distance of the sun from spectroscopic 
 observations, long anticipated, but only to be 
 achieved after great efforts at conquering 
 instrumental difficulties. We have seen how 
 the spectroscope is able to measure the 
 velocity in the line of sight between the earth 
 and a star. This velocity necessarily varies 
 
CELESTIAL MEASUREMENTS 155 
 
 with the season of the year, and the variable 
 part due to the motion of the earth round the 
 sun can be disentangled from the constant 
 part due to the combined velocities of our sun 
 and of the star in space. Hence one finds the 
 ratio between the velocity of the earth and 
 the velocity of light that is to say one finds 
 the constant of aberration by a new and direct 
 method ; and the well known values for the 
 velocity of light and the length of the year 
 give us eventually the distance of the sun, 
 which by this new and quite independent 
 method comes out in almost precise agreement 
 with the old. 
 
 The result which emerges from a very big 
 enterprise such as the Eros campaign may be 
 expressed in four figures, which are usually 
 written thus : 
 
 Solar Parallax from Eros = 8".806. . 
 
 For the moment one is left wondering how 
 this can be called a determination of the 
 sun's distance. Even if the sign for seconds of 
 arc is misread as inches, which is only too 
 common, the result has the appearance of 
 being absurd. The fact is that the astronomer 
 does not want the distance of the sun expressed 
 in miles ; for his purposes the numbers are 
 
156 ASTRONOMY 
 
 inconveniently big and the whole thing in 
 the wrong shape. What he really cares about 
 is the ratio between the equatorial radius of 
 the earth and the mean distance of the earth 
 from the sun. This small quantity is the sine 
 of a certain angle ; the angle is what we 
 ordinarily call the Solar Parallax, and it is 
 independent of any reference to miles or kilo- 
 metres. These units, more or less arbitrarily 
 defined by standard bars of platinum, are 
 indeed purely mundane, and have no real 
 place in celestial measurements. One may 
 very well question how far it is desirable to 
 use them for celestial distances, though there 
 is no difficulty in doing so if it is wished. 
 Geodesy has measured the radius of the earth's 
 equator, and makes it about 6,378 kilometres. 
 Using this value we find at once from the above 
 value for the solar parallax that the mean dis- 
 tance of the sun from the earth is 149,400,000 
 kilometres, or 92,830,000 miles. 
 
 An interesting by-product of an investiga- 
 tion such as we have described, is a value for 
 the mass of the moon in terms of that of the 
 earth. This quantity becomes involved in our 
 problem in the following way, which illustrates 
 very clearly the complexity of measurements 
 made from a moving earth. We have been 
 
CELESTIAL MEASUREMENTS 157 
 
 speaking of observing the planet Eros night 
 and morning at a single observatory, or more 
 or less simultaneously at two observatories 
 far apart upon the earth's surface Greenwich 
 and the Lick Observatory for example, and 
 for simplicity we have spoken of the displace- 
 ment of the planet among the stars, as thus 
 viewed from different points, as though it 
 were the only displacement that the planet 
 suffered. But, in fact, the planet is moving 
 all the while in its own path round the sun, 
 and so is the earth ; while the motion of the 
 earth is complicated by the fact that the 
 earth has a partner, the moon. They turn 
 about one another in their progress much as 
 a heavy man waltzing with a little girl pro- 
 gresses round a ballroom, the light partner 
 turning in a larger circle, the heavy in a smaller. 
 
 In the case of the earth and the moon the one 
 
 
 
 is so much heavier than the other that the 
 common centre of gravity about which they 
 turn is within the surface of the earth, though 
 far from the centre ; but the effect of this 
 monthly oscillation of the earth is to pro- 
 duce a monthly irregularity in the apparent 
 motion of any planet viewed from its surface. 
 Disentangle this monthly oscillation in the 
 place of the planet, determine its amount, and 
 
158 ASTRONOMY 
 
 one can deduce the size of the oscillation in 
 the earth's motion which produces it. Thence 
 one derives the proportion between this 
 displacement and the distance of the moon, 
 and from that proportion the mass of the 
 moon in terms of the earth. The observations 
 of Eros gave for this proportion 1 : 81.5. 
 
 We have dealt at length with a single 
 problem of celestial movement in order to get 
 an idea of the manner in which such measures 
 are made, and of the way in which the results 
 are interwoven one with another ; of the 
 continual endeavour to refine and refine again 
 the important determinations of fundamental 
 quantities like the distance of the sun, and of 
 the fascinating enquiries which are opened up 
 when the results of different methods disagree. 
 With a foundation thus made secure, we may 
 proceed with the more confidence to build. 
 
CHAPTER VII 
 
 THE STARS 
 
 IN modern astronomy the stars have 
 individualities : we know them as near or 
 distant ; having large or small velocities among 
 their neighbours ; surpassingly brilliant of 
 surface or relatively dull ; single, or double to 
 the telescopic sight, or double only to the 
 searching scrutiny of the spectroscope ; 
 steadfast in light, or variable by eclipse, by 
 outburst of flame, by causes unknown. With 
 individuality they lose their name of fixed 
 stars. No longer can they be regarded as the 
 unchanging background before which the sun 
 and the moon and the planets move : as 
 useful points like the beacons of a survey, 
 pegs upon which to hang the delineation of 
 the country, uninteresting in themselves and 
 likely to disappear from the finished work 
 when they have served their purpose. We 
 are not, and never shall be, in a position to 
 neglect this aspect of the stars, expressed in the 
 
 159 
 
160 ASTRONOMY 
 
 word " fundamental," but we shall find that 
 in the light of modern methods and modern 
 results the fundamental star catalogue no 
 longer appeals to us as the last word in stellar 
 astronomy. The distinction is much the same 
 as the difference between the study of the 
 planets and the planetary theory. In the 
 latter the planets are treated as mere geome- 
 trical points in which is concentrated a certain 
 mass ; the planetary theory has no concern 
 with the physical state of the planets, their 
 structure, or their fitness for habitation. So 
 with the stars : we may think of them 
 as mere points in space, and be interested in 
 their positions, their distances, and their 
 proper motions or we may think of them as 
 suns, having different compositions, different 
 masses, and all kinds of different physical 
 properties. The first is astronomy in its 
 restricted sense ; the second is the modern 
 subject of astrophysics ; and though the two 
 have no sharp division, and are of course con- 
 tinually indebted one to another, we shall find 
 it natural to consider them in some measure 
 apart. 
 
 To begin then with astronomy proper the 
 science of star places and apparent motion, 
 often called astronomy of position. Its 
 
THE STARS 161 
 
 foundations were laid, it is believed, by 
 Hipparchus more than a century before Christ. 
 Hipparchus was probably the first who 
 constructed a catalogue of star places. The 
 catalogue has not come down to us under his 
 name, but there are good reasons for believing 
 that Ptolemy's famous catalogue in the 
 Almagest is founded upon an earlier catalogue 
 of Hipparchus. Right well, in that case, does 
 Hipparchus deserve the name of founder of 
 practical astronomy, for no sensible improve- 
 ment on that catalogue was made until the 
 days of Ulugh Beigh, who reigned at Samar- 
 cand, founded a great college and observatory, 
 and made a revision of Ptolemy's catalogue 
 which is dated 1437. Tycho Brahe's star cata- 
 logue, made with the greatly improved instru- 
 ments of his observatory at Uraniborg, was 
 published in 1602, the first of many farjious 
 catalogues that have come from round about 
 the Baltic. Then we come to the invention 
 of the telescope, and its application by Halley 
 to measurement as well as to description ; 
 to the names of Flamsteed and Bradley, 
 Lacaille and Lalande, Mayer, Piazzi, and 
 Bessel. By their labours was reared the 
 substructure, on which in a sense the whole 
 modern astronomy of position is carried. 
 
162 ASTRONOMY 
 
 Without attempting to follow in detail the 
 development of star cataloguing, we may 
 consider the main purposes to which it is 
 directed. Firstly, accurate places of the stars 
 form an essential basis of the useful appli- 
 cations of astronomy the determination 
 of time, navigation, surveying. Secondly, 
 accurate places of the stars are required as 
 reference points, to which the sun and moon, 
 planets and comets, are all referred. Without 
 good star places w r e can have no theory of the 
 motions in the solar system, no orbits of 
 planets or of comets. And thirdly, most 
 important for our present purpose, without 
 accurate catalogues of the stars we can know 
 nothing of the grander problems of the 
 universe, the motion of our sun among the 
 stars, or of the stars among themselves 
 
 When one compares a catalogue of to-day 
 with a catalogue made a hundred years ago, 
 one finds discordances between the places of 
 certain stars now and then. Some of these 
 are due to nothing but errors of observation 
 in one catalogue or in both. We have 
 means of estimating how large these errors 
 are likely to be, and can say approximately 
 what the average accidental discordance 
 should amount to. When we find a great 
 
THE STARS 163 
 
 number of observed differences much greater 
 than these, we must recognize that the stars 
 are not fixed ; that some, on the contrary, 
 have motions proper to themselves which 
 become quite evident in the course of fifty 
 or a hundred years. A most important pur- 
 pose of star cataloguing is to determine the 
 magnitude and direction on the celestial 
 sphere of these proper motions of the stars. 
 
 Having determined a great number of 
 proper motions we have next to enquire 
 whether they show any significant arrange- 
 ment. Suppose for the time being that the 
 real motions of the stars are haphazard, 
 distributed according to the laws of chance 
 with which mathematics can deal. Then, if 
 the sun is at rest, the proper motions of the 
 stars, as we see them, will be haphazard also, 
 and there are definite tests which can be 
 applied to see how nearly haphazard the 
 motions are. But if the sun is not at rest, 
 his movement will introduce at once a certain 
 degree of order into the otherwise random 
 proper motions. The nearer stars on either 
 hand will have a general tendency to move 
 backwards, that is to say, to appear to do so ; 
 they will, in fact, on the average, be left 
 behind, while the stars in the part of the sky 
 
 P2 
 
164 ASTRONOMY 
 
 to which the sun's motion is directed will be 
 very little or not at all affected ; and so will 
 the stars in the opposite quarter of the 
 sky. Working upon this principle, many 
 astronomers have made discussions of observed 
 proper motions, with a view to finding the 
 point towards which the sun is moving ; 
 or in technical language, to finding the " apex 
 of the sun's way." 
 
 As might be expected, the result depends 
 to some extent upon the selection of the stars 
 to be discussed. Different lists of stars give 
 somewhat different values for the position 
 of the apex, which means of course that one 
 has to use a great number of stars before their 
 individual peculiarities are averaged out, and 
 their motion in the mean is zero. There is, 
 however, a general consensus of result that 
 the motion of the sun is directed towards the 
 constellation Lyra, or at any rate towards that 
 region of the sky. 
 
 Ten years ago there was no suspicion that 
 the basis of this investigation might be 
 unsound, because of the fundamental assump- 
 tion that the real motions of the stars are 
 random. Seven years ago, at the Scientific 
 Congress held in connection with the St. 
 Louis Exhibition, Kapteyn, of Groningen in 
 
THE STARS 165 
 
 Holland, introduced to us the new idea of 
 " star-streaming " an idea which has upset 
 many of our previous notions. Kapteyn had 
 been engaged for many years on a series of 
 intricate researches into the arrangement 
 and the motions of the stars in the universe. 
 He had prepared an elaborate machinery of 
 theory and calculation which would work if 
 the stars were moving at random, and would 
 not work if they were not. On arriving at 
 the crucial point, where theory was confronted 
 with observation, the machine would not 
 work, and its manner of breakdown showed 
 at once what was wrong. The stars under 
 discussion were found to be moving in two 
 streams, crossing through one another and 
 pretty well mixed, but with an average 
 velocity so considerable that it destroyed any 
 possibility of treating the motions ol the 
 stars as random. 
 
 Since the announcement of this revolutionary 
 discovery much work has been done upon 
 the subject, particularly by British astrono- 
 mers. Eddington, Dyson, HougK and Halm 
 have examined the question from many 
 different points of view, and their results leave 
 no doubt that star- streaming is a real and 
 embarrassing fact. It can be represented in 
 
166 ASTRONOMY 
 
 several different ways, none of which is very 
 easy to understand. The broad result is 
 clear, however. We had been in error when 
 we assumed that the motions of the stars are 
 haphazard, with no general system super- 
 imposed upon their infinite variety. On the 
 contrary, we are now compelled to recognize 
 that considerable numbers of them are drifting 
 together, and that these drifts must have an 
 important influence upon all such discussions 
 as that of the movement of the sun in space. 
 Epoch-making as are these results, it appears 
 to the author that in one respect they have 
 been overrated. We begin to hear talk of the 
 stars being divided into two universes, as if 
 the stars which can be assigned to one drift 
 or another were representative samples of the 
 whole contents of the sky. It is far more 
 probable indeed we may say it is practically 
 certain that all the stars with a sensible 
 proper motion, all the stars, that is to say, 
 which belong to these drifts, are relatively 
 close neighbours of the sun. The bulk of the 
 stars in the sky, and especially the stars of 
 the Milky Way, are untouched by these 
 discussions, because they have no determined 
 proper motions to be discussed. In the course 
 of centuries we may discover that there are 
 
THE STARS 167 
 
 a thousand drifts a thousand universes, if 
 one will. Meanwhile we shall do well to avoid 
 the false suggestion of completeness implied 
 by the statement that there are two. 
 
 The greatest impediment to the study of 
 star distribution is the extreme difficulty of 
 determining a star's distance from the sun. 
 The principle is easy : the nearer stars should 
 appear to oscillate in a yearly period, as the 
 earth moves round the sun ; the range of the 
 oscillation should give the distance of the 
 star. The practical difficulties of turning 
 this simple principle to account are due to 
 the immense distances of even the nearer 
 stars, which make the oscillations infinitesimal. 
 Measurements for the determination of a star's 
 distance are generally made nowadays upon 
 a photographic plate, taken with a telescope 
 of considerable focal length, so that the plates 
 are on a large scale. The Cambridge Obser- 
 vatory photographic telescope, for example, is 
 nearly twenty feet long ; a degree on its plates 
 is about four inches. Now if that telescope 
 were photographing the star which is nearest 
 the sun, its image would in the course of the 
 year oscillate a thousandth of an inch on each 
 side of its mean place ; this is all the effect 
 produced by the displacement of the earth 
 
168 ASTRONOMY 
 
 across the 186 million miles which is the 
 diameter of the earth's orbit. And this is for 
 the very nearest star. Very few stars 
 perhaps two or three hundred in the whole 
 sky are within ten times its distance ; very 
 few stars, then, will be displaced a ten- 
 thousandth of an inch either way by the 
 earth's revolution in a year. In fact their 
 distances are so great that the nearer stars are 
 on the borders of possibility, while the great 
 bulk of the stars in the sky are utterly beyond 
 the reach of this method of measurement. 
 And since at present for the generality of 
 stars there is no other way available, it seems 
 that we are debarred from extending our 
 knowledge in this most desirable of directions. 
 It was thought at one time that a rough 
 estimate of a star's distance might be deduced 
 from its brightness ; but the relatively few 
 measurements we have show that this idea 
 is illusory. Most of the bright stars are 
 quite out of reach ; some few of the faint 
 ones are among the sun's nearer neighbours. 
 Hence, though no doubt it is true that on the 
 whole the fainter stars are the more distant, 
 for any particular star or star group its bright- 
 ness tells us nothing about its distance. 
 The only criterion of nearness is proper 
 
THE STARS 169 
 
 motion. If a star has a large proper motion 
 there is some chance that it is within measur- 
 able distance ; so that stars with large proper 
 motions get selected for measurement. But 
 it is evident that another star without proper 
 motion may be just as near, and will escape 
 observation because there is no reason to 
 select it as promising some return for a great 
 deal of work. This is doubly unfortunate, 
 because it gives to all the statistics of stellar 
 distances a bias which can scarcely be 
 avoided. 
 
 Many have tried to find some way of 
 picturing the distances which separate star 
 from star in the sun's neighbourhood. Perhaps 
 there is no way better than to imagine a 
 model in which the sun is represented by a 
 grain of sand one-hundredth of an inch in 
 diameter, and the earth by a quite invisible 
 speck one inch away. Upon this scale the 
 nearest star will be another grain of sand some 
 four miles away, and the other stars will be 
 scattered at somewhat greater distances apart. 
 To this incredible sparseness are the stars 
 reduced when we try to look at them in three 
 dimensions. 
 
 While the model is in mind, let us anticipate 
 a result of spectroscopic work which gives us 
 
170 ASTRONOMY 
 
 the velocity of the sun in space four radii 
 of the earth's orbit per year, or four inches 
 on our model. At this rate the sun would 
 take some seventy thousand years to reach 
 his nearest neighbour. Now so far as is 
 known we have no real reason for assuming 
 that the stars are gathered anywhere much 
 more closely than this, except of course in the 
 star clusters. We shall do well, therefore, 
 to view with scepticism any theories which 
 require crowding or collision among the stars. 
 Despite the richness of the sky, the emptiness 
 of space is its most striking character. 
 
 From time to time a new star will suddenly 
 blaze up, telling of some sort of catastrophe 
 in the heavens, and inspiring the sensational 
 press to enquire whether such a conflagration 
 might not overwhelm us any night. Having 
 no taste for the lurid, we will dismiss this 
 last idea without anxiety ; but the general 
 question of what happens when a new star 
 appears is well worth discussion, in a more 
 cautious and sober spirit than that displayed 
 by the collision-mongers. 
 
 The appearance of a new star is not an 
 exceedingly rare event. Every year or two 
 they find, on the stores of photographs which 
 are accumulated at the Harvard College 
 
THE STARS 171 
 
 Observatory, evidence that a star has appeared 
 suddenly, declined gradually, and generally 
 disappeared again before it had been recog- 
 nized. Occasionally, when the star is bright, 
 it is caught before its decline has proceeded 
 very far, and once, by good fortune, the most 
 splendid new star of modern times was 
 detected when it was still upon the upgrade. 
 The new star in Perseus Nova Persei was 
 found in February of the year 1901 by Dr. 
 Anderson, of Edinburgh. This accomplished 
 observer not an astronomer but an " astro- 
 phile " he calls himself has a habit all too 
 rare among astronomers proper, of getting 
 out of bed every hour or two of a cloudy night, 
 to see if the sky has cleared. On February 
 21, 1901, the sky cleared after midnight, and 
 Dr. Anderson began his regular work of 
 comparing the sky with star charts, field by 
 field, in the hope of discovering new variables. 
 He once described to the author how, becoming 
 tired at the telescope, at twenty minutes to 
 three in the morning he leaned back to rest a 
 moment, and saw in the constellation Perseus 
 the bright new star. Unable to believe that 
 so bright an object could have escaped 
 previous notice, he made diplomatic enquiries 
 early next morning at the Royal Observatory 
 
172 ASTRONOMY 
 
 Edinburgh, and learned that they had no 
 news of any special interest. Then, said he, he 
 knew that he was all right. 
 
 At the time of discovery the star was of 
 magnitude 2.7; that is to say, about half as 
 bright as the Pole Star. A photograph taken 
 only twenty-eight hours before, by Stanley 
 Williams at Brighton, showed stars nearly 
 to the twelfth magnitude, but no trace of the 
 Nova, whose light must have increased, 
 therefore, at least four thousand times during 
 that interval. In another day and a half it 
 had become the brightest star in the northern 
 sky ; then it began to diminish, at first 
 rapidly, afterwards more slowly, with large 
 fluctuations which embarrassed alternately 
 those who sought to show the new star, and 
 those who wished to point out that it had 
 disappeared. By June it had become invisible 
 to the naked eye, and thenceforward it faded 
 slowly in the way characteristic of new stars. 
 
 Until the invention of the spectroscope 
 very little could be made of these phenomena ; 
 to observe the amount of light, and its colour, 
 did not carry the observer very far towards 
 explaining what had caused the sudden 
 outburst. The new possibilities of light 
 analysis have shown that the sequence of 
 
THE STARS 173 
 
 events in the life of a new star is generally as 
 follows : When it is bright, a great part of 
 its light comes from outbursts of glowing gas ; 
 as it fades, it becomes a gaseous nebula ; 
 in its final stage it is a very faint star of no 
 special characteristics. The very prompt 
 discovery of Nova Persei enabled the astro- 
 nomers of Harvard to photograph it while it 
 was still increasing in brightness to the 
 general surprise they found a spectrum like 
 that of the stars in Orion, with no bright 
 lines. This first chapter is unique, and it 
 adds very much to the difficulty of explaining 
 a new star, a difficulty already grave enough. 
 We must, in fact, admit that the evidence 
 has not as yet received any satisfactory 
 interpretation. Actual collision between two 
 stars is always possible, though extremely 
 unlikely. Did it occur, there would be 
 inevitably a terrific production of intensely 
 hot vapour as the energy of the collision was 
 changed into heat. We should expect that 
 analysis of its light would show bright lines 
 from the first. As a fact, they did not appear 
 for several days, and seem to have been the 
 result and not the cause of the brilliance. 
 
 All kinds of theories have been put forward 
 to account for the sudden appearance of these 
 
174 ASTRONOMY 
 
 new stars most of them of the catastrophe 
 or collision class. Perhaps the most plausible 
 is the suggestion that a new star is due to the 
 passage of a faint, cool, and almost dark star 
 through a nebula, in which case we must 
 suppose that the surface of the star becomes 
 suddenly heated by friction, and thus the 
 outburst of light is produced. On this theory 
 a new star would be a kind of celestial meteor. 
 Without going into the very complex facts 
 revealed by the spectroscope, we may say 
 that no satisfactory explanation of those facts 
 has been put forward ; the light analysis 
 defies interpretation. 
 
 But it is by no means improbable that the 
 eventual solution will be found in regarding 
 new stars simply as exaggerated cases of 
 variable stars. We have in such a star as 
 Mira Ceti a case quite as difficult to explain, 
 which attracts rather less attention only 
 because the outburst happens every 330 
 days, more or less. The brightness of this star 
 is continually varying, and at maximum it is 
 anything from a hundred to a thousand times 
 as bright as at minimum. Another variable 
 star of long period, x Cygni, has a total range of 
 nearly ten magnitudes, or shines 10,000 times 
 as bright on some days as on others. 
 
THE STARS 175 
 
 We can hardly believe that a catastrophe 
 or collision affects such stars every three or 
 four hundred days ; there must be some more 
 normal explanation of the light-outbursts, 
 though none as yet has been established. 
 
 It cannot be said, therefore, that the out- 
 bursts of light in a new star are so immensely 
 greater than those in some of the variables, 
 that they must without doubt be ascribed to 
 different causes. Yet it is not easy to assign 
 them to the same cause, because the spectra 
 of new stars are very different from the spectra 
 of long period variables. The latter, when 
 they are bright, show exceedingly brilliant lines 
 of hydrogen in their spectra. The former 
 show the hydrogen lines, but they show also 
 other lines, quite as bright, whose origin is 
 unknown. And most curious and inexplicable, 
 each principal bright line is accompanied by 
 a dark absorption line alongside it, which is 
 the characteristic spectrum of all new stars. 
 It is not, however, quite unique. There is 
 one star in the constellation Cygnus which 
 has it permanently, and this one star is a 
 great asset, because it suggests, nay compels 
 us to believe, that the new star spectrum is 
 not necessarily associated with a catastrophe. 
 Some day, perhaps, we shall see a way of 
 
176 ASTRONOMY 
 
 combining these facts into a single story, and 
 of finding the new stars a proper place at the 
 extremity of our classification of variables. 
 
 What would make this the more natural 
 is the fact, well established, that the longer the 
 period of a variable star, the less is it disposed 
 to follow regular laws ; the shorter the 
 period, the more precisely does the star repeat, 
 time after time, its fluctuations in light. So 
 great has been the amount of information 
 collected within the last few years, so compli- 
 cated are the classifications proposed to fit 
 the observed facts, that within the limits of a 
 book like this we can do no more than indicate 
 how wide is their variety. One has classes of 
 stars which increase quickly and diminish 
 slowly ; others, less numerous, which do the 
 reverse. Some have irregularities during 
 the ascent, some during the descent Some 
 have double, unequal maxima, separated by 
 equal minima. Some have prolonged maxima, 
 and minima quickly passed ; in others the 
 minimum is prolonged, and the maximum 
 no sooner reached than the light begins 
 again to decline. Of all these types there 
 is only one that is certainly and completely 
 explained the Algol-type of eclipsing vari- 
 able, named from the bright star Algol in 
 
THE STARS 177 
 
 Perseus which was first remarked and has 
 been most completely studied. In this type 
 the star is normally at full brightness at 
 regular intervals its brightness fails and quickly 
 recovers. It is now fully established that this 
 is due to an eclipse. The star has a dark 
 companion revolving round it, like a gigantic 
 planet, very close. The plane of the com- 
 panion's orbit is so placed that our sun lies 
 nearly upon it, from which the consequence 
 follows that at each revolution the star is 
 partially eclipsed. 
 
 A particular interest attaches to these 
 Algol stars, because Algol was the first member 
 of the class which has now become enormously 
 important under the name Spectroscopic 
 Binaries. A binary star is a double star in 
 which the components are observed to revolve 
 about their common centre of gravity. We 
 shall deal presently with the cases in which 
 both stars can be seen in the telescope. A 
 spectroscopic binary is a star which under 
 the highest telescopic power remains single, 
 but which with the spectroscope is proved 
 to be a pair of stars in revolution. To this, 
 the most remarkable of all the doings of 
 the spectroscope, we must devote a brief 
 explanation. 
 
178 ASTRONOMY 
 
 The position in the spectrum of any line, 
 bright or dark, depends upon the number per 
 second of the corresponding waves of light 
 which enter the spectroscope. If the star 
 and the spectroscope are approaching one 
 another, the number of the waves of light 
 from the star which the spectroscope meets is 
 naturally increased ; if the star is receding, 
 the number is diminished. It follows from 
 this that if a star is alternately advancing and 
 receding, the lines in its spectrum are shifted 
 alternately towards the violet and the red ends 
 of the spectrum. The shift can be measured 
 and the measurement can be translated into 
 the statement that the star's velocity is 
 variable by so many miles per second. It was 
 in this way that Vogel showed that Algol 
 receded and advanced exactly in the way that 
 it should do if it were revolving about the 
 common centre of itself and a companion. 
 
 In the Algol pair the companion is dark, 
 or at least gives very little light compared 
 with Algol itself. But this is not always so. 
 Somet mes both stars of a spectroscope binary 
 are bright. Then the spectrum belonging to 
 the one is shifted towards the violet while the 
 spectrum of the other is shifted towards the 
 red ; in this case all the lines in the compound 
 
THE STARS 179 
 
 spectrum belonging to both components are 
 alternately double and single. Such a doub- 
 ling was recognized by Pickering in two stars 
 in the year 1889, about the time that Vogel 
 published his observations of Algol. 
 
 Since then an immense piece of work has been 
 done in some of the principal observatories 
 of the world, in examining all the brighter 
 stars one by one, to search for variable 
 velocities ; and a surprising degree of success 
 has rewarded this search. One star out of 
 every four or five examined proves to be a 
 spectroscopic binary, and the proportion 
 seems to grow steadily larger which after all 
 is to be expected when one remembers that 
 the appliances for detecting variable velocity 
 grow year by year more powerful and more 
 delicate. Remarkable instrumental refine- 
 ments are demanded in this work. A great 
 step forward was made by Campbell at the 
 Lick Observatory, when he devised means for 
 keeping his spectroscope at a constant 
 temperature during the hours required for 
 photographing a spectrum. The most perfect 
 apparatus at work is probably that attached 
 to the Victoria telescope of the Royal Obser- 
 vatory at the Cape, where by a system of 
 electrical control the prism box can be 
 
180 ASTRONOMY 
 
 maintained at a constant temperature, 
 automatically, for as many days or weeks as 
 may be desired. 
 
 The time is not yet ripe for the formation 
 of an opinion as to the final significance of 
 this remarkable fact, the large proportion of 
 spectroscopic binaries among the stars. From 
 one point of view it is a triumphant vindication 
 of the mathematicians, who have shown how 
 easily an elastic globe of gas, like a star, may 
 become unstable, and after passing through 
 a sequence of changes become of a pear-shaped 
 figure which is preliminary to separation into 
 two parts. From the mathematical analysis 
 of the problem this ought to happen some- 
 times ; observation has now shown that it 
 happens very frequently once out of three or 
 four times, perhaps. From another point of 
 view the discovery of so many spectroscopic 
 binaries disturbs the idea, drawn from our 
 own solar system, that the function of a star 
 is to nourish with heat and light a family of 
 planets. The terrible problem of the motion 
 of a planet round a pair of suns has not yet 
 been solved, but it seems quite unlikely that 
 such a planet could pursue an equable way 
 conducive to the development of life upon 
 its surface. 
 
THE STARS 181 
 
 The same method which allows us to detect 
 variability in the motion of a star, allows us 
 to measure the velocity, whether variable or 
 not, with which that star is approaching or 
 receding from the sun ; while, when enough 
 of these velocities have been determined, we 
 may derive at once the direction and the 
 velocity of the sun's motion in space. It 
 will be remembered that other methods gave 
 us a determination of the direction, but were 
 not powerful to show the velocity. The 
 spectroscope possesses what the other methods 
 lacked, and makes it evident that the sun, 
 with all the planets, is moving with a speed of 
 some twelve miles a second towards the 
 neighbourhood of the constellation Lyra. 
 By a very recent extension of this work 
 Hough and Halm at the Cape have been able 
 to separate the spectroscopic, or radial, 
 velocities of the stars into several drifts, 
 partly confirming and partly denying the 
 evidence obtained from proper motions, to 
 which we have already referred. The subject 
 is too new and too complex for discussion here, 
 where one may not talk the simple mathe- 
 matical language in which alone these things 
 become intelligible. 
 
 Now to return to those binary stars which 
 
182 ASTRONOMY 
 
 are visibly double, and which can be measured 
 and studied by the telescope without the aid 
 of the spectroscope. Sir William Herschel 
 began to measure such pairs in the hope that 
 he might detect a yearly oscillation of one 
 star with respect to the other, from which, 
 in the way we have already discussed, the 
 distance of the brighter might be determined. 
 It did not occur to him that the two stars of 
 a pair, though of very different magnitudes, 
 might be at the same distance, and in fact a 
 pair of suns. His measures served, however, 
 as the foundation of what has become a great 
 branch of stellar astronomy, the measurement 
 of double stars and the calculation of the 
 orbits of all those pairs which prove to be in 
 revolution. When it was first shown that 
 such pairs obeyed the laws of gravitation in 
 their motions, it seemed a great thing to have 
 proved that those laws were no local regula- 
 tions of the solar system, but had universal 
 force. Now that the spectroscope has shown 
 that the most distant stars are formed of very 
 much the same materials as the sun and the 
 earth, we should find it strange if they were 
 not subject to the same laws of gravitation. 
 It would, in fact, be more interesting if one 
 could find a pair of stars whose motions were 
 
THE STARS 183 
 
 demonstrably not in accordance with those 
 laws ; but this has never been done. When 
 irregularities of motion exist, they can be 
 explained very plausibly as due to the dis- 
 turbing action of some third, invisible body. 
 
 The measurement and calculation of binary 
 stars is a relatively simple matter, requiring 
 a good telescope without over elaborate acces- 
 sories, but requiring above all a perfect 
 eye and delicate hand for its successful 
 prosecution. The days are not long past when 
 a man with a genius for double star work 
 could pick up new discoveries with a compara- 
 tively small instrument. Within recent years 
 the northern hemisphere of the sky has been 
 so thoroughly searched by the great telescope 
 on Mount Hamilton, that it has become hard 
 to find anything which that telescope has 
 overlooked. The southern hemisphere* is not 
 nearly so thoroughly explored, and very much 
 remains to be done there in double stars, as 
 in many other branches of astronomy. 
 
 Yet it must not be imagined that there is 
 not plenty of work left for telescopes in the 
 north. The very close pairs, which demand 
 large telescopes for their observation, demand 
 also very frequent observation, for many of 
 them are in rapid motion, completing their 
 
184 ASTRONOMY 
 
 revolutions in relatively few years. The 
 wider pairs, which are easier to measure, are 
 less likely to be in rapid motion, but must not 
 on that account be neglected. Still wider 
 pairs may not be in orbital motion at all, but 
 merely two stars, very far apart, which chance 
 to be in nearly the same line of sight from the 
 earth. None the less, they are worth measuring 
 from time to time, for these measures serve 
 to detect straightforward proper motion in 
 the nearer or faster star. 
 
 One might reasonably enquire how such 
 measures could be valuable, since only the 
 difference between the motions of the two 
 stars could be observed. The answer depends 
 upon the view which one may take as to the 
 " background " of stars, as we have come to 
 call it in recent years ; and different astron- 
 omers view the background in different lights. 
 Some there are who hold the view that none 
 of the stars may be called fixed : that all are 
 in relative motion, which no one can deny, 
 and more, that the motions of all are probably 
 large enough to be distinguishable after a few 
 centuries of observation. Others there are 
 who see things in a different perspective : 
 they feel that the stars which show any 
 measurable motion are relatively few, and 
 
THE STARS 185 
 
 comparatively near the sun, while the great 
 majority of the fainter stars lie at distances 
 so vast that for a few centuries they may be 
 considered fixed. The time has not come to 
 decide between these two opinions, which 
 affect, nevertheless, for better or for worse, 
 a great many of the programmes of work 
 laid out for the next twenty years. In 
 particular they affect any estimates which 
 may be formed of the ultimate value of that 
 magnificent scheme, the Photographic Chart 
 and Catalogue of the sky, planned in 1887 at 
 Paris, and pursued since then at eighteen 
 observatories. To appreciate the bearings 
 of some of the questions involved we must go 
 back to star cataloguing, forget the stars' 
 physical individualities, and regard them once 
 more as mere points upon the celestial sphere. 
 Since the first star catalogues were *made 
 before the invention of the telescope, they were 
 necessarily limited to the stars visible to the 
 naked eye ; and since it was desired to observe 
 stars in twilight, to obtain the place of the sun, 
 the brightest stars were naturally selected 
 for especial care. The possibility of observing 
 bright stars with a telescope in full daylight, 
 and the necessity of so doing, have maintained 
 them in the select list of fundamental stars, 
 
186 ASTRONOMY 
 
 in spite of the fact that a very bright star is 
 much harder to observe than a moderately 
 faint one, and the observation affected with 
 particular errors. It thus happens that all 
 successive catalogues of precision, as they are 
 termed, start with the brightest stars as 
 fundamenta , and refer the fainter stars to 
 them. And further, it follows that the most 
 important quantities in astronomy, such as 
 the constant of precession, or the obliquity of 
 the ecliptic, rest at the bottom upon the places 
 of a small list of bright stars, nearer to us, no 
 doubt, than the average fainter star, and 
 affected by larger proper motions. 
 
 Now it is practically impossible to observe 
 with the meridian circle the instrument used 
 for star cataloguing stars fainter than mag- 
 nitude nine and a half ; while the number of 
 stars down to that limit is already so large 
 that it has been found impossible to deal 
 with more than a selection of them. For 
 the places of stars fainter than this, we must 
 rely on measurements made on photographs. 
 But a photograph will not determine the 
 absolute places of stars, as the meridian circle 
 can. It will do no more than refer the faint 
 stars to such brighter stars already catalogued 
 as may happen to be upon the plate. Thus 
 
THE STARS 187 
 
 the faint stars are referred to others of inter- 
 mediate brightness, and those again to the 
 bright fundamental stars. There are several 
 steps, at each of which it is easy to accumulate 
 error indeed very hard, if not impossible, to 
 avoid doing so. 
 
 One cannot help feeling that this process of 
 working down from the bright to the faint, 
 from the foreground to the background, 
 though it was doubtless an inevitable process, 
 is nevertheless far from being ideal. It 
 would have been far more satisfactory to 
 begin at the background, nearly if not quite 
 immovable, and to work up to the shifting 
 foreground ; the precise gain depends of course 
 in great measure on the degree of assurance 
 that there really is a background practically 
 fixed. Such advantage we may gain by using 
 photography. Consider two plates taken fifty 
 years apart, showing a f ew stars of intermediate 
 brightness and a great number of faint ones. 
 We wish to find what movements have taken 
 place among those stars in the course of the 
 fifty years. Two courses are open. We can 
 fit each plate on to the celestial sphere by 
 means of the few intermediate stars, whose 
 relations to the fundamental stars are more 
 or less imperfectly known ; or on the other 
 
188 ASTRONOMY 
 
 hand we may fit the two plates together 
 directly by the large number of faint stars, so as 
 to discover the degree of misfit of the brighter. 
 Without entering further into details, it may 
 be clear that in the first instance we are 
 working down from bright to faint ; in the 
 second we are working up from faint to bright. 
 
 For some years it has been the belief of the 
 author that we have in the second a method 
 of great power and promise, capable of 
 avoiding most of the snares and pitfalls which 
 beset the first course ; and certain to give us 
 independent information of the most valuable 
 kind, so soon as a sufficient time shall have 
 passed since photography was applied to 
 recording the places of the stars. In the great 
 Astrographic Chart and Catalogue we have the 
 material for putting this method to the test. 
 
 The original idea was to make a photo- 
 graphic map of the sky. The brothers Henry, 
 who combined in a singular way the dual 
 roles of instrument makers and astronomers, 
 had constructed about the year 1884, and had 
 used with great success, a photographic 
 telescope at the Paris Observatory. Filled 
 with enthusiasm at the results obtained, the 
 astronomers of France called a congress at 
 Paris in 1887 to prepare plans for an inter- 
 
THE STARS 189 
 
 national undertaking : no less a scheme than 
 photographing the whole sky down to a 
 limit of magnitude 14. They proposed to 
 divide the sky into eighteen zones, and to 
 find eighteen observatories prepared to take 
 nearly equal shares in the business. 
 
 It was a remarkable feature of this enter- 
 prise that it was undertaken with hardly any 
 preliminary knowledge of the difficulties to be 
 overcome. The brothers Henry had shown 
 the possibility of charting the sky with an 
 instrument of a certain type, and the photo- 
 graphic dry plate, which had not been available 
 for earlier workers in the same field, was now 
 an established success. But there was no 
 general experience at the disposal of the Con- 
 gress, and few of its members had ever taken 
 an astronomical photograph. Under such 
 circumstances it was inevitable that .some 
 initial mistakes should be made, and that 
 resolutions should be formed and adopted, 
 only to be broken. Few would deny now that 
 it was a mistake to adopt the ordinary 
 photographic object glass for making a chart. 
 But out of the chart grew the much more 
 important project of the catalogue, to give 
 the places of all stars down to the eleventh 
 magnitude, measured upon the photographic 
 
190 ASTRONOMY 
 
 plates ; and for this purpose the instrument 
 actually adopted was probably the best. The 
 estimate of the time required to finish the 
 work was very naturally over sanguine. 
 At the time of writing (June, 1911), twenty- 
 four years after the inception of the scheme, 
 only one observatory Greenwich has fin- 
 ished and published its share of the chart ; 
 and two observatories Greenwich and Oxford 
 have practically finished their catalogues. 
 In other observatories great progress has been 
 made, and large sections have been published, 
 but the end is not yet in sight. 
 
 Putting aside the chart of stars to the four- 
 teenth magnitude whose utility is obvious 
 let us consider for a moment the great photo- 
 graphic catalogue of all stars down to magni- 
 tude 11, which when complete will contain 
 the places of some two million stars. Not one 
 of these places can be determined entirely 
 by photography ; all must depend upon a 
 sufficiency of stars on each plate observed in 
 the older way with the meridian circle. What 
 the adoption of photography for star cata- 
 loguing has done is therefore briefly this, 
 that it has relieved the meridian observers of 
 the necessity to build large instruments and 
 to strain after fainter and fainter stars. 
 
THE STARS 191 
 
 Speaking broadly, meridian observers may 
 now stop at some point about the ninth 
 magnitude, or rather brighter. If they can 
 supply the places of these stars, photography 
 can tie up to them all the fainter stars, practi- 
 cally to any degree of faintness desired. 
 
 But photography demands a very high 
 degree of precision in these stars of reference, 
 and meridian observers are hard put to it to 
 furnish the accuracy desired. The Astro- 
 graphic Congress has therefore to take into its 
 counsels all the men who are engaged on the 
 meridian observation, that by a united effort 
 of organization and economy, the whole 
 co-operation may proceed harmoniously to 
 the desired end. At the last meeting of the 
 Congress, in 1909, this enlarged scheme was 
 put into working order, and with it the 
 Astrographic Congress became in effect an 
 alliance binding a great part of the world's 
 astronomical forces truly a splendid develop- 
 ment of an undertaking to make a single 
 photographic chart of the sky. All honour 
 to the judicious guidance of the successive 
 directors of the Paris Observatory, to the 
 enlightened and liberal support of the French 
 Government, and to the powerful assistance 
 of the famous Academy of Sciences of Paris ! 
 
192 ASTRONOMY 
 
 And honour also to the participating observa- 
 tories, who with less powerful backing have 
 laboured to make the enterprise truly inter- 
 national, and each country's share worthy of 
 the whole. 
 
 The immediate aim of the International 
 Committee of the Chart of the Heavens is to 
 produce a chart and a catalogue. Its inevit- 
 able destiny is to produce, during the present 
 century, a second chart and catalogue, to 
 discover all the movements that are taking 
 place among the fainter stars. Then will 
 arise the question of the background, the 
 question which has been raised before in this 
 chapter. We have seen how the astronomy of 
 the stars has become involved with star drifts 
 and eddies, which threaten the foundations 
 of much that had been reckoned secure. It 
 is the great merit of the new photographic 
 method that it enables us to start at the other 
 end of the scale, when we are determining 
 the motions of the stars. The fashion has been 
 in the past, not by caprice but by necessity, to 
 make the faint stars depend upon the bright ; 
 at a future day the comparison of photographs 
 taken then, with photographs taken now, 
 will make it possible to regaj-i the bright 
 stars as passing hither and thither before a 
 
THE STARS 193 
 
 distant and nearly if not quite stationary 
 background. That, in the author's belief, is 
 the right way to look at the problem ; he will 
 be honest and confess that few agree with 
 him. 
 
 We cannot close a chapter on the stars 
 without reference to the attempt which has 
 been made to classify them in order of evolu- 
 tion ; yet there is little that a mere astronomer 
 can venture to say, when astro-physicists are 
 by no means agreed. The early work of 
 Secchi and Vogel suggested a classification of 
 stars by their spectra. Some gave light that 
 had suffered strong absorption in passing 
 through atmospheres of hydrogen, and appar- 
 ently of little else. In others, like our sun, 
 the vapours of heavier metals played a large 
 part in the absorption of light. A third series 
 had banded spectra, suggestive of lowered 
 temperatures. At first the classification was 
 merely for convenience, and was simple. As 
 instruments became powerful, and differences 
 of detail could be recognized, the old classifica- 
 tion was found inadequate, and new ones were 
 proposed, depending upon minutiae which are 
 for the expert to appreciate. When many 
 have been content to accumulate facts, a few 
 r e sought to arrange the stars, by their 
 
194 ASTRONOMY 
 
 spectra, in order,, and to say : These ar 
 young, and are still growing hotter ; these 
 have reached their greatest intensity ; these 
 others have passed their prime, and those 
 again are on their way to extinction. No one 
 doubts that such a classification will be 
 possible in the end ; but it is more than 
 doubtful if the end is yet in sight. 
 
CHAPTER VIII 
 
 THE NEBULAE 
 
 AT several points in the sky the unaided eye 
 can see something hazy and ill- defined, which 
 is certainly not a star, and scarcely looks like 
 a group of small stars close together. One is 
 in the sword of Orion, another in the constella- 
 tion Andromeda, a third in Hercules ; and 
 there are others in the southern sky. One at 
 least, that in Andromeda, had been known 
 to the Arabian astronomers, if not to the 
 Greeks. Galileo's telescope discovered that 
 in Orion, and Halley rather more than two 
 hundred years ago found the third. All three 
 are easily seen without telescopic aid when 
 one knows where to look. As telescopes were 
 gradually improved these patches of light were 
 carefully studied to see what could be made 
 of them, and the heavens were scanned to 
 discover more objects of the same kind. A 
 French astronomer, Messier, was foremost in 
 the search, so that in the year 1776 he was 
 
 G2 195 
 
196 ASTRONOMY 
 
 able to publish a catalogue of 103 of these 
 objects, and they are still for convenience and 
 brevity often referred to by their numbers in 
 this celebrated little catalogue. Some of these 
 Messier himself could see were nothing but 
 clusters of faint stars close together ; others 
 could not be resolved into stars, and these 
 were the nebulce proper (Latin, nebula 
 cloud). 
 
 The most famous and the most enduring 
 w r ork of the Herschels is their catalogue of 
 nebulae, recorded in the course of their survey 
 of the sky from pole to pole. When William 
 Herschel began his methodical examination of 
 the sky, field by field, scarcely more than a 
 hundred nebulae were known. When his son, 
 Sir John, long after his return from the Cape 
 and the southern skies, was able to bring 
 together into one general catalogue (dated 
 1864) every nebula and star cluster that was 
 known, their number exceeded five thousand ; 
 and all but a small fraction had been found by 
 his father and himself 
 
 The nebulae and the star-clusters were 
 catalogued together, for the time had not 
 ome to draw any clear line of distinction 
 between them. In early days it had often 
 happened that what had appeared to one 
 
THE NEBULAE 197 
 
 observer as a filmy patch of light had been 
 resolved into clearly separate stars by the 
 more powerful instrument of another. What 
 more reasonable than to suspect that every 
 nebula could be so resolved, had we telescopes 
 sufficiently powerful ? So far as was then 
 known, stars, and stars only, shine by their 
 own light. Until proof to the contrary was 
 obtained the simplest, and therefore the prefer- 
 able, explanation of the nebulae was to suppose 
 that they were clouds of stars, but so im- 
 mensely distant that the stars could not be 
 separated one from another. 
 
 Thus each builder of a greater telescope 
 strove after the resolvability of nebulae. But 
 the nebulae in general refused to be resolved. 
 Some of their secrets they gave up, one by 
 one. It became more and more evident that 
 they were by no means all of the same 'kind. 
 In some the light was curdled, like the filmiest 
 cirrus cloud high up on a fine summer day ; 
 others were made of wisps and streaks like a 
 gauze veil torn to shreds ; others again were 
 clear cut and sharp in outline, like pale ghosts 
 of planets. Sharply separated from these 
 were other nebulae of a most remarkable form 
 first revealed by Lord Rosse's great telescope 
 at Parsonstown in Ireland nebulae in which 
 
198 ASTRONOMY 
 
 spiral arms were wound about a central 
 condensation. So exceedingly difficult to see, 
 so much more difficult to understand and 
 explain was this complicated structure, that 
 astronomers with feebler instruments were not 
 unnaturally at first a little disinclined to accept 
 as reality the fantastic spirals that Lord 
 Rosse and his assistants drew at the eyepiece 
 of the great telescope. That nebulae should 
 be resolved was indeed to be expected, but 
 that they should be resolved not into stars, 
 but into spiral arms still nebulae, was extra- 
 ordinary, almost beyond belief. 
 
 Somewhat after the middle of the nineteenth 
 century, then, the problem of the nebulae 
 was so far from being solved that it had 
 become much more complicated, and appar- 
 ently more hopeless of solution. The limits 
 of telescope construction had been reached 
 so far at least as mere power to gather 
 more and more light was in question. Great 
 indeed as has been the advance in instrument 
 construction since that day, the advance, as 
 we have already seen, has been along the lines 
 of greater perfection, and for sheer grasp of 
 light no telescope has yet surpassed that 
 constructed by Lord Rosse some seventy years 
 ago. The next great step in our knowledge 
 
THE NEBULA 199 
 
 of the nebulae was to come by the application 
 of a new principle, of analysing the light 
 collected by the telescope, of making it reveal 
 by that analysis the nature of the source from 
 which it had come. 
 
 We have already seen something of how this 
 spectrum analysis is applied to the stars, and 
 how it has shown that the stars are suns, and 
 that they are like our own sun in being 
 surrounded by layers of vapours which absorb 
 and cut out certain of the rays, so revealing 
 what substances they are which are vaporized 
 above the glowing surface. The light of the 
 stars shows on analysis that most of the rays 
 are present, but that some have been absorbed 
 in their passage through the overlying vapours 
 in more technical language, we have learned 
 in a preceding chapter that stars give a 
 continuous spectrum crossed by dark absorp- 
 tion lines. Here then we have the power to 
 discover if the nebulae are merely masses of 
 stars at a great distance. 
 
 The crucial experiment was made by Sir 
 William Huggins. In the year 1864 he 
 turned his spectroscope to a nebula in the 
 constellation Draco and read its secret at a 
 glance. The light was not starlight, but 
 something very different ; light which came 
 
200 ASTRONOMY 
 
 from glowing gas, shining in space by itself. 
 In a short while came the explanation of the 
 fact that the nebula in Orion could never be 
 resolved into stars. It was not made of stars 
 but was an immense cloud of self-luminous 
 gases of hydrogen, and the gas which we 
 now know as helium, which since then has 
 been found in small quantities upon earth, 
 and another gas which is still unknown except 
 by the green ray which it contributes to the 
 spectrum of the gaseous nebulae. 
 
 We can easily picture to ourselves the 
 enthusiasm with which this achievement was 
 received. Here was a difficulty which had 
 puzzled several generations of astronomers 
 now completely solved, and by the simplest 
 of means. Further, the discovery gave us the 
 new conception, that a simple gas, not 
 condensed about a solid body, but free in 
 space, and excessively rarefied, can shine by 
 its own light and let us see in what arrange- 
 ment it is disposed. True, the conception is 
 not without difficulties, for we are unable to 
 imitate in our laboratories the physical 
 condition of the gas which is thus self-luminous 
 in space. A very small quantity of hydrogen 
 can be enclosed in a tube and excited by an 
 electric discharge so that it emits light of the 
 
THE NEBUUE 201 
 
 same kind as that which comes to us from 
 the hydrogen in a nebula. But it is altogether 
 impossible to suppose that the hydrogen in 
 the laboratory tube and the hydrogen in the 
 nebula are under conditions even remotely 
 similar. The former is rarefied, as we under- 
 stand rarefaction in the laboratory, but it is 
 immensely more condensed than the latter. 
 The former is comparatively warm ; the latter 
 is exposed to the absolute cold of space. The 
 former is put into a state of intense agitation 
 and made to glow by the electric discharge; 
 the mechanism which excites the latter may 
 be electric, but we have no knowledge as to 
 how it is operated. When, therefore, we are 
 told that the spectroscope has shown the 
 Orion nebula to be a mixture of several self- 
 luminous gases, we are elated for a moment 
 at the brilliance of the discovery, but the'next 
 moment we are sobered by the reflection that 
 the solution of one problem has but proposed 
 to us several new ones, each more difficult to 
 solve. 
 
 Let us pause for a moment and make some 
 attempt to realize precisely what we mean 
 when we say that the density of the Orion 
 nebula must be inconceivably small. It 
 covers quite a considerable area of the sky. 
 
202 ASTRONOMY 
 
 The part that can be photographed readily is 
 roughly half a degree in diameter the appar- 
 ent breadth of the moon. And it is immensely 
 distant. How distant we have not at present 
 any means of determining ; but we shall 
 probably be well within the mark if we take it 
 at 150 times the distance of our nearest 
 neighbour among the stars. Now the sun is 
 about one and a quarter times as dense as 
 water, or about 1000 times as dense as the air 
 under average conditions at the surface of the 
 earth. A simple calculation shows us that if 
 we assume the Orion nebula spherical its 
 volume, compared with the volume of the 
 sun, is expressed by the number 58 followed 
 by twenty-one noughts. Suppose now that we 
 took a thousand million suns, each as large as 
 our own, and expanded them until they were 
 of the density of air. They would then occupy 
 a space equivalent to a million million of our 
 sun's. That would cancel twelve of the above 
 twenty-one zeros, and we are left with the 
 result that a thousand million suns expanded 
 into the size of the Orion nebula would have 
 a density one fifty-eight thousand millionth 
 that of air at sea level upon the earth. And 
 even this density, inconceivably small as it is, 
 is probably far too great. There are good 
 
THE NEBULAE 203 
 
 grounds for believing that the whole amount 
 of matter within range of our telescopes does 
 not equal a thousand million suns. It is 
 vastly improbable that any sensible fraction 
 of that mass is contained in the Orion nebula. 
 Hence we can hardly doubt that the real 
 density of the gas in the nebula is immensely 
 less than what we have found above, and we 
 may begin to realize how powerless are our 
 brains and our language to express what is 
 really involved in the statement that the 
 Orion nebula is a region of glowing gas. 
 
 But it is time that we took up again the 
 thread of our story. And we must not run 
 away with the idea that all the nebulae are 
 gaseous, for this is far from being the case. 
 We have pictured to ourselves the delight 
 with which Sir William Huggins examined his 
 first nebula and found that it was gaSeous. 
 We may now imagine with what surprise he 
 found, on turning to the great nebula in 
 Andromeda, that it was not gaseous. Gaseous 
 nebulae, indeed, have proved to be the excep- 
 tion rather than the rule. Of the many 
 thousand nebulae which are now catalogued, it 
 is probable that only a few hundred are 
 gaseous. We must be careful to say, probable, 
 for the truth is that most nebulae are so faint 
 
204 ASTRONOMY 
 
 that their light is not sufficient for analysis. 
 We may, however, very properly argue 
 something from appearance ; and when we find 
 that the nebulae which are certainly gaseous 
 differ very much in appearance from those 
 which the spectroscope seems to tell us are 
 certainly not ; when further we find that a 
 very great majority of the nebulae belong in 
 appearance to the latter class ; then we may 
 say with some confidence that the majority, 
 the so-called white nebulae, are very different 
 from the small minority, the gaseous green 
 nebulae. 
 
 The power to discriminate, to classify the 
 nebulae by their apparent structure, has come 
 with the employment of photography. The 
 eye and the pencil are incompetent in their 
 attempts to portray such very faint and deli- 
 cate objects ; the eye cannot distinguish with 
 any certainty what even in the largest tele- 
 scopes is on the borders of invisibility ; the 
 pencil of the most skilful draughtsman fails to 
 represent those slightest variations of light and 
 shade which are all important. How well the 
 sensitive photographic place succeeds where 
 eye and pencil fail may be seen in any one of 
 the many collections of astronomical photo- 
 graphs which are now available, and to which 
 
THE NEBULJE 205 
 
 reference will be made in the list at the end 
 of this volume. 
 
 We have spoken already of the famous 
 achievement of Lord Rosse's telescope the 
 discovery of the spiral nebulae. We have 
 spoken also of the natural half incredulity 
 with which this surprising discovery was 
 received. It was embarrassing, this complica- 
 tion of structure, so unlike anything else in 
 heaven or in earth. But there was the 
 consolation that the strange spiral form was 
 rare and exceptional. It was curious and 
 interesting and most impossible to explain, yet 
 at any rate there was no reason to suppose, 
 twenty years ago, that the spiral nebula was 
 a common feature of the universe. Only a 
 few were known, and if they could not be 
 understood, they could at least be set apart 
 as rare creatures, outside the ordinary run of 
 things. 
 
 By the application of photography this 
 comfortable belief was shattered. In the 
 early eighties Common showed that with the 
 invention of the dry plate nebular photography 
 had become possible. By the middle eighties 
 Isaac Roberts had begun his steady syste- 
 matic photography of all the brighter nebulae ; 
 ten years later Keeler with the Crossley 
 
206 ASTRONOMY 
 
 reflector at the Lick Observatory made a 
 great advance towards perfection of results ; 
 and in the last few years Ritchey has shown 
 what great results can be obtained by courage 
 and consummate skill in instrument construc- 
 tion. 
 
 The workers in this field of photography 
 have not been many, for it demands uncommon 
 qualities of skill and persistence. Even with 
 the rapid plates of to-day the exposures must 
 run into hours. Be the instrument ever so 
 perfectly built and driven, it is still required 
 that the astronomer shall keep incessant 
 watch upon a guiding star, to ensure that there 
 shall be not the slightest wandering and 
 consequent deformation of the images upon 
 the plate. To maintain this exacting watch 
 night after night, year in and year out, 
 whenever the sky is clear and the moon is 
 absent, makes demands on resolution and 
 temper which few can meet. Hence the 
 successful photographers of nebulas may be 
 counted on the fingers. When they have 
 overcome the difficulties of tuning up their 
 instruments to the highest pitch, they have 
 still to contend with the more formidable 
 difficulties of the weather ; and until one 
 has tried one will scarcely believe how difficult 
 
THE NEBULAE 207 
 
 it is to obtain four complete hours of serene, 
 transparent sky. The hours spent upon the 
 plates which succeed bear but a small ratio 
 to the hours that are wasted by unsteadiness 
 of the air, by premature clouding, by dewing 
 of the mirror, or by the hundred other ills 
 that vex the soul of the celestial photographer, 
 especially in those pleasant but changeable 
 climates where civilized man has generally 
 made his home. 
 
 But the revolution in our knowledge of the 
 nebulae which these photographs have made 
 is very remarkable. At the very outset they 
 confirmed to the full the accuracy of Lord 
 Rosse's early observation, that many of the 
 brightest and finest nebulae are formed of 
 spiral arms, generally two. They spring from 
 a central condensation at two diametrically 
 opposite points, wind with more or less 
 perfectly regularity around the centre, and are 
 studded along their length with bright knots 
 that look as if the nebulous stuff were there 
 condensing into stars. Year by year the 
 number of these spirals has grown. They 
 have rapidly outnumbered the gaseous 
 nebulae ; it begins to look as if the chances are 
 in favour of nearly all the white nebulae 
 proving themselves spirals, so soon as they 
 
208 ASTRONOMY 
 
 may be successfully photographed. And most 
 remarkable of all, nearly every plate of a 
 large spiral shows a number of smaller 
 uncatalogued nebulae of which many are spiral 
 also ; there are even regions in the sky where 
 the nebulae are more plentiful than the stars. 
 It would be premature to estimate the 
 number of the nebulae which are within 
 reach of existing telescopes and plates ; but 
 it seems likely that the number will run into 
 hundreds of thousands. Whoever essays to 
 fashion a theory of the universe structure 
 must be prepared to reckon with a hundred 
 thousand spiral nebulae ; and at the same 
 time he has to admit that he knows scarcely 
 anything of them beyond their shapes and 
 their apparent places in the sky. How far 
 away there are, how large they are, of what 
 they are made, whether movement is in 
 progress in those spiral arms, whether the 
 knots are becoming more condensed, whether 
 the condensation will eventually produce a 
 real star, no one can say. No one has as yet 
 formulated any mathematical theory of the 
 structure ; no one has any real grounds for 
 affirming that the spiral nebulae are members 
 of our stellar universe ; still less, in the opinion 
 of the author, is it legitimate to suppose that 
 
THE NEBULAE 209 
 
 they are distant universes of stars outside 
 our own. 
 
 This hypothesis that each spiral nebula is a 
 distinct and separate universe of stars, 
 comparable perhaps with the whole system 
 which forms the Milky Way, has undoubtedly 
 a grandiose attraction. But there is a good 
 scientific rule that an hypothesis which cannot 
 be supported or tested by facts, is not a very 
 valuable asset. Unfortunately in this case 
 there are hardly any facts available. Consider 
 first what the spectroscope has to say as to 
 the spirals. In the few cases where there 
 is light enough to make a proper examination, 
 the quality of the light seems to be the same 
 as that of the stars : technically, their spectra 
 are continuous, with dark absorption lines. 
 Were we sure that the source was intensely 
 hot, as the stars are, we might take this as 
 proof that the spirals are really formed of 
 condensed bodies something like stars. But 
 it is a mistake to suppose that white light 
 necessarily has a hot source. The glow-worm, 
 the firefly, and the innumerable minute but 
 intensely luminous creatures that live in the 
 surface layers of the sea are sufficient to prove 
 the contrary. Hardly anything is known, it 
 seems, of the way in which these creatures 
 
210 ASTRONOMY 
 
 produce their light ; but it is so completely 
 out of proportion to the size of their mechanism 
 that we may be sure it is produced very 
 economically, with but little expenditure of 
 energy. If then we prefer caution to extrava- 
 gance, we may very well prefer to believe 
 that the spiral nebulae shine white in a way that 
 we do not understand, rather than jump to 
 the conclusion that they must be masses of 
 distant and perhaps only partly developed 
 stars. 
 
 The distribution of the spiral nebulae is very 
 curious. In some parts of the sky they are 
 as numerous as the stars ; in others they seem 
 to be entirely absent. In the northern sky 
 they are found in greatest number where 
 the stars are few, far removed from the region 
 of the Milky Way, and from this fact some 
 have been led to draw far-reaching conclusions 
 which we shall find it more convenient to 
 discuss in a later chapter. For the moment 
 let us confine ourselves to bare facts, which 
 are a little difficult to present without the aid 
 of elaborate diagrams. We must appeal to 
 the reader's knowledge of the geography 
 of our terrestrial sphere, to provide a rough 
 illustration of the way in which the nebulae 
 are arranged upon the celestial. The Milky 
 
THE NEBULAE 211 
 
 Way may be considered very conveniently 
 as the equator of the sky ; the region of the 
 Milky Way we may call the celest al tropics. 
 What then of the spiral or white nebulae ? 
 They are decidedly arctic. It is somewhat 
 the fashion to speak of them as so peculiarly 
 arctic in their distribution that they are 
 definitely condensed at the pole, and become 
 fewer and fewer as one goes towards the 
 equator. It seems, however, to the author 
 that the facts are against this very formal 
 view, and that we shall be very much nearer 
 the truth if we liken the distribution of the 
 spiral nebulae to the distribution of perpetual 
 snow upon the earth. Perpetual snow, no 
 doubt, is rather conspicuously related to the 
 poles, but it is very far from being distributed 
 regularly with regard to them. Greenland 
 and Iceland make big unsymmetrical patches 
 on one side ; on another the Rockies carry the 
 snow down towards the equator. The Cau- 
 casus and the Himalayas, the Andes, Ruwen- 
 zori, and the snow mountains of New Guinea 
 give us patches of ice far down into temperate 
 and tropical latitudes. And how does our 
 comparison fare in the south ? We have not 
 much perpetual snow in the southern hemi- 
 sphere of the earth, because there is not much 
 
212 ASTRONOMY 
 
 land until we reach the Antarctic continent. 
 We do not for the moment want it to represent 
 the spiral nebulae of the southern sky, because 
 as a matter of fact we know very little about 
 them. There is not as yet a photographic 
 reflecting telescope at work in the south. 
 
 Having once established this idea of speaking 
 geographically of the white nebulae, we shall 
 find it convenient to deal with the green or 
 gaseous nebulae in the same way. And here 
 the facts are much more simple, for the gaseous 
 nebulae are entirely tropical ; scarcely any 
 big ones, and no little ones at all are found 
 outside the tropics. This rule is true of the 
 small planetary or almost stellar globes of 
 gas which have of late years been found in 
 considerable numbers at the Harvard College 
 Observatory. It is even more conspicuously 
 true of the immense extended nebulae in the 
 Milky Way which are represented on the 
 photographs of Professor Barnard and Pro- 
 fessor Max Wolf. These objects are much 
 the largest things in the sky, extending each 
 over quite a number of square degrees. To 
 call them nebulae hardly does justice to their 
 size, as indeed Sir William Herschel must 
 have felt when he brought them together in a 
 special list, not of nebulae, but of " regions 
 
THE NEBULAE 213 
 
 affected with nebulosity." Of late years this 
 happy phrase has been revived, and for the 
 first time they have been worthily portrayed. 
 Before the advent of photography this was 
 quite impossible, for the simple reason that 
 they are too big to be seen in a visual telescope, 
 very much in the same way that the zodiacal 
 light and the Gegenschein are also too big. For 
 it must not be overlooked that the field of 
 view of a telescope is practically limited to a 
 single degree, and in general the larger the 
 telescope the smaller the field of view. Hence 
 it is no paradox to say that an object is too 
 big to be seen in a telescope. 
 
 When a nebula more than fills the field of 
 view, one is conscious only that the back- 
 ground is dimmed and that the faint stars 
 appear dulled for want of contrast. But by 
 sweeping the instrument backwards " and 
 forwards one is able to trace out the nebulous 
 region to this extent, that one can say : Here 
 the background is clear and black ; there it is 
 full of vague light and the stars are dulled. 
 The camera, with its big field ten or fifteen 
 degrees square, has superseded this unsatis- 
 factory groping, and is now able to let us 
 stand back, so to speak, and take in at a 
 glance these wonderful regions. Let us 
 
214 ASTRONOMY 
 
 imagine ourselves looking at one of Professor 
 Barnard's pictures of the Milky Way in the 
 constellation Scorpio, near the star lettered p . 
 We are struck at once by a curious dark lane, 
 almost empty of stars, which runs across the 
 plate for several degrees, and appears to be 
 blacker than the background of sky. For a 
 moment we wonder how this is possible, and 
 then we notice that over the greater part of 
 the plate the background is not really black 
 at all. We are in one of the regions affected 
 with nebulosity. 
 
 To obtain the spectrum of such a very faint 
 and extraordinarily diffuse object has proved 
 impossible up to the present ; but all analogy 
 is on our side if we assume that the nebulosity 
 is of the gaseous kind. There are, therefore, 
 immense regions of space filled with a luminous 
 haze ; and we may go back to our geographical 
 comparison to find a parallel. On the high 
 veldt in South Africa the air is marvellously 
 clear, and it is almost impossible for a visitor 
 from misty England to realize that mountains 
 apparently just outside the town are really 
 fifty miles away. But further north, in 
 tropical Africa, the case is very different. 
 There the air is almost continually filled 
 with a thick haze, the Harmattan, which is 
 
THE JNEBUL.E 215 
 
 eloquently described in the doleful reports of 
 the surveyors. So it is in the sky. Away 
 from what we have called the tropics, the 
 Milky Way belt, the background is clear ; 
 but extensive regions in the tropics are filled 
 with a luminous haze which partially dims the 
 stars. And what are we to make of those 
 dark lanes which interpose apparently between 
 us and the background, blotting out haze and 
 stars alike ? They remind us that space may 
 be filled with the invisible as well as with the 
 visible, and they introduce us to the difficult 
 question of the absorption of light in space. 
 Whether it is possible to detect a universal 
 absorption of light or want of transparency 
 we must discuss in another chapter. But 
 we shall do well to learn from these extensive 
 nebulae that there is plenty of evidence of 
 local fogs ; and we have a ready means of 
 deciding whether the dark fog is gaseous 
 the same gas as the luminous part of the 
 nebulae or solid dust. The former may cut 
 out the nebula, but cannot do much towards 
 dimming the stars. The latter will be equally 
 effectual on both ; and since both nebulae 
 and stars are in some cases equally affected 
 we shall have no difficulty in coming to the 
 conclusion, at least provisionally, that the 
 
2ie ASTRONOMY 
 
 dark lanes in the Milky Way are due to 
 something like dust interposed between us and 
 the stars. 
 
 In one celebrated case we had independent 
 evidence of the existence of such streams of 
 dust. Many will remember the " new star 
 of the new century " which blazed up in the 
 constellation Perseus early in 1901. In the 
 space of a few hours it rose from insignificance 
 to a splendour greater than any other star in 
 the northern sky, and within a few weeks it 
 was gone again. This veritable light explo- 
 sion sent a blaze of light speeding through 
 space at the rate of nearly two hundred 
 thousand miles a second, able to illuminate 
 suddenly anything that came in its path. 
 And something of the kind was seen to happen. 
 The direct flood of light reached us early in 
 the year. In the autumn it was discovered 
 that at some distance from the star now 
 grown comparatively dim patches of nebulae 
 were appearing, and moving away from the 
 star. Now motion in a nebula was unheard of, 
 and motion so quick as this was unimaginable, 
 unless it was the motion of the light itself 
 that we could observe. The explanation is 
 not altogether free from difficulty, but it 
 seems to the author that the moving nebula 
 
THE NEBULAE 217 
 
 round Nova Persei is best explained by the 
 idea that the light explosion travelling out- 
 wards illuminated drifts of celestial dust as it 
 passed, and thus allowed us to see them for 
 the moment. 
 
 In a chapter dealing with the nebulae it 
 would be impossible to pass in silence the 
 celebrated Nebular Hypothesis ; and yet it is 
 not clear that there is much of value to be 
 said at the present time in a work such as this. 
 For in recent years the subject has been highly 
 controversial. Laplace, the eminent author 
 of the hypothesis, never worked it out in 
 detail ; he contented himself with sketching 
 in a general kind of way how a sun and a 
 system of planets circling round him might be 
 formed by the condensation of a nebula. On 
 the mathematical side the difficulties are 
 formidable, and they do not tend to disappear 
 with years. It is natural, then, that specula- 
 tive astronomers should be on the look-out 
 for some visible example of the processes 
 which Laplace imagined ; that they should 
 scrutinize the details of the nebulae as they 
 are revealed by photography ; and should 
 seize on this or that detail as a verification of 
 the hypothesis. Now to make a system of 
 bodies out of one nebula, condensation must 
 
218 ASTRONOMY 
 
 take place about a number of centres. We 
 can find nothing of the kind in the gaseous 
 nebulae ; but the spirals are very conspicu- 
 ously condensed about centres which are 
 strung along the spiral arms like beads upon 
 a string. We cannot hope within a reasonable 
 time to see signs of the actual operation of the 
 process. There are, however, an immense 
 number of spiral nebulae in all apparent stages 
 of condensation, and it is not very difficult to 
 pick out a sequence which serves to illustrate 
 a supposed succession of stages in the process. 
 So far, so good. But the cautious astronomer 
 is more than justified in feeling some doubts 
 whether it is legitimate to appeal to the spiral 
 nebulae to illustrate a theory which was devised 
 to explain the formation, not of a gigantic 
 system of stars, but a single star and its 
 attendant planets. We have laid stress before 
 upon the enormous size of the spirals, even 
 when one takes the most modest estimate of 
 their distance from us. What we see in 
 them may be the evolution of a star system ; 
 it can hardly be a solar system. And it is a 
 well-known principle in mechanism that a 
 device which will work perfectly on one scale, 
 will not work at all upon another. We appeal 
 to gravitation as the moulding force when 
 
THE NEBULAE 219 
 
 things are reasonably close together ; but it is 
 by no means certain that gravitation is the 
 sole or even the most potent force in controlling 
 an immensely extended nebula. For these 
 reasons it may be said to be rash even to the 
 point of danger, to summon the nebulae as 
 witnesses to the Nebular Hypothesis 
 
CHAPTER IX 
 
 THE MILKY WAY 
 
 ON a clear but moonless night in July or 
 August, as soon as it gets dark, suppose that 
 we lie back in a garden-chair and study the 
 sky from the zenith down towards the south. 
 The most conspicuous feature of the sky is the 
 broad irregular band of milky light that 
 stretches through Cygnus, Aquila, Monoceros, 
 down to Sagittarius and the constellations 
 further south. We notice at once that it is 
 not uniform, but lies in bright patches with 
 dark intervals between. We see also that it 
 is straight, or to be more precise, that it lies 
 along a great circle of the sky. If we travel 
 south we find that this straightness continues 
 through the southern constellations, and 
 brings it back again to the north, along that 
 part of the Milky Way through Cassiopeia, 
 Perseus, and Auriga, that we in the Northern 
 hemisphere see in the winter months. So far 
 as we may speak precisely of such a broad 
 
 220 
 
THE MILKY WAY 221 
 
 and irregular belt, we may say that it divides 
 the whole sky into two equal halves a fact 
 which immediately arrests our attention as 
 likely to give a clue to the enquiry we have 
 always in mind, the problem of the structure 
 of the Universe. 
 
 In the northern sky there is nothing apart 
 from the Milky Way which resembles it at all ; 
 but early travellers to the south brought back 
 news of wonders not only on earth, but in 
 the sky also. The learned doctor Pietro of 
 Abano has preserved for us the tale told him 
 by Marco Polo, that in the south he had seen 
 46 a star as big as a sack," which he described 
 as being like a piece of cloud, and having a 
 long tail. This early reference to the Great 
 Cloud of Magellan is made doubly interesting 
 by a figure reputed to be drawn by Marco 
 Polo himself, which, though not very successful 
 in representing the most wonderful objefct in 
 the whole sky, is worthy of all respect as the 
 first attempt to portray a celestial wonder for 
 the benefit of those who had never seen it. 
 The Clouds of Magellan for there are two of 
 them, the greater and the smaller are named 
 by us after the great Portuguese navigator, 
 from the account which his fellow adventurers 
 gave on their return from the famous journey 
 
222 ASTRONOMY 
 
 which cost their leader his life in the Philip- 
 pines. They are the objects, far more than 
 the Southern Cross, which interest an 
 astronomer visiting the south for the first 
 time, for he has learned that the greater of 
 the clouds is a veritable celestial museum, 
 crowded with the most beautiful specimens 
 of star clusters, nebulae, and stars of curious 
 kind ; and he is anxious to see what has, 
 within recent years, been regarded as the key 
 which may unlock the whole secret of the 
 Universe. Let us examine the line of thought 
 which leads to such a fascinating possibility. 
 
 It was one of the earliest discoveries of the 
 telescope that the Milky Way can be resolved 
 into stars. Along its central line the stars 
 lie thick ; towards each side their numbers 
 fall off, until at its poles they are relatively 
 few. This is the main fact which invests the 
 Milky Way, or the Galaxy as we may prefer 
 to call it, with a profound significance. This 
 fact is responsible for the one feature that is 
 common to all theories of the Universe, that 
 it is greatest and richest and most extended 
 in the Galactic plane. 
 
 The grand survey of the sky which was 
 made by the Herschels, father and son, 
 provided the foundation upon which all but 
 
THE MILKY WAY 223 
 
 the most recent investigations were based. 
 They determined by their " star gauges " or 
 counts, the way in which the richness of the 
 sky in stars varied from place to place ; and 
 they catalogued many thousands of star 
 clusters and nebulae. When all this material 
 was discussed and examined, there emerged 
 several important principles. 
 
 (1) As one approaches the Galaxy the 
 number of stars per square degree continually 
 increases. 
 
 (2) The star clusters (other than the globular 
 clusters, which are nearly confined to one half 
 of the sky) lie closely along the Galaxy, and 
 evidently form part of it. 
 
 (8) The nebulae, on the other hand, seem in 
 general to avoid the Galaxy, the region where 
 they are found in greatest number being about 
 the northern galactic pole. 
 
 (4) But star clusters and nebulae, elsewhere 
 seemingly antagonistic, are brought together 
 in profusion in the great cloud of Magellan. 
 
 Upon these facts the philosopher Herbert 
 Spencer based his celebrated argument, that 
 the stars and the nebulae, just because they 
 seem to avoid one another, must for that 
 very reason be opposite parts of one system ; 
 were they independent of one another, 
 
224 ASTRONOMY 
 
 they would not avoid one another so 
 markedly 
 
 The very fact of avoidance, according to 
 this argument, is convincing evidence that 
 the stars and the nebulae are not indifferent 
 to one another. We must look then for some 
 great cause which has been potent to set the 
 stars on the one hand, the nebulae on the other, 
 in one immense ordered system embracing all 
 the universe, and following laws which we 
 may hope to understand by study of their 
 effects. 
 
 This argument has had a great success for 
 many years, so that at last it needs a distinct 
 effort to ask ourselves: Are the facts as to 
 the distribution of the stars and nebulae so 
 clear cut and definite that they admit only of 
 this one interpretation ? For a long time it 
 has seemed to the author that this is not the 
 case, and in a lecture delivered before the 
 British Association meeting at Bloemfontein 
 in 1905, he ventured to try the experiment of 
 denying that there is only this one conclusion 
 possible. To a great extent one's judgment 
 on such a question must depend upon tempera- 
 ment. One man will be seeking always to 
 trace how a complex and delicate system may 
 have evolved from something simple and 
 
THE MILKY WAY 225 
 
 widespread. Another will instinctively prefer 
 to believe that there is diversity in the 
 Universe, and that no single controlling 
 principle has moulded all its constituents as 
 essential and intimately related parts of a 
 single mechanism. In short there may be 
 diversities now because there were diversities 
 to start with, however that start may have 
 been made. 
 
 Let us, however, before we attempt to pur- 
 sue this enquiry, try to see how the immense 
 growth of our knowledge since Herbert 
 Spencer's day may have undermined the 
 foundations of his argument. In the first 
 place, when Spencer wrote, the nebulae, if 
 classified at all, were divided into irresolvable 
 nebulae, and those which it was supposed 
 showed some sign of resolvability. More 
 recent enquiry, and especially the application 
 of the spectroscope and the photographic 
 plate to the study of the nebulae, has shown 
 that the distinction drawn fifty years ago is 
 illusory. No nebula is resolvable ; but we 
 have a new line of distinction to draw between 
 the gaseous and the white or spiral nebulae. 
 The larger gaseous nebulae do not by any 
 means follow the same law of distribution as 
 the spirals ; on the contrary, they are found 
 
226 ASTRONOMY 
 
 most often in dense regions of the Milky Way, 
 entangled with the stars, and sometimes, 
 associated with matter absorbing the light 
 of the stars beyond them. Such are the 
 great extended nebulae in Scorpio and Ophiu- 
 chus which have been so magnificently shown 
 on the portrait lens photographs made by 
 Barnard at Mount Hamilton and Mount 
 Wilson. 
 
 The smaller gaseous nebulas, of round and 
 definite form, the so-called planetary and 
 stellar nebulae, are not so markedly confined 
 to the Milky Way, but at any rate show no 
 tendency to avoid it. We must, therefore, 
 exclude them from any statement that the 
 stars and the nebulae are oppositely distri- 
 buted. 
 
 The gaseous nebulae, are, however, a small 
 minority in the total ; by far the greater part 
 belong to the class of white nebulae, of which 
 the most conspicuous are spiral, and which 
 very probably are really all spiral. In 
 the last chapter we have seen how the 
 number of these is increasing by leaps and 
 bounds as the well known objects are photo- 
 graphed with powerful reflecting telescopes. 
 In the face of such a fact, it becomes im- 
 possible to say with certainty what is the 
 
THE MILKY WAY 227 
 
 distribution of spirals in the sky, for a dozen 
 plates may conceivably reveal a thousand 
 new nebulae, in a part of the sky supposed to 
 be nearly free from them. The probability 
 that this will happen is not however very 
 great, for it seems that the new nebulae are 
 most frequent precisely in those parts of the 
 sky where the well-known nebulae are most 
 thickly strewn. A recent study of the dis- 
 tribution of spiral nebulae made by the author 
 shows, in fact, very little sign that the newly 
 discovered small nebulae differ at all in their 
 distribution from the brighter and better 
 known. So far as one can see, the main facts 
 about the distribution of spirals are indicated 
 by the fifty brightest, and are confirmed 
 without sensible modification as one brings 
 into account list after list of new discoveries. 
 This study does, however, show clearly that 
 the older views as to the distribution of the 
 white nebu'ae require very considerable modi- 
 fication. It is usually said that they cluster 
 closely about the poles of the Galaxy, and 
 avoid the Galaxy itself. The clustering near 
 the northern pole is certainly very marked ; 
 that about the southern is so little marked that 
 it can scarcely be said to exist. Moreover, the 
 spiral nebulae do not markedly avoid all parts 
 
 H2 
 
228 ASTRONOMY 
 
 of the Galaxy, but only certain stretches of 
 it, and it is quite evident that when Sir William 
 Herschel wrote that the nebulae lie in a zone of 
 the sky roughly at right angles to the Galaxy, 
 he was more nearly right than were those who 
 held the more modern view, that they are 
 clustered about the Galactic poles. 
 
 In the chapter on Nebulae we have given 
 a geographical illustration of the distribution 
 of spirals, comparing them with the snow 
 mountains of the world. A more formal 
 statement may be obtained from our figure, 
 in which the celestial sphere is supposed to be 
 projected upon a cube circumscribing it, and 
 so arranged that the Milky Way runs along 
 the centres of the middle line of squares into 
 which the cube is unfolded. The poles of the 
 Galaxy will then lie in the centres of the 
 top and bottom squares. For convenience of 
 reference we letter the Galactic polar squares 
 P and Q, while the four squares in which the 
 Milky Way runs are lettered K L M N. We 
 can now express what is known of the distri- 
 bution of spiral nebulae very briefly : 
 
 They are exceedingly numerous in P. 
 
 They are fairly numerous in Q, L, and N. 
 
 There are a few in M. 
 
 There are none at all in K 
 
THE MILKY WAY 
 
 229 
 
 K 
 
 M 
 
 Q 
 
 N 
 
 This last is a very striking fact, that in a 
 space of one-sixth of the whole sky there is 
 not at the moment of writing a single spiral 
 nebula known ; doubtless further photographic 
 search may bring to light a few ; but it is 
 significant that whereas long exposure photo- 
 graphs with a powerful reflecting telescope 
 reveal great numbers of new spirals in pther 
 parts of the sky, in this great region they have 
 up to the present produced none. 
 
 With the aid of this diagram we may now 
 put very briefly our knowledge of how other 
 classes of objects behave, with reference to the 
 Galaxy 
 
 The stars are immensely more numerous 
 along the Galaxy, in the squares K L M N 
 
230 ASTRONOMY 
 
 than in the polar squares P and Q. So far 
 as the brighter stars are concerned the stars 
 in our catalogues, down to about the tenth 
 magnitude, let us say the numbers contin- 
 ually increase as we get nearer the central 
 line. They increase gradually, and they are 
 congregated more thickly towards some parts 
 of the line than towards others. They are 
 concentrated upon the plane of the Milky 
 Way, and yet they do not make it. This 
 rather curious result has not always received 
 the attention which it deserves. On the 
 contrary, it would seem that many writers 
 have been over-ready to argue thus : The 
 Milky Way is composed of masses of stars ; 
 the stars in our catalogues are concentrated 
 towards the Milky Way ; it must be they 
 that form it. In reality this argument is quite 
 unsound, as can be proved very easily. We 
 have only to remember that in the visible 
 Milky Way there is a great dark rift up in the 
 constellation Cygnus ; and then to look for it 
 in the celebrated chart on which Proctor 
 plotted the 300,000 brighter stars in the 
 northern sky. The general course of the 
 Milky Way can be traced easily enough by 
 the conspicuous crowding of the stars along it, 
 but the rift is nowhere to be seen. The expla- 
 
THE MILKY WAY 231 
 
 nation of this strange fact is not difficult, 
 however. The visible cloud forms of the 
 Milky Way are due to immense masses of stars 
 fainter than those which have as yet been 
 catalogued. 
 
 The recognition of these star clouds is due 
 in great measure to the success of Barnard in 
 photographing them with the portrait lens. 
 In looking at these admirable pictures one 
 sees at once the characteristic feature of a 
 star cloud, its almost sharp edges. As one 
 travels down towards the Milky Way the 
 stars increase gradually in number ; then 
 suddenly one runs into a star cloud, and the 
 richness increases tenfold. The conclusion 
 is almost obvious : the Galaxy is no single 
 structure, no single mass of stars grouped 
 according to a definite law pervading the 
 whole, but is an assemblage of some few 
 hundred more or less distinct clouds of ^ stars, 
 tumbled roughly into one plane. 
 
 Among these clouds of stars there is 
 considerable diversity. Some seem to consist 
 of stars only ; in others there are immense 
 stretches of the vague and almost shapeless 
 nebula which we recognize as having all the 
 appearance of gaseous nebula, though its 
 exceeding faintness has prevented, up to the 
 
232 ASTRONOMY 
 
 present, the gathering of any very definite 
 evidence upon this point. In some places 
 the nebula is traversed by the long dark lanes 
 which have been already described, character- 
 istic especially of the Milky Way in the regions 
 of Scorpio and Sagittarius, and to all appear- 
 ance cutting out the light of the star clouds. 
 These dark lanes present a most interesting 
 problem, far easier to state than to solve. Is 
 the obstructing medium just on the near side 
 of the star cloud, or is it very much nearer to 
 us ? When direct evidence is wanting we 
 must content ourselves with the very general 
 line of argument that the dark lanes are 
 apparently in the gaseous nebula, and the 
 nebula is not found except in apparent 
 connection with the star clouds ; we may for 
 the present suppose, then, that the connection 
 is not only apparent but real, and that these 
 great nebulae are part of the distant structure 
 of star clouds. 
 
 There are also closely associated with the 
 Milky Way stars of a peculiar type the Type 
 V of the early Harvard classification. These 
 stars are not common, only about one hundred 
 being known in all ; and they show a remark- 
 ably close concentration on the central line 
 of the Milky Way or along certain stretches 
 
THE MILKY WAY 233 
 
 of the central line ; for their distribution in 
 longitude is far from uniform. Placed as 
 they are almost exactly upon a great circle 
 of the sky, they show that there is something 
 extraordinarily flat in their arrangement, and 
 one naturally expects to find them in the 
 dense star clouds along the Galaxy. It has 
 been remarked, however, by Newcomb that 
 they display no such preference. Some are 
 found in the star clouds, and others in the 
 vacant spaces between the clouds. Why, 
 then, they should be arranged in their very 
 well marked plane remains for the present 
 one of the most interesting puzzles that we 
 have to solve. 
 
 A considerable bunch of these Type V stars 
 is found in the greater Cloud of Magellan, 
 associated with the globular star clusters, 
 the loose star clusters, the star clouds, and the 
 numerous nebulae which make up that wonder- 
 ful object. If one charts the fainter stars 
 one can trace in the result the course of the 
 Milky Way, and the two Clouds of Magellan. 
 If one charts the loose star clusters or the 
 Type V stars one can do the same ; the Milky 
 Way and the greater Cloud of Magellan are 
 full of them. If one charts the nebulae in 
 general of which, as we now believe, the 
 
234 ASTRONOMY 
 
 greater part are spirals, one finds the Milky 
 Way zone made rather conspicuous by their 
 absence, but the greater Cloud of Magellan 
 made brilliant by their presence in great 
 numbers. So that an inspection of the chart 
 gives rise to the belief that here we have 
 brought together objects which in other parts 
 of the sky seem to avoid one another. This 
 is the fact to which we referred at the outset, 
 the suggestion of the idea which has found 
 some favour in recent years, that we may seek 
 in the greater Cloud of Magellan for the key 
 to our long standing puzzle, why nebulae and 
 stars should avoid one another. 
 
 It will not have escaped the notice of the 
 reader that the crucial question is, are the 
 nebulae in the cloud spirals, or are they 
 gaseous. If, like the majority of the nebulae, 
 they are spirals, then we must consider it 
 remarkable that they are associated here 
 with Milky Way objects. But if on the other 
 hand they are gaseous nebulae, then there is 
 nothing remarkable in their association with 
 objects which are equally with them charac- 
 teristic of the Milky Way. It may seem 
 strange that on such an important point we 
 should still be left in doubt ; but it must 
 be remembered that the only instrument 
 
THE MILKY WAY 235 
 
 which is thoroughly competent to decide the 
 matter is the modern reflecting telescope 
 adapted to photography, and there is not yet 
 such an instrument in the southern hemisphere. 
 It is, however, possible for instruments of 
 other kinds to do something to decide the 
 problem : they have picked up spiral nebulae 
 in other parts of the southern sky, but never 
 in the greater Cloud of Magellan. Negative 
 testimony such as this must of course be used 
 with all reserve. But on a recent study of all 
 available evidence it does seem to the writer 
 that the balance of probability lies in the 
 direction of finding that those nebulae are 
 gaseous, like the extensive nebulae of the 
 Milky Way ; that they have intruded them- 
 selves on our notice because they are 
 particularly bright and particularly numerous 
 in that region, to such an extent that they 
 have reversed the rule which holds elsewhere 
 the rule that spirals are very much more 
 common than gaseous nebulae. 
 
 If in our attempt to disentangle the threads 
 of a complicated problem we have laid too 
 much stress upon the doubts and difficulties 
 which beset it, we must excuse ourselves upon 
 the ground that a difficulty defined is half- 
 way solved. The difficulty in this case is 
 
236 ASTRONOMY 
 
 perfectly definite. In the comparatively few 
 observatories of the south no opportunity has 
 yet been found to instal a photographic 
 reflecting telescope, and to do for the southern 
 hemisphere of the sky what Isaac Roberts and 
 Keeler have done for the northern. Such a 
 telescope is not in fact or has not been in the 
 past an instrument usually found in a 
 Government or University observatory. In an 
 earlier chapter we have already remarked 
 upon this, and have shown that the mirror 
 has often done its best work in the hands of 
 enthusiastic amateurs. For the moment the 
 great success of the five-foot reflector on 
 Mount Wilson has rather obscured this fact ; 
 but it must be remembered that Professor 
 Ritchey was an enthusiastic instrument- 
 maker for many years before he became pro- 
 fessionally associated with an observatory, 
 and his present success should encourage 
 rather than dissuade an amateur who might 
 be contemplating filling that void in our 
 knowledge of the southern nebulae which for 
 the moment leaves incomplete our survey of 
 the Milky Way. 
 
 We have found reason for interpreting thus 
 our present knowledge of the Milky Way : 
 It is made of a great number of more or less 
 
THE MILKY WAY 237 
 
 distinct and separate star clouds, tumbled 
 roughly into one plane. Probably a few of 
 the nearer clouds supply that condensation 
 of stars towards the galactic plane which has 
 an undoubted relation with the Milky Way, 
 and yet is not responsible for its visible 
 aspect. Perhaps the stars in the parts of 
 space nearer our sun do but form one of those 
 star clouds. At least it is certain, from the 
 precision with which the Milky Way divides 
 the sky into two equal halves, that the sun 
 lies pretty close to that plane. But can we go 
 further, and say whether our own star cloud 
 is towards the middle, or near the borders of 
 the whole assemblage. Probably we can. 
 The question whether or no the universe has 
 an infinite extent is a very difficult one, and 
 its investigation is much complicated by the 
 probability that light is gradually extinguished 
 in its passage through space, so that our 
 range of telescopic vision may be bounded. 
 If that is so, then of course an observer at 
 any point of a very large universe must 
 imagine himself at the centre, since he can see 
 for a limited but equal extent in any direction, 
 and the signs which he takes as evidence 
 the falling away of star density near the 
 limits of his vision are merely the results of the 
 
238 ASTRONOMY 
 
 extinction of light in its passage through 
 space. Apart, however, from this difficulty, 
 the near equality of brightness of the Milky 
 Way all round its circumference seems to 
 assure us that we are situated somewhere 
 towards the centre of the visible universe ; 
 more than that it is impossible to say 
 
 But let us avoid most carefully the error 
 into which more than one writer has fallen, 
 of imagining that this result gives us any 
 pre-eminence of position in the universe. 
 Our sun seems to be one of the stars that 
 form one of the star clouds lying towards the 
 centre of the visible universe, but it is in no 
 sense more favourably placed than any one 
 of ten thousand other suns. There is no justi- 
 fication whatever for asserting that modern 
 astronomy assures us we occupy a particularly 
 favourable, perhaps a unique position : not 
 the slightest vestige of an excuse for con- 
 cluding that our own earth, owing to that 
 unique position, may very probably be the 
 only abode of life. 
 
CHAPTER X 
 
 ASTRONOMY IN DAILY USE 
 
 THERE can be no doubt that a science gains 
 very much when it is able to play a practical 
 part in everyday life ; when it can, so to 
 speak, earn its own living. Only a few men 
 have the intellectual capacity to revel in the 
 intricate theories which are beautiful of their 
 kind as exquisite works of art. A very much 
 larger number find keen pleasure in the beauty 
 of the facts and of the simple laws which govern 
 the universe, and there is no danger that its 
 study will ever be neglected for want of 
 widespread interest and general curiosity to 
 know as much as possible of all the bodies in 
 the sky. But it is of real importance that the 
 science of astronomy can earn its living by 
 rendering direct practical service to all man- 
 kind. We propose, in this last chapter, to 
 see what these services are. 
 
 The most obvious, and the one most taken 
 for granted, is the supply of accurate time, 
 which yearly becomes of greater importance 
 in industry and commerce. The day is past 
 
 239 
 
240 ASTRONOMY 
 
 when each city can keep its own local time. 
 Railways and telegraphs have made it impera- 
 tive that as far as possible a country should 
 keep one standard time throughout, while 
 the great increase in the international traffic 
 has made it at least very desirable that when 
 there must be a change, the change shall be 
 an exact hour. Further the early predom- 
 inance of British shipping and the wide use of 
 British admiralty charts made the meridian 
 of Greenwich the natural zero to adopt when 
 it became a question of agreeing upon some 
 one meridian for general use. Thus it has 
 come about that a great part of the civilized 
 world bases its time on Greenwich, in so far 
 that the time which the official observatories 
 send daily or hourly by telegraph differs by 
 an exact number of hours occasionally with 
 an odd half hour thrown in from the time 
 which Greenwich itself distributes throughout 
 England 
 
 It is a common mistake to suppose that the 
 time is determined by observing the sun at 
 noon. The mistake is natural, for the time 
 we keep is mean solar time. Nevertheless, it 
 is determined not from the sun directly, but 
 from the stars, which is possible because we 
 now know very accurately the relative position 
 
ASTRONOMY IN DAILY USE 241 
 
 of the san and of the stars at each instant 
 and is preferable, not only because it is both 
 easier and more accurate to observe a star 
 than to observe the sun, but also because the 
 use of the stars makes it possible to determine 
 the time whenever either by night or day the 
 sky is clear for a few minutes. Had we to 
 depend in England upon catching the sun 
 exactly as it crossed the meridian, it is probable 
 that our clocks would sometimes be rather 
 seriously wrong wrong at any rate by a 
 second or two, which from the point of view 
 of strict time-keeping is very serious. A 
 determination of time, then, consists in observ- 
 ing one of the well-known " clock " stars, as 
 it crosses the wires in the field of view of the 
 transit instrument mounted in the meridian. 
 The observer has a kind of telegraph key in 
 his hand. As the star comes to a wire he taps 
 the key, and a record is made on the chjono- 
 graph sheet, alongside the signals which the 
 clock itself is making automatically every 
 second. It will be readily understood, without 
 going into details, that this observation give* 
 the time as shown by the clock at which the 
 star crossed the meridian. The clock star list 
 in the Nautical Almanac gives the time which 
 the clock should have shown at that instant 
 
242 ASTRONOMY 
 
 a comparison between the two gives the error 
 of the clock. 
 
 Now it is nothing to the discredit of the 
 clock that it has an error. No clock will go 
 perfectly right for ever ; no clock likes being 
 interfered with more than is absolutely neces- 
 sary ; and no clock of the highest precision 
 can be set backwards or forwards by less than 
 one whole minute without unnecessarily 
 interfering with it. Hence the clock of which 
 we are speaking is rarely right. It gradually 
 accumulates an error one way or the other until 
 it is a whole minute wrong and then the 
 minute hand is altered. " Clocks," said the 
 first lecturer on astronomy whom the author 
 ever heard, " clocks always have errors ; you 
 observe them and allow for them, as you do 
 with your friends." 
 
 The clock of which we have been speaking 
 is a sidereal clock, keeping star time, which is 
 very convenient in the observatory, but gains 
 a whole day a year on the solar time which is 
 more convenient outside. It is, however, an 
 easy matter to compare the sidereal with the 
 mean time clock. The Nautical Almanac tells 
 what should be the difference between the 
 two at any instant, and the error of one being 
 known you have the error of the other. Now 
 
ASTRONOMY IN DAILY USE 243 
 
 this latter error must be corrected before the 
 time signal goes out. The correction is 
 probably only a fraction of a second, but we 
 require a delicate and certain way of applying 
 this to the clock with as little disturbance as 
 possible. It can be done by adding or taking 
 away little weights from a dish on the pendu- 
 lum rod ; but a prettier way is to use some 
 arrangement of electro-magnets to hurry or 
 retard the swing of the pendulum until the 
 desired correction is obtained. Thus it is 
 possible to keep right within a small fraction 
 of a second the clock at Greenwich which sends 
 the time signals to the General Post Office. 
 These signals are transmitted hourly or daily 
 over the telegraph system to every post office, 
 and can easily be made to control public 
 clocks or drop public time balls wherever the 
 local authorities are sufficiently keen to pro- 
 vide the small outlay necessary to give^their 
 city really accurate time. 
 
 The recent development of wireless tele- 
 graphy has made possible a remarkable 
 extension of this time service. At stated 
 times, both by night and day, the wireless 
 stations at the Eiffel Tower in Paris, and at 
 Nordreich on the German coast, send out 
 time signals which can be picked up over a 
 
244 ASTRONOMY 
 
 great part of Europe and far out into the 
 Atlantic. We shall see immediately the 
 immense value of this new development. 
 
 On land, in a civilized country, the ordinary 
 man is apt to take the provision of time as a 
 matter of course, and hardly realizes that he 
 is indebted to astronomy for it. At sea he 
 can hardly fail to know that the ship's officers 
 are constantly taking " sights " of the sun, 
 moon, or stars, and are depending upon these 
 observations to find the place of the ship. 
 But there is a great deal of misapprehension 
 as to the actual process, for it is commonly 
 supposed that the observation of the sun at 
 noon gives the complete position of the 
 ship, and enables the captain to set the ship's 
 clock. In reality the noon observation gives 
 only the latitude, which it does directly and 
 simply. The equally important longitude 
 must be found by an indirect and much more 
 elaborate process. Early in the morning, 
 and again late in the afternoon, the navigating 
 officer takes a " sight " of the sun which 
 after a brief calculation gives him the local 
 time. Now the longitude, east or west of 
 Greenwich, is the amount by which this local 
 time at the ship is fast or slow of Greenwich 
 time which the ship must carry with her, or 
 
ASTRONOMY IN DAILY USE 245 
 
 obtain in some way independently. She 
 carries it with her by means of chronometers 
 which have been brought to an astonishing 
 degree of perfection since the days when the 
 celebrated John Harrison gained the prize of 
 20,000 offered by the Government for a time- 
 keeper which would keep the time at sea 
 within specified limits of error. But chrono- 
 meters, even the best, sometimes go wrong, 
 and we have provided an alternative, that the 
 ship may obtain the Greenwich time 
 independently. The historical method of so 
 doing is by " lunar distances," or more briefly, 
 by " lunars " ; that is to say, by measuring 
 with the sextant the angular distance of the 
 moon from a bright star or planet, and after 
 a somewhat difficult calculation comparing 
 the result with the distance predicted in the 
 Nautical Almanac. On the first proposal of 
 this method it was found impracticable, for 
 neither the motion of the moon, nor the places 
 of the stars, were known with anything like 
 sufficient accuracy. To remedy this defect 
 King Charles the Second founded the Royal 
 Observatory at Greenwich, for the benefit of 
 his seamen, and since that day the first duty 
 of Greenwich Observatory has always been 
 to improve the catalogues of star places and to 
 
246 ASTRONOMY 
 
 observe at all opportunities the position of the 
 moon. 
 
 Of late years the famous method of " lunars " 
 has fallen completely into disuse ; partly 
 because chronometers have become more 
 reliable ; partly because steam has made 
 voyages so much more rapid that the chrono- 
 meters have in any case less time to go wrong ; 
 and partly because the vibration of a fast 
 steamship is so great that it is impossible to 
 make observations from her bridge with the 
 required nicety. Finally, the establishment 
 of the wireless service of time signals has 
 already made it possible for a ship to pick up 
 Greenwich time when hundreds of miles at sea, 
 or to compare one ship's chronometers with 
 another's whenever the two come within 
 wireless range. The day is probably not far 
 distant when such a service will be almost 
 world-wide. 
 
 Besides the processes of finding the latitude 
 and longitude, which in their effect, at any 
 rate, are familiar to all, there is another 
 observation made at sea of which one hears far 
 less, but which is equally important, and even 
 more frequently performed ; the determination 
 of the error of the compass. As ships become 
 larger and faster, an error of half a degree in 
 
ASTRONOMY IN DAILY USE 247 
 
 the compass becomes more and more important 
 and at the same time the difficulty of keeping 
 the compass free from the disturbing attraction 
 of the iron in the ship becomes more and more 
 serious. Hence it arises that in a well run 
 ship the most frequent astronomical observa- 
 tion made is the sight of the sun with the 
 standard compass, which gives the deviation 
 of that compass from the true north. 
 
 It will be sufficiently obvious that what 
 applies to the mariner applies with slight 
 modifications to the explorer on land. To find 
 where he is he must make his observations of 
 the sun and the stars as the mariner does, and 
 the first rude maps of an unsurveyed country 
 depend very largely upon such astronomical 
 positions. Afterwards comes the surveyor 
 with his more precise methods ; but he again 
 has to rely upon astronomical observations 
 for a fundamental need. With his theodolite 
 and plane table, it is true, he might make a 
 map which would show the whole country's 
 topography without using sun or star. But in 
 that case the map would be lacking in one 
 serious respect there would be nothing to 
 show where in the world it was, nor which way 
 up it should be viewed. The surveyor has, so 
 to speak, to pin his map down upon the world, 
 
248 ASTRONOMY 
 
 in. the right place, and in the right orientation, 
 and this he can do only by the methods of 
 field astronomy. 
 
 Moreover, he relies in yet another way upon 
 astronomy, for in the process of his map- 
 making he needs to know the size and the shape 
 of the earth. Geodesy is the science which 
 combines the highest and most refined processes 
 of survey with some of the most delicate oper- 
 ation of astronomy. Without going into details 
 it is easy to see how the geodesist must 
 proceed. Suppose, for simplicity, that by his 
 survey processes he has measured the distance 
 between two points, one of which is due north 
 of the other. The distance between these 
 two points he expresses in terms of the standard 
 unit of length adopted by his country. He 
 determines in effect that the distance he has 
 measured upon the earth is so many times the 
 distance between two fine lines on the standard 
 bar which is preserved with all imaginable care 
 as the absolute standard of length. He next 
 determines astronomically the latitudes of his 
 two points ; that is to say, he finds that they 
 are so many degrees, minutes, and seconds 
 of arc apart. It needs now but a very elemen- 
 tary knowledge of geometry to see that when 
 we know a length in degrees, and also let us say 
 
ASTRONOMY IN DAILY USE 249 
 
 in feet, it is easy to find in feet the size of the 
 whole circle, of which the part measured is 
 an arc ; it is easy, that is to say, if the curve 
 is really a circle. But is it ? That can be 
 determined only by measuring different arcs, 
 at different distances from the equator. When 
 this is done, it is found that a degree near 
 the poles contains more feet than a degree 
 near the equator ; the immediate inference 
 is that the earth is not truly a sphere, but is 
 somewhat flattened at the poles ; and the 
 amount of the flattening is determined from 
 the discordances in the measured lengths of a 
 degree in different latitudes. The process is 
 as old as the time of Ptolemy the Alexandrian, 
 who actually measured in a rough way the size 
 of the earth from his observations in Egypt. 
 
 After a regrettable interval of some eighteen 
 centuries work has now been resumed in 
 Egypt, which finds itself on the line of the 
 great meridian arc 30 E. of Greenwich, 
 passing over the greatest north and south 
 extent of land. From the northern shore of 
 the lovely harbour of Hammerfest, near the 
 North Cape, down to the mouth of the Danube, 
 was measured in the middle of the last century 
 by the joint efforts of Russia, Norway, and 
 Sweden. Starting from the south of South 
 
250 ASTRONOMY 
 
 Africa, many years later, the various govern- 
 ments of South Africa, British and Boer, have 
 measured up to near Lake Tanganyika, to the 
 borders of German territory. North of that 
 the British have begun again, on the Uganda- 
 Congo boundary ; and British officers have 
 been responsible for the new work in Egypt, 
 Much remains to be done, in tropical Africa, 
 where the actual field work is terribly exacting, 
 and through Asia Minor and Turkey, where the 
 political difficulties may prove more formidable 
 than the natural. Gradually, however, both 
 along the 30th meridian, and in India, in the 
 States and Canada, in Central Asia and in 
 Central America, the surveyors are measuring 
 their triangles and observing their latitudes 
 astronomically, adding step by step to the 
 accumulating knowledge of the true size and 
 shape of the earth. 
 
 A curious outcome of these researches 
 deserves a passing mention. One would 
 hardly expect that the science which is 
 concerned with regions far outside the earth 
 could tell us much of the inside of the earth 
 itself. Yet it is the fact that nearly all our 
 knowledge of the earth's interior is derived 
 from astronomy and sciences closely allied. 
 The complicated phenomenon of the " varia- 
 
ASTRONOMY IN DAILY USE 251 
 
 tion of latitude " seems to show that the 
 earth as a whole is about as rigid as steel ; 
 the tidal investigations of Lord Kelvin and 
 Sir George Darwin are against the old idea 
 that the interior is molten ; and a most 
 interesting chapter of modern geodesy has 
 established the fact that the great mountain 
 masses of the world, the Caucasus, the Rockies, 
 the Tibet plateau, are, nevertheless, balanced 
 by defects of density below them, just as if 
 they were icebergs floating in the sea. 
 
 We have no space to follow further afield the 
 ways in which astronomy is ever lending a 
 helping hand in the operations of other 
 sciences or in good government and administra- 
 tion ; but we may, in conclusion, spend a few 
 moments in considering how our everyday 
 lives are affected by stern astronomical facts. 
 We have in the last few years seen a serious 
 attempt to promote " daylight saving " by 
 putting the clock forward an hour during the 
 summer months. It appears to the author 
 that the advocates of this scheme do not 
 always recognize the hard facts about the 
 partition of daylight between summer and 
 winter. The truth is that in any country 
 more than fifty degrees from the equator, the 
 winter day and the summer night are incon- 
 
252 ASTRONOMY 
 
 veniently short ; when one approaches the 
 latitude 60 there is practically no darkness 
 for two or three months in summer. For 
 such countries the summer daylight is more 
 than ample ; the difficulty indeed is of the 
 opposite kind, that it is hard to get darkness 
 in which to sleep. For a period of six or 
 seven weeks in spring it would be pleasant 
 to have a longer evening ; for the cor- 
 sponding period in late summer the benefit 
 would be more doubtful, when the day is apt 
 to be too hot, and the cool dusk of the evening 
 is to be desired rather than postponed. Thus a 
 change which might be beneficial in spring and 
 of more doubtful advantage in late summer, 
 becomes almost disadvantageous at mid- 
 summer, and this because the length of the 
 day is already so exaggerated in the higher 
 latitudes. Man does not like going to sleep by 
 daylight or getting up in the dark. To some 
 extent he has already to do both when he 
 lives as far north as England or Canada ; he 
 will have to do more of both if the clock is 
 put forward an hour for nearly half the year. 
 No alteration of the clock can change the 
 unchangeable fact of astronomy, that the plane 
 of the equator is inclined nearly 23j to the 
 plane in which the earth moves round the sun. 
 August, 1911. 
 
NOTE ON BOOKS 
 
 THE number of books, especially of elementary and 
 popular books, dealing with the many sides of Astronomy, 
 is so great that it is difficult to make a fair selection. 
 
 By far the best text-book is Young's General Astronomy 
 (Ginn & Co.), which gives a concise and simply expressed 
 account of both theory and practice. An excellent 
 smaller book is the Primer of Astronomy by Sir Robert 
 Ball (Cambridge University Press). The same author's 
 Popular Guide to the Heavens (G. Philip & Son), contains 
 star maps, good photographs of nebulae and star clusters, 
 and much information useful to the amateur astronomer. 
 A small book by the late J. E. Gore, The Stellar Heavens 
 (Chat to & Windus), gives a valuable summary description 
 of the most interesting objects among the stars and 
 nebulae. A most valuable account of modern theories 
 of the structure of the universe is to be found in the late 
 Professor Simon Newcomb's book, The Stars .- a Study 
 of the Universe (John Murray). For an elementary, but 
 not easy, account of tidal phenomena the student must 
 read Professor Sir George Darwin's work, The Tides (John 
 Murray). An excellent small book on spectroscopy is 
 The Spectroscope and its Work (S.P.C.K.), by Professor 
 H. F. Newall. 
 
 For the history of our subject, we must refer to the late 
 Professor Grant's History of Physical Astronomy (out of 
 print, but in all good libraries) , and to the late Miss Agnes 
 Clerke's well-known History of Astronomy in the XlXth 
 Century (A. & C. Black). The same author's The Herschels 
 253 
 
254 NOTE ON BOOKS 
 
 and Modern Astronomy (Cassell & Co.) is good ; and the 
 Reminiscences of an Astronomer (Harper Bros.), by the 
 late Professor Simon Newcomb is a delightful book. 
 
 To keep abreast of recent work the student must refer 
 to the periodicals. The Observatory, published monthly, 
 edited unofficially from the Royal Observatory, Greenwich, 
 gives reports of the meetings of societies, reviews and 
 notices of recent work, and general astronomical news. 
 The Journal of the British Astronomical Association, 
 the Publications of the Astronomical Society of the Pacific, 
 and the Journal of the Royal Astronomical Society of 
 Canada, are all full of interesting matter, largely unmathe- 
 matical. The Monthly Notices of the Royal Astronomical 
 Society, and The Astrophysical Journal are more severely 
 technical, but contain also the best celestial photographs. 
 
INDEX 
 
 ABERRATION of Light, 15, 148 
 Absorption of Light in Space. 215. 
 
 232 
 
 Adams, 17, 117 
 Airy, 17 
 Algol, 177 
 Ancient Buildings, Astronomical 
 
 Significance of, 8 
 Anderson, 171 
 Apex of Sun's Way, 164, 181 
 Aristotle, 9 
 Astrophysics, 31, 160 
 Auroras and Sunspots, 45 
 
 Backlund, 115, 137 
 Barnard, 212, 214, 226 
 Bessel, 17, 161 
 Bradley, 15, 161 
 Brown, 129 
 
 Cambridge Observatory, 150, 167 
 
 Campbell, 179 
 
 Cape, Royal Observatory, 146, 
 
 154, 179, 181 
 Clocks, 242 
 
 Clockwork for Telescopes, 22 
 Coeiostat, 32 
 Comet, Biela's, 108 
 
 Composition of, 108-111 
 
 Encke's, 115, 136 
 
 Halley's, 93-103 
 
 Method of Discovery, 111 
 
 Morehouse's, 107 
 
 Periodic, 114 
 
 The " Daylight," 1910, 102 
 
 The Tail of a, 99, 103-107 
 
 Common, 205 
 
 Compass, Error of, 246 
 Computing, 33 
 Copernicus, 13 
 Cowell, 134 
 
 Cowell & Crommelin, Work on 
 Halley's Comet, 91, 94, 97 
 
 Darwin, 61, 251 
 Day, Length of, 135 
 " Daylight Saving," 251 
 Dehra Dun Observatory, 47 
 Deslandres, 56 
 Downing, 117 
 Dyson, 165 
 
 Earth, Mass of, 147 
 
 , Radius of, 156 
 
 Eddington, 165 
 Elements, Permanence of, 57 
 Eros, Solar Parallax from, 149-154 
 , The Planet, 72 
 
 Fotheringham, 134 
 Fowler, 109 
 
 Galaxy, 222, 227 
 Galileo, 13, 195 
 Geodesy, 156, 248 
 
 Glass, Optical, 24 
 
 Gravitation, Discovery of Theory, 
 
 , a Source of Heat, 40 
 
 , in Solar System, 125 
 
 , Problem of Three Bodies, 
 
 125 
 
 , Test of Theory, 130 
 
 Greek Astronomy, 9 
 
 Greenwich Observatory, 47, 90, 
 
 107, 190, 240, 245 
 
 Hale, 56 
 
 Halley, 14, 94-97, 161, 195 
 
 Halm, 165, 181 
 
 Harrison, John, 245 
 
 Harvard College Observatory 
 
 90, 170, 173 
 Helmholtz, 41 
 Henry, 188 
 Herschel, William, 16, 182, 196, 
 
 222 
 
 , John, 17, 196, 222 
 
 Hill, 129 
 
 Hipparchus, 9, 161 
 Hough, 165, 181 
 Huggins, 17, 199 
 Hull, 104 
 
 Janssen, 55 
 Johnstone-Stoney, 117 
 Jupiter, The Planet, 72 
 
 , Comet Family of, 114 
 
 , New Satellites of, 89 
 
 Kapteyn, 164 
 
 Keeler, 205, 236 
 
 Kelvin, 251 
 
 Kepler, 12 
 
 Kiel, Central Bureau, 112 
 
 Kodaikanal Observatory, 47 
 
 Laplace, 217 
 
 Latitude at Sea, 244 * 
 
 , Variation of, 250 
 
 Leonid Meteors, 93 
 
 Leverrier, 17, 117 
 
 Lick Observatory, 90, 179, 183, 
 
 206 
 
 Light, Pressure of, 54, 104-108, 137 
 Lockyer, 55 
 Loewy, 152 
 
 Longitude at Sea, 244 
 Lowell, Theories of Mars, 81-86 
 Lunar Distances, 245 
 
 Magellan, Clouds of, 221, 233-235 
 Magnetism and Sunspots, 46 
 Marius, Simon, 13 
 Mars Atmosphere of, 76 
 
 Canals, 77-80 
 
 Habitability of, 67, 75 
 
 Lowell's Theories, 81-86 
 
 Solar Parallax from, 145 
 
 Mauritius Observatory, 47 
 
256 
 
 INDEX 
 
 Melotte, 90 
 
 Mercury, 67 
 
 Meridian Circle, 27 
 
 Messier, 195 
 
 Meteors, Nature of, 115, 120 
 
 , Observation of, 118 
 
 , Radiant point of, 116 
 
 , Stationary Radiants, 119 
 
 , The Leonids, 116-118 
 Milky Way, Photographs of, 30, 
 
 212-214, 226 
 , Structure of, 220-222, 230, 
 
 236 
 
 Minor Planets, 69-71 
 Moon, Features of, 64 
 
 , Mass of, 156-158 
 
 , Motion of, 124, 127-129 
 
 , Past History, 61 
 
 Mount Wilson Solar Observatory, 
 
 25, 47, 236 
 
 Nautical Almanac, 125, 241 
 
 Navigation, 244 
 
 Nebulae. Classification of, 204 
 
 Density of, 201 
 
 Distribution of, 210-212, 
 
 226-230 
 
 Gaseous, 199-204, 212-215 
 
 Spectra of, 199 
 
 Spiral, 198, 207-212, 218, 
 
 226-229, 234 
 
 , Supposed Resolvability, 197, 
 
 225 
 
 Nebular Hypothesis, 217 
 Neptune. 17, 74, 87, 88 
 Nevill, 134 
 Newall, 43 
 
 Newcomb, 125, 131, 132 
 Newton, 14, 95, 96 
 JNova Persei, 171 
 Nutation, 15 
 
 Observatory, Equipment of, 27-36 
 Observatories, Solar, 47 
 Optical Work, 21 
 
 Paris, Astrographic Congress, 150 
 
 188, 191 
 
 , Observatory, 152, 188, 191 
 
 Perrine, 90 
 
 Phoebe, Satellite of Saturn, 89 
 Photography, Astronomical, 19-21 
 
 28-30, 30-32, 36, 186 
 
 , of Nebulas 204-206 
 
 Photographic Chart of the Sky, 
 
 30, 185, 188-192 
 
 Plates, S4 
 
 Photographs, Study of, 35 
 
 Pickering, W. H., 90 
 
 Planet beyond Neptune, Search 
 
 for, 87 
 Planets, Theory of Motions, 125- 
 
 127 
 
 Proctor, 230 
 Ptolemy, 9, 161, 249 
 
 Radio-activity, 42 
 Ritchey, 236 
 Roberts, Isaac, 205, 236 
 Rosse, Lord, 17, 197, 205, 207 
 
 Satellites, 65, 88-92 
 
 Saturn, 73 
 
 Schiaparelli, 77 
 
 Secchi, 193 
 
 South Kensington, Solar Physics 
 
 Observatory, 47 
 Spectroheliograph, 45, 56 
 Spectrum Analysis, 17, 18, 31 
 Spencer, Herbert, 223-225 
 Stars, Binary and Double, 181-184 
 
 , Catalogues of, 161, 185 
 
 , Distance of, 167-169 
 
 , Distribution of, 229-232 
 
 , Fundamental. 159, 186 
 
 , New or Temporary, 170-174 
 
 , Proper motions of, 163-166 
 
 , Spectra of, 193 
 
 , Spectroscopic Binary, 177- 
 
 180 
 
 , Streams of, 165, 181 
 
 , Type V, 232 
 
 , Variable, 174-177 
 
 Sun, Ancient Eclipses, 132-135 
 
 , Chromosphere of, 49, 57 
 
 , Corona of, 49, 53-55 
 
 , Distance of, 139-158 
 
 , Eclipses of, 48-53 
 
 , Heat of, 38, 42 
 
 , Motion in Space of, 170 
 
 , Place in the Universe of, 238 
 
 , Prediction of Eclipses, 123 
 
 -, Prominences on, 55 
 
 Sunspots, 44-47 
 Surveying, 247 
 
 Telescope, Invention of, 12 
 
 , Equatorial, 28 
 
 , Horizontal, 32 
 
 , Limits to size of, 22-26, 198 
 
 , Photographic, 21 
 
 , Reflecting, 16, 236 
 
 , Vertical, 33 
 
 Tides, Cause of, 58 
 
 , Effect on Moon, 61 
 
 , Prediction of, 59 
 
 Time, 239-242 
 
 Tycho Brahe, 10-12, 102, 161 
 
 Ulugh Beigh, 161 
 Universe, Extent of, 237 
 Uranus, 74, 87-88, 117 
 
 Venus, 68 
 
 , Transit of, 142-145 
 
 Vogel, 193 
 
 Williams, Stanley, 172 
 Wireless Telegraphy, 243 
 Witt, 149 
 Wolf, Max, 212 
 
 Yerkes Observatory, 23 
 
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