LIBRARY 
 
ASTRONOMICAL DISCOVERY 
 
C T H F 
 
ASTRONOMERS ROYAL. 
 
ASTRONOMICAL 
 DISCOVERY 
 
 BY 
 
 HERBERT HALL TURNER, D.Sc., F.R.S. 
 
 SAVILIAN PROFESSOR OF ASTRONOMY IN THE 
 UNIVERSITY OF OXFORD 
 
 OF THF 
 
 UNWEPT 
 
 Of 
 
 .' iro 
 
 WITH PLATES 
 
 LONDON 
 EDWARD ARNOLD 
 
 41 & 43 MADDOX STREET, W. 
 1904 
 
 (All rights reserved) 
 
' 
 
 ASTWfr 
 LIBR> 
 
 1 
 
TO 
 
 EDWARD EMERSON BARNARD 
 i 
 
 ASTRONOMICAL DISCOVERER 
 
 THESE PAGES ARE INSCRIBED IN MEMORY OF 
 NEVER-TO-BE-FORGOTTEN DAYS SPENT WITH HIM AT THE 
 
 YERKES OBSERVATORY 
 
 OP 
 THE UNIVERSITY OF CHICAGO 
 
PBEFACE 
 
 THE aim of the following pages is to illustrate, by 
 the study of a few examples chosen almost at 
 random, the variety in character of astronomical 
 discoveries. An attempt has indeed been made 
 to arrange the half-dozen examples, once selected, 
 into a rough sequence according to the amount of 
 ''chance" associated with the discovery, though 
 from this point of view Chapter IV. should come 
 first ; but I do not lay much stress upon it. 
 There is undoubtedly an element of "luck" in 
 most discoveries. " The biggest strokes are all 
 luck," writes a brother astronomer who had done 
 me the honour to glance at a few pages, " but a 
 man must not drop his catches. Have you ever 
 read Montaigne's essay ' Of Glory ' ? It is worth 
 reading. Change war and glory to discovery 
 and it is exactly the same theme. If you are 
 looking for a motto you will find a score in 
 it." Indeed even in cases such as those in 
 Chapters V. and VI., where a discovery is made 
 by turning over a heap of rubbish declared such 
 by experts and abandoned accordingly we in- 
 stinctively feel that the finding of something 
 valuable was especially " fortunate." We should 
 scarcely recommend such waste material as the 
 best hunting ground for gems. 
 
viii PREFACE 
 
 The chapters correspond approximately to a 
 series of six lectures delivered at the University 
 of Chicago in August 1904, at the hospitable 
 invitation of President Harper. They afforded me 
 the opportunity of seeing something of this 
 wonderful University, only a dozen years old and 
 yet so amazingly vigorous ; and especially of its 
 observatory (the Yerkes observatory, situated 
 eighty miles away on Lake Geneva), which is 
 only eight years old and yet has taken its place 
 in the foremost rank. For these opportunities I 
 venture here to put on record my grateful thanks. 
 
 In a portion of the first chapter it will be 
 obvious that I am indebted to Miss Clerke's 
 " History of Astronomy in the Nineteenth 
 Century " ; in the second to Professor R. A. 
 Sampson's Memoir on the Adams MSS. ; in 
 the third to Rigaud's " Life of Bradley." There 
 are other debts which I hope are duly acknow- 
 ledged in the text, My grateful thanks are due 
 to Mr. F. A. Bellamy for the care with which he 
 has read the proofs ; and I am indebted for per- 
 mission to publish illustrations to the Royal 
 Astronomical Society, the Astronomer Royal, the 
 editors of The Observatory, the Cambridge 
 University Press, the Harvard College Obser- 
 vatory, the Yerkes Observatory, and the living 
 representatives of two portraits. 
 
 H. H. TURNER. 
 
 UNIVERSITY OBSERVATORY, OXFORD, 
 November 9, 1904. 
 
CONTENTS 
 
 CHAPTER I 
 
 PAGE 
 
 URANUS AND EROS i 
 
 CHAPTER II 
 THE DISCOVERY OF NEPTUNE 38 
 
 CHAPTER III 
 
 BRADLEY'S DISCOVERIES OF THE ABERRATION OF LIGHT 
 
 AND OF THE NUTATION OF THE EARTH'S AXIS . 86 
 
 CHAPTER IV 
 
 ACCIDENTAL DISCOVERIES . . . . . .121 
 
 CHAPTER Y 
 
 SCHWABE AND THE SUN-SPOT PERIOD . . . -155 
 
 CHAPTER VI 
 
 THE VARIATION OF LATITUDE 177 
 
 INDEX 221 
 

ASTRONOMICAL DISCOVERY 
 
 CHAPTER I 
 URANUS AND EROS 
 
 DISCOVERY is expected from an astronomer. The Popular 
 lay mind scarcely thinks of a naturalist nowadays discovery, 
 discovering new animals, or of a chemist as find- 
 ing new elements save on rare occasions ; but it 
 does think of the astronomer as making dis- 
 coveries. The popular imagination pictures him 
 spending the whole night in watching the skies 
 from a high tower through a long telescope, occa- 
 sionally rewarded by the finding of something new, 
 without much mental effort. I propose to compare 
 with this romantic picture some of the actual facts, 
 some of the ways in which discoveries are really 
 made ; and if we find that the image and the reality 
 differ, I hope that the romance will nevertheless 
 not be thereby destroyed, but may adapt itself to 
 conditions more closely resembling the facts. 
 
 The popular conception finds expression in the Keats' 
 lines of Keats : 
 
 Then felt I like some watcher of the skies 
 When a new planet swims into his ken. 
 
 Keats was born in 1795, published his first 
 volume of poems in 1817, and died in 1821. At 
 
2 ASTRONOMICAL DISCOVERY 
 
 the time when he wrote the discovery of planets 
 was comparatively novel in human experience. 
 Uranus had been found by William Herschel in 
 1781, and in the years 1800 to 1807 followed the 
 first four minor planets, a number destined to 
 remain without additions for nearly forty years. 
 It would be absurd to read any exact allusion into 
 the words quoted, when we remember the whole 
 circumstances under which they were written ; 
 but perhaps I may be forgiven if I compare them 
 especially with the actual discovery of the planet 
 Uranus, for the reason that this was by far the 
 largest of the five far larger than any other planet 
 known except Jupiter and Saturn, while the 
 others were far smaller and that Keats is using 
 throughout the poem metaphors drawn from the 
 first glimpses of " vast expanses " of land or water. 
 Perhaps I may reproduce the whole sonnet. 
 His friend C. C. Clarke had put before him 
 Chapman's " paraphrase " of Homer, and they 
 sat up till daylight to read it, "Keats shouting 
 with delight as some passage of especial energy 
 struck his imagination. At ten o'clock the next 
 morning Mr. Clarke found the sonnet on his 
 breakfast-table." 
 
 SONNET XI 
 
 On first looking into Chapman's " Homer " 
 
 Much have I travell'd in the realms of gold, 
 And many goodly states and kingdoms seen ; 
 Round many western islands have I been 
 Which bards in fealty to Apollo hold. 
 
URANUS AND EROS 3 
 
 Oft of one wide expanse had I been told 
 
 That deep-brow'd Homer ruled as his demesne ; 
 
 Yet did I never breathe its pure serene 
 
 Till I heard Chapman speak out loud and bold : 
 
 Then felt I like some watcher of the skies 
 
 When a new planet swims into his ken ; 
 
 Or like stout Cortez when with eagle eyes 
 
 He star'd at the Pacific and all his men 
 
 Look'd at each other with a wild surmise 
 
 Silent, upon a peak in Darien. 
 
 Let us then, as our first example of the way in 
 which astronomical discoveries are made, turn to discovery 
 the discovery of the planet Uranus, and see how 
 it corresponds with the popular conception as 
 voiced by Keats. In one respect his words are 
 true to the life or the letter. If ever there was a 
 " watcher of the skies," William Herschel was 
 entitled to the name. It was his custom to watch 
 them the whole night through, from the earliest 
 possible moment to daybreak ; and the fruits of 
 his labours were many and various almost beyond 
 belief. But did the planet " swim into his ken" ? 
 Let us turn to the original announcement of his 
 discovery as given in the Philosophical Transac- 
 tions for 1781. 
 
ASTKONOMICAL DISCOVERY 
 PHILOSOPHICAL TRANSACTIONS, 1781 
 
 XXXII. ACCOUNT OF A COMET 
 
 BY MB. HERSCHEL, F.R.S. 
 
 (Communicated by Dr. Watson, jun., of Bath, F.R.S.) 
 Read April 26, 1781 
 
 "On Tuesday the I3th of March, between ten 
 
 Announce- -A. i,-i T 
 
 ment. and eleven in the evening, while I was exam- 
 ining the small stars in the neighbourhood of 
 H Geminorum, I perceived one that appeared 
 visibly larger than the rest; being struck with 
 its uncommon magnitude, I compared it to H 
 Geminorum and the small star in the quartile 
 between Auriga and Gemini, and finding it to be 
 so much larger than either of them, suspected it 
 to be a comet. 
 
 " I was then engaged in a series of observations 
 on the parallax of the fixed stars, which I hope 
 soon to have the honour of laying before the 
 Koyal Society; and those observations requiring 
 very high powers, I had ready at hand the several 
 magnifiers of 227, 460, 932, 1536, 2010, &c., all 
 which I have successfully used upon that occasion. 
 The power I had on when I first saw the comet 
 was 227. From experience I knew that the 
 diameters of the fixed stars are not proportionally 
 magnified with higher powers as the planets are ; 
 therefore I now put on the powers of 460 and 932, 
 and found the diameter of the comet increased in 
 proportion to the power, as it ought to be, on a 
 
URANUS AND EROS 5 
 
 supposition of its not being a fixed star, while the 
 diameters of the stars to which I compared it 
 were not increased in the same ratio. Moreover, 
 the comet being magnified much beyond what its 
 light would admit of, appeared hazy and ill-defined 
 with these great powers, while the stars preserved 
 that lustre and distinctness which from many 
 thousand observations I knew they would retain. 
 The sequel has shown that my surmises were well 
 founded, this proving to be the Comet we have 
 lately observed. 
 
 " I have reduced all my observations upon this 
 comet to the following tables. The first contains 
 the measures of the gradual increase of the comet's 
 diameter. The micrometers I used, when every 
 circumstance is favourable, will measure extremely 
 small angles, such as do not exceed a few seconds, 
 true to 6, 8, or 10 thirds at most ; and in the 
 worst situations true to 20 or 30 thirds ; I have 
 therefore given the measures of the comet's 
 diameter in seconds and thirds. And the parts 
 of my micrometer being thus reduced, I have also 
 given all the rest of the measures in the same 
 manner ; though in large distances, such as one, 
 two, or three minutes, so great an exactness, for 
 several reasons, is not pretended to." 
 
 At first sight this seems to be the wrong refer- called 
 ence, for it speaks of a new comet, not a new comet 
 planet. But it is indeed of Uranus that Herschel 
 is speaking ; and so little did he realise the full 
 
6 ASTRONOMICAL DISCOVERY 
 
 magnitude of his discovery at once, that he 
 announced it as that of a comet ; and a comet 
 the object was called for some months. Attempts 
 were made to calculate its orbit as a comet, and 
 broke down ; and it was only after much work 
 of this kind had been done that the real nature 
 of the object began to be suspected. But far 
 more striking than this misconception is the 
 display of skill necessary to detect any peculiarity 
 in the object at all. Among a number of stars 
 one seemed somewhat exceptional in size, but 
 the difference was only just sufficient to awaken 
 
 other suspicion in a keen-eyed Herschel. Would any 
 other observer have noticed the difference at all ? 
 Certainly several good observers had looked at 
 
 at ail. the object before, and looked at it with the care 
 necessary to record its position, without noting 
 any peculiarity. Their observations were re- 
 covered subsequently and used to fix the orbit of 
 the new planet more accurately. I shall remind 
 you in the next chapter that Uranus had been 
 observed in this way no less than seventeen times 
 by first-rate observers without exciting their 
 attention to anything remarkable. The first 
 occasion was in 1690, nearly a century before 
 Herschel'.s grand discovery, and these chance 
 observations, which lay so long unnoticed as 
 in some way erroneous, subsequently proved to 
 be of the utmost value in fixing the orbit of the 
 new planet. But there is even more striking 
 testimony than this to the exceptional nature of 
 
URANUS AND EROS 7 
 
 Herschel's achievement. It is a common experience 
 in astronomy that an observer may fail to notice in 
 a general scrutiny some phenomenon which he can 
 see perfectly well when his attention is directed 
 to it : when a man has made a discovery and 
 others are told what to look for, they often see 
 it so easily that they are filled with amazement 
 and chagrin that they never saw it before. Not 
 so in the case of Uranus. At least two great 
 astronomers, Lalande and Messier, have left on 
 record their astonishment that Herschel could 
 differentiate it from an ordinary star at all ; for 
 even when instructed where to look and what 
 to look for, they had the greatest difficulty in 
 finding it. I give a translation of Messier' s words, 
 which Herschel records in the paper already quoted 
 announcing the discovery : 
 
 " Nothing was more difficult than to recognise 
 it ; and I cannot conceive how you have been 
 able to return several times to this star or comet ; 
 for absolutely it has been necessary to observe it 
 for several consecutive days to perceive that it 
 was in motion." 
 
 We cannot, therefore, fit the facts to Keats' NO 
 version of them. The planet did not majesti- 
 cally reveal itself to a merely passive observer: ken 
 rather did it, assuming the disguise of an ordinary 
 star, evade detection to the utmost of its power ; 
 so that the keenest eye, the most alert attention, 
 the most determined following up of a mere 
 
8 ASTRONOMICAL DISCOVERY 
 
 hint, were all needed to unmask it. But is the 
 romance necessarily gone ? If another Keats 
 could arise and know the facts, could he not 
 coin a newer and a truer phrase for us which 
 would still sound as sweetly in our ears ? 
 
 Though I must guard against a possible misconception. 
 
 happenat I do not mean to convey that astronomical dis- 
 coveries are not occasionally made somewhat in 
 the manner so beautifully pictured by Keats. 
 Three years ago a persistent " watcher of the 
 skies/' Dr. Anderson of Edinburgh, suddenly 
 caught sight of a brilliant new star in Perseus ; 
 though here "flashed into his ken" would per- 
 haps be a more suitable phrase than " swam." 
 And comets have been detected by a mere glance 
 at the heavens without sensible effort or care on 
 the part of the discoverer. But these may be 
 fairly called exceptions ; in the vast majority of 
 cases hard work and a keen eye are necessary to 
 make the discovery. The relative importance of 
 these two factors of course varies in different cases ; 
 for the detection of Uranus perhaps the keen eye 
 may be put in the first place, though we must not 
 forget the diligent watching which gave it oppor- 
 tunity. Other cases of planetary discovery may 
 be attributed more completely to diligence alone, 
 as we shall presently see. But before leaving 
 
 Name Uranus for them I should like to recall the 
 circumstances attending the naming of the planet. 
 Herschel proposed to call it Georgium Sidus 
 in honour of his patron, King George III., and 
 
URANUS AND EROS 9 
 
 as the best way of making his wishes known, 
 wrote the following letter to the President of the 
 Royal Society, which is printed at the beginning 
 of the Philosophical Transactions for 1783. 
 
 A Letter from WILLIAM HERSCHEL, Esq., F.R.S., 
 to Sir JOSEPH BANKS, Bart., P.R.S. 
 
 " SIR, By the observations of the most eminent 
 astronomers in Europe it appears that the new 
 star, which I had the honour of pointing out to 
 them in March 1781, is a Primary Planet of our 
 Solar System. A body so nearly related to us by 
 its similar condition and situation in the un- 
 bounded expanse of the starry heavens, must often 
 be the subject of conversation, not only of astro- 
 nomers, but of every lover of science in general. 
 This consideration then makes it necessary to 
 give it a name whereby it may be distinguished 
 from the rest of the planets and fixed stars. 
 
 " In the fabulous ages of ancient times, the 
 appellations of Mercury, Venus, Mars, Jupiter, 
 and Saturn were given to the planets as being the 
 names of their principal heroes and divinities. In 
 the present more philosophical era, it would hardly 
 be allowable to have recourse to the same method, 
 and call on Juno, Pallas, Apollo, or Minerva for 
 a name to our new heavenly body. The first 
 consideration in any particular event, or remark- 
 able incident, seems to be its chronology : if in 
 any future age it should be asked, when this last 
 
to ASTRONOMICAL DISCOVERY 
 
 found planet was discovered? It would be a 
 very satisfactory answer to say, ' In the reign of 
 King George the Third.' As a philosopher then, 
 the name GEORGICTM SIDUS presents itself to me, 
 as an appellation which will conveniently convey 
 the information of the time and country where 
 and when it was brought to view. But as a 
 subject of the best of kings, who is the liberal 
 protector of every art and science ; as a native 
 of the country from whence this illustrious 
 family was called to the British throne ; as a 
 member of that Society which flourishes by the 
 distinguished liberality of its royal patron ; and, 
 last of all, as a person now more immediately 
 under the protection of this excellent monarch, 
 and owing everything to his unlimited bounty ; I 
 cannot but wish to take this opportunity of ex- 
 pressing my sense of gratitude by giving the name 
 Georgium Sidus, 
 
 Sidus. 
 
 Georgium Sidus 
 
 - jam nunc assuesce vocari, 
 
 Virg. Georg. 
 
 to a star which (with respect to us) first began to 
 shine under his auspicious reign. 
 
 " By addressing this letter to you, Sir, as Presi- 
 dent of the Royal Society, I take the most 
 effectual method of communicating that name to 
 the literati of Europe, which I hope they will 
 receive with pleasure. I have the honour to be, 
 with the greatest respect, Sir, your most humble 
 and most obedient servant, W. HERSCHEL." 
 
URANUS AND EROS 11 
 
 This letter reminds us how long it was since a 
 new name had been required for a new planet, 
 to find a similar occasion Herschel had to go to 
 the almost prehistoric past, when the names of 
 heroes and divinities were given to the planets. 
 It is, perhaps, not unnatural that he should have 
 considered an entirely new departure appropriate 
 for a discovery separated by so great a length of 
 time from the others; but his views were not 
 generally accepted, especially on the Continent. 
 Lalande courteously proposed the name of Her- Herschel. 
 schel for the new planet, in honour of the dis- 
 coverer, and this name was used in France ; but 
 Bode, on the other hand, was in favour of retain- 
 ing the old practice simply, and calling the new 
 planet Uranus. All three names seem to have 
 been used for many years. Only the other day 
 I was interested to see an old pack of cards, used 
 for playing a parlour game of Astronomy, in which 
 the name Herschel is used. The owner told me 
 that they had belonged to his grandfather; and 
 the date of publication was 1829, and the place 
 London, so that this name was in common use 
 in England nearly half a century after the actual 
 discovery; though in the " English Nautical Al- 
 manac " the name "the Georgian" (apparently pre- 
 ferred to Herschel's Georgium Sidus) was being 
 used officially after 1791, and did not disappear 
 from that work until 1851 (published in 1847.) 
 
 It would appear to have been the discovery of Uranus 
 Neptune, with which we shall deal in the next adopted. 
 
12 ASTRONOMICAL DISCOVERY 
 
 chapter, which led to this official change ; for in 
 the volume for 1851 is included Adams' account 
 of his discovery with the title 
 
 "Ox THE PERTURBATIONS OF UKANUS," 
 
 and there was thus a definite reason for avoiding 
 two names for the same planet in the same work. 
 But Le Verrier's paper on the same topic at the 
 same date still uses the name "Herschel" for the 
 planet. 
 
 The discovery of Neptune, as we shall see, was 
 totally different in character from that of Uranus. 
 The latter may be described as the finding of 
 something by an observer who was looking for 
 anything ; Neptune was the finding of something 
 definitely sought for, and definitely pointed out 
 by a most successful and brilliant piece of metho- 
 dical work. But before that time several planets 
 had been found, as the practical result of a 
 definite search, although the guiding principle 
 was such as cannot command our admiration to 
 quite the same extent as in the case of Neptune. 
 To explain it I must say something of the relative 
 sizes of the orbits in which planets move round 
 the sun. These orbits are, as we know, ellipses ; 
 but they are very nearly circles, and, excluding 
 refinements, we may consider them as circles, with 
 the sun at the centre of each, so that we may 
 talk of the distance of any planet from the sun 
 Bode's as a constant quantity without serious error. Now 
 if we arrange the planetary distances in order, we 
 
URANUS AND EROS 
 
 shall notice a remarkable connection between the 
 terms of the series. Here is a table showing this 
 connection. 
 
 TABLE OP THE DISTANCES OF THE PLANETS FROM 
 THE SUN, SHOWING " BODE's LAW." 
 
 
 Distance from 
 
 "Bode'sLaw" 
 
 Name of Planet. 
 
 Sun, taking 
 that of Earth 
 as 10. 
 
 (originally formulated 
 by Titius, but brought 
 into notice by Bode). 
 
 Mercury . 
 
 4 
 
 4+ o= 4 
 
 Venus 
 
 7 
 
 4+ 3= 7 
 
 The Earth 
 
 10 
 
 4+ 6 = 10 
 
 Mars 
 
 15 
 
 4+ 12= 16 
 
 ( ) 
 
 ( ) 
 
 4+ 24 = 28 
 
 Jupiter 
 
 52 
 
 4+ 48= 52 
 
 Saturn 
 
 95 
 
 4+ 96=100 
 
 Uranus . 
 
 192 
 
 4+192 = 196 
 
 If we write down a series of 4*3, and then add 
 the numbers 3, 6, 12, and so on, each formed by 
 doubling the last, we get numbers representing 
 very nearly the planetary distances, which are 
 shown approximately in the second column. But 
 three points call for notice. Firstly, the number 
 before 3 should be ij, and not zero, to agree with 
 the rest. Secondly, there is a gap, or rather was Gap in 
 a gap, after the discovery of Uranus, between suggest- 
 Mars and Jupiter ; and thirdly, we see that when J^wn 
 Uranus was discovered, and its distance' from the P lanet - 
 sun determined, this distance was found to fall 
 in satisfactorily with this law, which was first 
 stated by Titius of Wittenberg. This third fact 
 naturally attracted attention. No explanation of 
 
i 4 ASTRONOMICAL DISCOVERY 
 
 the so-called "law" was known at the time; nor 
 is any known even yet, though we may be said 
 to have some glimmerings of a possible cause ; 
 and in the absence of such explanation it must 
 be regarded as merely a curious coincidence. But 
 the chances that we are in the presence of a mere 
 coincidence diminish very quickly with each new 
 term added to the series, and when it was found 
 that Herschel's new planet fitted in so well at 
 the end of the arrangement, the question arose 
 whether the gap above noticed was real, or 
 whether there was perhaps another planet which 
 had hitherto escaped notice, revolving in an orbit 
 represented by this blank term. This question 
 had indeed been asked even before the discovery 
 of Uranus, by Bode, a young astronomer of Berlin ; 
 and for fifteen years he kept steadily in view this 
 idea of finding a planet to fill the vacant interval. 
 The search would be a very arduous one, involv- 
 ing a careful scrutiny, not perhaps of the whole 
 heavens, but of a considerable portion of it along 
 the Zodiac : too great for one would-be discoverer 
 single-handed; but in September 1800 Bode suc- 
 Search ceeded in organising a band of six German astro- 
 nomers (including himself) for the purpose of 
 conducting this search. They divided the Zodiac 
 into twenty-four zones, and were assigning the 
 zones to the different observers, when they were 
 startled by the news that the missing planet had 
 been accidentally found by Piazzi in the constella- 
 tion Taurus. The discovery was made somewhat 
 
URANUS AND EROS 15 
 
 dramatically on the first evening of the nineteenth 
 century (January i, 1801). Piazzi was not look- Acciden- 
 ing for a planet at all, but examining an error 
 made by another astronomer ; and in the course 
 of this work he recorded the position of a star of 
 the eighth magnitude. Eeturning to it on the 
 next night, it seemed to him that it had slightly 
 moved westwards, and on the following night this 
 suspicion was confirmed. Remark that in this 
 case no peculiar appearance in the star suggested 
 that it might be a comet or planet, as in the case 
 of the discovery of Uranus. We are not unfair 
 in ascribing the discovery to pure accident, al- 
 though we must not forget that a careless observer 
 might easily have missed it. Piazzi was anything 
 but careless, and watched the new object assidu- 
 ously till February nth, when he became danger- 
 ously ill; but he had written, on January 23rd, to 
 Oriani of Milan, and to Bode at Berlin on the 
 following day. These letters, however, did not 
 reach the recipients (in those days of leisurely 
 postal service) until April 5th and March 2Oth 
 respectively ; and we can imagine the mixed feel- 
 ings with which Bode heard that the discovery 
 which he had contemplated for fifteen years, and 
 for which he was just about to organise a diligent 
 search, was thus curiously snatched from him. 
 
 More curious still must have seemed the mtelli- Hegel's 
 gence to a young philosopher of Jena named fc 
 Hegel, who has since become famous, but who 
 had just imperilled his future reputation by pub- 
 
1 6 ASTRONOMICAL DISCOVERY 
 
 lishing a dissertation proving conclusively that the 
 . number of the planets could not be greater than 
 seven, and pouring scorn on the projected search 
 of the half-dozen enthusiasts who were proposing 
 to find a new planet merely to fill up a gap in a 
 numerical series. 
 
 The The sensation caused by the news of the dis- 
 
 lost again, covery was intensified by anxiety lest the new 
 planet should already have been lost; for it 
 had meanwhile travelled too close to the sun 
 for further observation, and the only material 
 available for calculating its orbit, and so predict- 
 ing its place in the heavens at future dates, was 
 afforded by the few observations made by Piazzi. 
 Was it possible to calculate the orbit from such 
 slender material ? It would take too long to ex- 
 plain fully the enormous difficulty of this problem, 
 but some notion of it may be obtained, by those 
 unacquainted with mathematics, from a rough 
 analogy. If we are given a portion of a circle, 
 we can, with the help of a pair of compasses, 
 complete the circle : we can find the centre from 
 which the arc is struck, either by geometrical 
 methods, or by a few experimental trials, and 
 then fill in the rest of the circumference. If the 
 arc given is large we can do this with certainty 
 and accuracy ; but if the arc is small it is difficult 
 to make quite sure of the centre, and our drawing 
 may not be quite accurate. Now the arc which 
 had been described by the tiny planet during 
 Piazzi's observations was only three degrees ; and 
 
URANUS AND EROS 17 
 
 if any one will kindly take out his watch and 
 look at the minute marks round the dial, three 
 degrees is just half a single minute space. If 
 the rest of the dial were obliterated, and only 
 this small arc left, would he feel much confidence 
 in restoring the obliterated portion ? This problem 
 gives some idea of the difficulties to be encountered, 
 but only even then a very imperfect one. 
 
 Briefly, the solution demanded a new mathe- Gauss 
 matical method in astronomy. But difficulties hovTto 
 are sometimes the opportunities of great men, findlt - 
 and this particular difficulty attracted to astro- 
 nomy the great mathematician Gauss, who set 
 himself to make the best of the observation avail- 
 able, and produced his classical work, the Theoria 
 Motus, which is the standard work for such cal- 
 culations to the present day. May we look for a 
 few moments at what he himself says in the pre- 
 face to his great work ? I venture to reproduce 
 the following rough translation (the book being 
 written in Latin, according to the scientific usage 
 of the time) : 
 
 EXTRACT FKOM THE PREFACE TO THE 
 Theoria Motus. 
 
 " Some ideas had occurred to me on this sub- The 
 ject in September 1801, at a time when I was 
 occupied on something quite different; ideas 
 which seemed to contribute to the solution of the 
 great problem of which I have spoken. In such 
 cases it often happens that, lest we be too much 
 
1 8 ASTRONOMICAL DISCOVERY 
 
 distracted from the attractive investigation on 
 which we are engaged, we allow associations of 
 ideas which, if more closely examined, might 
 prove extraordinarily fruitful, to perish from 
 neglect. Perchance these same idea-lets of mine 
 would have met with this fate, if they had not 
 most fortunately lighted upon a time than which 
 none could have been chosen more favourable for 
 their preservation and development. For about 
 the same time a rumour began to be spread abroad 
 concerning a new planet which had been detected 
 on January ist of that year at the Observatory of 
 Palermo ; and shortly afterwards the actual obser- 
 vations which had been made between January ist 
 and February nth by the renowned philosopher 
 Piazzi were published. Nowhere in all the 
 annals of astronomy do we find such an impor- 
 tant occasion ; and scarcely is it possible to 
 imagine a more important opportunity for point- 
 ing out, as emphatically as possible, the impor- 
 tance of that problem, as at the moment when 
 every hope of re-discovering, among the innumer- 
 able little stars of heaven, that mite of a planet 
 which had been lost to sight for nearly a year, 
 depended entirely on an approximate knowledge 
 of its orbit, which must be deduced from those 
 scanty Observations. Could I ever have had a 
 better opportunity for trying whether those idea- 
 lets of mine were of any practical value than if I 
 then were to use them for the determination of 
 the orbit of Ceres, a planet which, in the course 
 
URANUS AND EROS 19 
 
 of those forty-one days, had described around the 
 earth an arc of no more than three degrees? and, 
 after a year had passed, required to be tracked 
 out in a region of the sky far removed from its 
 original position? The first application of this 
 method was made in the month of October 1801, 
 and the first clear night, when the planet was 
 looked for by the help of the ephemeris I had 
 made, revealed the truant to the observer. Three 
 new planets found since then have supplied fresh 
 opportunities for examining and proving the effi- 
 cacy and universality of this method. 
 
 "Now a good many astronomers, immediately 
 after the rediscovery of Ceres, desired me to publish 
 the methodswhichhad been used in my calculations. 
 There were, however, not a few objections which 
 prevented me from gratifying at that moment 
 these friendly solicitations, viz. other business, 
 the desire of treating the matter more fully, and 
 more especially the expectation that, by continu- 
 ing to devote myself to this research, I should 
 bring the different portions of the solution of the 
 problem to a more perfect pitch of universality, 
 simplicity, and elegance. As my hopes have been 
 justified, I do not think there is any reason for 
 repenting of my delay. For the methods which I 
 had repeatedly applied from the beginning ad- 
 mitted of so many and such important variations, 
 that scarcely a vestige of resemblance remains 
 between the method by which formerly I had 
 arrived at the orbit of Ceres and the practice 
 
20 ASTRONOMICAL DISCOVERY 
 
 which I deal with in this work. Although in- 
 deed it would be alien to my intention to write 
 a complete history about all these researches 
 which I have gradually brought to even greater 
 perfection, yet on many occasions, especially 
 whenever I was confronted by some particularly 
 serious problem, I thought that the first methods 
 which I employed ought not to be entirely sup- 
 pressed. Nay, rather, in addition to the solutions 
 of the principal problems, I have in this work 
 followed out many questions which presented 
 themselves to me, in the course of a long study 
 of the motions of the heavenly bodies in conic 
 sections, as being particularly worthy of attention, 
 whether on account of the neatness of the analysis, 
 or more especially by reason of their practical 
 utility. Yet I have always given the greater care 
 to subjects which I have made my own, merely 
 noticing by the way well-known facts where con- 
 nection of thought seemed to demand it." 
 
 These words do not explain in any way the 
 methods introduced by Gauss, but they give us 
 some notion of the flavour of the work. Aided 
 by these brilliant researches, the little planet was 
 found on the last day of the year by Von Zach at 
 Gotha, 'and on the next night, independently, by 
 Olbers at Bremen. But, before this success, there 
 had been an arduous search, which led to a curious 
 consequence. Olbers had made himself so fami- 
 liar with all the small stars along the track which 
 was being searched for the missing body, that he 
 
URANUS AND EROS 21 
 
 was at once struck by the appearance of a stranger Another 
 near the spot where he had just identified Ceres, found! 
 At first he thought this must be some star which 
 had blazed up to brightness ; but he soon found 
 that it also was moving, and, to the great bewilder- 
 ment of the astronomical world, it proved to be 
 another planet revolving round the sun at a dis- 
 tance nearly the same as the former. This was 
 an extraordinary and totally unforeseen occurrence. 
 The world had been prepared for one planet ; but 
 here were two ! 
 
 The thought occurred to Olbers that they 
 were perhaps fragments of a single body which H ypo - 
 
 , , , * , , F , , . . thesis of 
 
 had been blown to pieces by some explosion, and many 
 that there might be more of the pieces ; and he 
 therefore suggested as a guide for finding others 
 that, since by the known laws of gravitation, 
 bodies which circle round the sun return perio- 
 dically to their starting-point, therefore all these 
 fragments would in due course return to the point 
 in the heavens where the original planet had 
 exploded. Hence the search might be most pro- 
 fitably conducted in the neighbourhood of the 
 spot where the two first fragments (which had 
 been named Ceres and Pallas) had already been 
 found. We now have good reason to believe 
 that this view is a mistaken one, but nevertheless 
 it was apparently confirmed by the discovery of 
 two more bodies of the same kind, which were 
 called Juno and Vesta ; the second of these being 
 found by Olbers himself after three years' patient 
 
22 
 
 ASTRONOMICAL DISCOVERY 
 
 work in 1807. Hence, although the idea of 
 searching for a more or less definitely imagined 
 planet was not new, although Bode had conceived 
 it as early as 1785, and organised a search on this 
 plan, three planets were actually found before the 
 first success attending a definite search. Ceres, 
 as already remarked, was found by a pure acci- 
 dent ; and the same may be said of Pallas and 
 Juno, though it may fairly be added that Pallas 
 was actually contrary to expectation. 
 
 MINOR PLANETS, 1801 TO 1850. 
 
 Number. 
 
 Name. 
 
 - 
 Discoverer. 
 
 Date. 
 
 I 
 
 Ceres 
 
 Pia/zi 
 
 1801 
 
 2 
 
 Pallas 
 
 Gibers 
 
 1802 
 
 3 
 
 Juno 
 
 Harding 
 
 1804 
 
 4 
 
 Vesta 
 
 Olbers 
 
 1807 
 
 5 
 
 Astraea 
 
 Hencke 
 
 1845 
 
 6 
 
 Hebe 
 
 Hencke 
 
 1847 
 
 7 
 
 Iris 
 
 Hind 
 
 1847 
 
 8 
 
 Flora 
 
 Hind 
 
 1847 
 
 9 
 
 Metis 
 
 Graham 
 
 1848 
 
 10 
 
 Hygeia 
 
 De Gasparis 
 
 1849 
 
 ii 
 
 Parthenope 
 
 De Gasp;iris 
 
 1850 
 
 12 
 
 Victoria 
 
 Hind 
 
 1850 
 
 13 
 
 Egeria 
 
 De Gasparis 
 
 1850 
 
 Here now is a table showing how other bodies 
 were gradually added to this first list of four, but 
 you will see that no addition was made for a long 
 time. Not that the search was immediately aban- 
 doned ; but being rewarded by no success for some 
 years, it was gradually dropped, and the belief 
 gained ground that the number of the planets 
 
i 
 
 By permission of Messrs. Macmillan & Co. 
 
 I. I. C. ADAMS. 
 
 II. A. GRAHAM. 
 
 DISCOVERER OF THE NINTH MINOR PLANET (.METIS). 
 
URANUS AND EROS 23 
 
 was at last complete. The discoverers of Uranus 
 and of these first four minor planets all died before 
 any further addition was made ; and it was not 
 until the end of 1845 that Astraea was found by 
 an ex-postmaster of the Prussian town of Driessen, Hencke' 
 by name Hencke, who, in spite of the general search, 
 disbelief in the existence of any more planets, set 
 himself diligently to search for them, and toiled 
 for fifteen long years before at length reaping his 
 reward. Others then resumed the search ; Hind, 
 the observer of an English amateur astronomer 
 near London, found Iris a few weeks after Hencke 
 had been rewarded by a second discovery in 1847, 
 and in the following year Mr. Graham at Markree 
 in Ireland (who is still living, and has only just 
 retired from active work at the Cambridge Obser- 
 vatory) found Metis ; and from that time new 
 discoveries have been added year by year, until 
 the number of planets now known exceeds 500, 
 and is steadily increasing. 
 
 You will see the great variety characterising 
 these discoveries ; some of them are the result 
 of deliberate search, others have come accident- 
 ally, and some even contrary to expectation. Of 
 the great majority of the earlier ones it may be 
 said that enormous diligence was required for 
 each discovery ; to identify a planet it is necessary 
 to have either a good map of the stars or to know 
 them thoroughly, so that the map practically exists 
 in the brain. We need only remember Hencke's 
 fifteen years of search before success to recognise 
 
24 ASTRONOMICAL DISCOVERY 
 
 what vast stores of patience and diligence were 
 
 required in carrying out the search. But of late 
 
 The years photography has effected a great revolution 
 
 graphic in this respect. It is no longer necessary to do 
 method. more than get what g ir R obert Ba u has called a 
 
 " star-trap," or rather planet-trap. If a photograph 
 be taken of a region of the heavens, by the methods 
 familiar to astronomers, so that each star makes a 
 round dot on the photographic plate, any suffi- 
 ciently bright object moving relatively to the stars 
 will make a small line or trail, and thus betray its 
 planetary character. In this way most of the 
 recent discoveries have been made, and although 
 diligence is still required in taking the photo- 
 graphs, and again in identifying the objects thus 
 found (which are now very often the images of 
 already known members of the system), the tedious 
 scrutiny with the eye has become a thing of the past. 
 
 TABLE SHOWING THE NUMBER OF MINOR PLANETS DISCOVERED 
 IN EACH DECADE SINCE 1850. 
 
 1 80 1 to 1850 altogether 13 discoveries. 
 1851 to 1860 , 49 
 
 1861 to 1870 
 1871 to 1880 
 1881 to 1890 
 1891 to 1900 
 In 1901 
 
 49 > 
 1 08 
 
 83 
 1 80 announcements. 
 
 36 
 
 1902 50 
 
 1903 4i 
 
 Total 609 
 
 [N.J2. Many of the more recent announce- 
 ments turned out to refer to old discoveries.] 
 
URANUS AND EROS 25 
 
 The known number of these bodies has accord- 
 ingly increased so rapidly as to become almost 
 an embarrassment ; and in one respect the embar- 
 rassment is definite, for it has become quite 
 difficult to find names for the new discoveries. Scarcity 
 We remember with amusement at the present 
 time that for the early discoveries there was 
 sometimes a controversy (of the same kind 
 as in the case of Uranus) about the exact 
 name which a planet should have. Thus when 
 it was proposed to call No. 12 (discovered in 
 1850, in London, by Mr. Hind) "Victoria," 
 there was an outcry by foreign astronomers 
 that by a subterfuge the name of a reigning 
 monarch was again being proposed for a planet, 
 and considerable opposition was manifested, 
 especially in America. But it became clear, 
 as other discoveries were added, that the list 
 of goddesses, or even humbler mythological 
 people, would not be large enough to go round 
 if we were so severely critical, and must 
 sooner or later be supplemented from sources 
 hitherto considered unsuitable ; so, ultimately, 
 the opposition to the name Victoria was with- 
 drawn. Later still the restriction to feminine 
 names has been broken through ; one planet has 
 been named Endyniion, and another, of which we 
 shall presently speak more particularly, has been 
 called Eros. But before passing to him you 
 
26 ASTRONOMICAL DISCOVERY 
 
 may care to look at some of the names selected 
 for others : 
 
 No. Name. 
 
 248 . . Lameia 
 
 250 . . Bettina 
 
 261 . Prymno 
 
 264 . . Libussa 
 
 296 . . Phaetusa 
 
 340 . . Eduarda 
 
 341 . . California 
 350 . . Ornamenta 
 357 . . Ninina 
 385 . . Ilmatar 
 
 No. Name. 
 
 389 . . ludustria 
 
 391 . . Ingeborg 
 
 433 Eros 
 
 443 . . Photograpliica 
 
 457 . . Alleghenia 
 
 462 . . Eri phyla 
 
 475 . . Ocllo 
 
 484 . . Pitt>burghia 
 
 503 . . Evelyn 
 
 Bettina. In connection with No. 250 there is an interest- 
 ing little history. In the Observatory for 1885, 
 page 63, appeared the following advertisement : 
 " Herr Palisa being desirous to raise funds for his 
 intended expedition to observe the Total Solar 
 Eclipse of August 1886, will sell the right of 
 naming the minor planet No. 244 for ,50." The 
 bright idea seems to have struck Herr Palisa, who 
 had already discovered many planets and begun to 
 find difficulties in assigning suitable names, that 
 he might turn his difficulty into a source of profit 
 in a good cause. The offer was not responded to 
 immediately, nor until Herr Palisa had discovered 
 two more planets, Nos. 248 and 250. He found 
 names for two, leaving, however, the last dis- 
 covered always open for a patron, and on page 
 142 of the same magazine for 1886 the following 
 note informs us how his patience was ultimately 
 rewarded: "Minor planet No. 250 has been 
 
URANUS AND EROS 27 
 
 named ' Bettina ' by Baron Albert de Rothschild." 
 I have not heard, however, that this precedent has 
 been followed in other cases, and the ingenuity of 
 discoverers was so much overtaxed towards the 
 end of last century that the naming of their 
 planets fell into arrears. Recently a Commission, 
 which has been established to look after these 
 small bodies generally, issued a notice that unless 
 the naming was accomplished before a certain 
 date it would be ruthlessly taken out of the hands 
 of the negligent discoverers. Perhaps we may 
 notice, before passing on, the provisional system Thepro- 
 which was adopted to fill up the interval required letter^ 
 for finding a suitable name, and required also for 
 making sure that the planet was in fact a new 
 one, and not merely an old one rediscovered. 
 There was a system of numbering in existence as 
 well as of naming, but it was unadvisable to 
 attach even a number to a planet until it was 
 quite certain that the discovery was new, for 
 otherwise there might be gaps created in what 
 should be a continuous series by spurious dis- 
 coveries being struck out. Accordingly it was 
 decided to attach at first to the object merely a 
 letter of the alphabet, with the year of discovery, 
 as a provisional name. The alphabet was, how- 
 ever, run through so quickly, and confusion was 
 so likely to ensue if it was merely repeated, that 
 on recommencing it the letter A was prefixed, 
 and the symbols adopted were therefore A A, 
 AB, AC, &c. ; after completing the alphabet 
 
28 ASTRONOMICAL DISCOVERY 
 
 again, the letter B was prefixed, and so on ; and 
 astronomers began to fear that they had before 
 them a monotonous prospect of continually add- 
 ing new planets, varied by no incident more excit- 
 ing than starting the alphabet over again after 
 every score. 
 
 Fortunately, however, on running through it 
 for the fifth time, an object of particular interest 
 
 Eros. was discovered. Most of these bodies revolve 
 at a distance from the sun intermediate between 
 that of Mars and that of Jupiter, but the little 
 planet which took the symbol DQ, and afterwards 
 the name of Eros, was found to have a mean 
 distance actually less than that of Mars, and 
 this gave it an extraordinary importance with 
 respect to the great problem of determining the 
 sun's distance. To explain this importance we 
 must make a small digression. 
 
 Transit of About the middle of the last century our 
 knowledge of the sun's distance was very rough, 
 as may be seen from the table on p. 32 ; but there 
 were in prospect two transits of Venus, in 1874 
 and 1882, and it was hoped that these would give 
 opportunities of a special kind for the measure- 
 ment of this important quantity, which lies at the 
 root of all our knowledge of the exact masses and 
 dimensions of not only the sun, but of the planets 
 as well. 
 
 The method may be briefly summarised thus : 
 An observer in one part of the earth would see 
 Venus cross the disc of the sun along a different 
 
URANUS AND EROS 29 
 
 path from that seen by another observer, as will 
 be clear from the diagram. If the size of the 
 
 EARTH 
 
 SUN 
 
 FIG. i. 
 
 earth, the distance of the sun, and the relative 
 distance of Venus be known, it can be calcu- 
 lated what this difference in path will be. 
 Now the relative distance of Venus is known 
 with great accuracy, from observing the time of 
 her revolution round the sun; the size of the 
 earth we can measure by a survey ; there remains, 
 therefore^ only one unknown quantity, the sun's 
 distance. And since from a knowledge of this we 
 could calculate the difference in path, it is easy 
 to invert the problem, and calculate the sun's 
 distance from the knowledge of the observed 
 difference in path. Accordingly, observers were 
 to be scattered, not merely to two, but to many 
 stations over the face of the earth, to observe the 
 exact path taken by Venus in transit over the sun's 
 disc as seen from their station ; and especially to 
 observe the exact times of beginning and ending 
 of the transit ; and, by comparison of their results, 
 
30 ASTRONOMICAL DISCOVERY 
 
 it was hoped to determine this very important 
 quantity, the sun's distance. It was known from 
 previous experience that there were certain diffi- 
 culties in observing very exactly the beginning 
 and end of the transit. There was an appear- 
 The ance called the " Black Drop," which had caused 
 
 "Black , , . . 
 
 Drop." trouble on previous occasions ; an appearance as 
 though the round black spot which can be seen 
 when Venus has advanced some distance over the 
 sun's disc was reluctant to make the entry and 
 clung to the edge or " limb " of the sun as it is 
 called, somewhat as a drop of ink clings to a 
 pen which is slowly withdrawn from an inkpot. 
 Similarly, at the end of the transit or egress, 
 instead of approaching the limb steadily the 
 planet seems at the last moment to burst out 
 towards it, rendering the estimation of the exact 
 moment when the transit is over extremely 
 doubtful. 
 
 These difficulties, as already stated, were known 
 to exist; but there is a long interval between 
 transits of Venus, or rather between every pair 
 of such transits. After those of 1874 and 1882 
 there will be no more until 2004 and 2012, so 
 that we shall never see another ; similarly, before 
 that pair of the last century, there had not been 
 any such occasion since 1761 and 1769, and no 
 one was alive who remembered at first hand 
 the trouble which was known to exist. It was 
 proposed to obviate the anticipated difficulties 
 by careful practice beforehand ; models were 
 
URANUS AND EROS 31 
 
 prepared to resemble as nearly as possible the 
 expected appearances, and the times recorded by 
 different observers were compared with the true 
 time, which could, in this case of a model, be 
 determined. In this way it was hoped that the 
 habit of each observer, his "personal equation" as 
 it is called, could be determined beforehand, and 
 allowed for as a correction when he came to 
 observe the actual transit. The result, however, 
 was a great disappointment. The actual appear- 
 ances were found to be totally different in 
 character from those shown by the model ; Failure. 
 chiefly, perhaps, because it had been impos- 
 sible to imitate with a model the effect of the 
 atmosphere which surrounds the planet Yenus. 
 Observers trained beforehand, using similar in- 
 struments, and standing within a few feet of each 
 other, were expected, after making due allowance 
 for personal equation, to give the same instant for 
 contact; but their observations when made were 
 found to differ by nearly a minute of time, and 
 after an exhaustive review of the whole material 
 it was felt that all hope of determining accurately 
 the sun's distance by this method must be given 
 up. The following table will show how much 
 was learned from the transits of Venus, and 
 how much remained to be settled. They left 
 the result in doubt over a range of about two 
 million miles. 
 
32 ASTRONOMICAL DISCOVERY 
 
 SUN'S DISTANCE, IN MILLIONS OF MILES, AS 
 FOUND BY DIFFERENT OBSERVERS 
 
 Before the Transits of Venus estimates varied 
 between 96 million miles (Gilliss and Gould, 
 1856) and 91 million (Winneche, 1863), a range 
 of 5 million miles. 
 
 The Transits of 1874 and 1882 gave results lying 
 between 93^ million (Airy, from British observa- 
 tions of 1874), 92^ million (Stone, from British 
 observations of 1882), and 91|- million (Puiseux, 
 from French observations), a range of if millions. 
 
 Gill's Heliometer results all lie very near 93 
 millions. The observations of Mars in 1877 give 
 about 100,000 miles over this figure : but the 
 observations of Victoria, Iris, and Sappho, which 
 are more trustworthy, all agree in giving about 
 100,000 miles less than the 93 millions. 
 
 It became necessary, therefore, to look to other 
 methods; and before the second transit of 1882 
 was observed, an energetic astronomer, Dr. David 
 Gill, had already put into operation the method 
 which may be now regarded as the standard 
 one. 
 
 Modem We have said that the relative distance of 
 
 fbr sun's Venus from the sun is accurately known from 
 
 distance, observations of the exact time of revolution. It 
 
 is easy to see that these times of revolution can 
 

 URANUS AND EROS 33 
 
 be measured accurately by mere accumulation. 
 We may make an error of a few seconds in noting 
 the time of return ; but if the whole interval 
 comprises 10 revolutions, this error is divided by 
 10, if 100 revolutions by 100, and so on; and by 
 this time a great number of revolutions of all the 
 planets (except those just discovered) have been 
 recorded. Hence we know their relative dis- 
 tances with great precision ; and if we can find 
 the distance in miles of any one of them, we 
 can find that of the sun itself, or of any other 
 planet, by a simple rule-of-three sum. By making 
 use of this principle many of the difficulties 
 attending the direct determination of the sun's 
 distance can be avoided ; for instance, since the 
 sun's light overpowers that of the stars, it is not 
 easy to directly observe the place of the sun 
 among the stars ; but this is not so for the planets. 
 We can photograph a planet and the stars sur- Photo- 
 rounding it on the same plate, and then by care- ^ 
 ful measurement determine its exact position 
 among the stars ; and since this position differs 
 slightly according to the situation of the observer 
 on the earth's surface, by comparing two photo- 
 graphs taken at stations a known distance apart 
 we can find the distance of the planet from the 
 earth ; and hence, as above remarked, the dis- 
 tance of the sun and all the other members of the 
 solar system. Or, instead of taking photographs 
 from two different stations, we can take from the 
 same station two photographs at times separated 
 
 c 
 
Ascen- 
 sion. 
 
 34 ASTRONOMICAL DISCOVERY 
 
 by a known interval. For in that interval the 
 station will have been carried by the earth's rota- 
 tion some thousands of miles away from its former 
 position, and becomes virtually a second station 
 separated from the first by a distance which is 
 known accurately when we know the elapsed 
 time. Again, instead of taking photographs, and 
 from them measuring the position of the planet 
 among the stars, we may make the measurements 
 on the planet and stars in the sky itself; and 
 Dr. Gin's since in 1878, when Dr. Gill set out on his enter- 
 tkmto" prise of determining the sun's distance, photo- 
 graphy was in its infancy as applied to astronomy, 
 he naturally made his observations on the sky 
 with an instrument known as a heliometer. He 
 made them in the little island of Ascension, which 
 is suitably situated for the purpose ; because, 
 being near the earth's equator, it is carried by 
 the earth's rotation a longer distance in a given 
 time than places nearer the poles, and in these 
 observations for "parallax," as they are called, 
 it is important to have the displacement of the 
 station as large as possible. For a similar reason 
 the object selected among the planets must be as 
 near the earth as possible ; and hence the planet 
 Mars, which at favourable times comes nearer to 
 us than any other superior planet 1 then known, 
 was selected for observation with the heliometer. 
 And now it will be seen why the discovery of 
 
 1 The inferior planet Venus conies closer, but is not visible 
 throughout the night. 
 
URANUS AND EROS 35 
 
 the little planet Eros was important, for Mars 
 was no longer the known planet capable of 
 coming nearest to us ; it had been replaced by 
 this new arrival. 
 
 Further, a small planet which is in appearance 
 just like an ordinary star has, irrespective of this 
 great proximity, some distinct advantages over a 
 planet like Mars, which appears as a round disc, 
 and is, moreover, of a somewhat reddish colour. 
 When the distance of an object of this kind from 
 a point of line such as a star is measured with the 
 heliometer it is found that a certain bias, some- 
 what difficult to allow for with certainty, is intro- 
 duced into the measures ; and our confidence in 
 the final results suffers accordingly. After his 
 observations of Mars in 1878, Dr. David Gill was 
 sufficiently impressed with this source of error 
 to make three new determinations of the sun's 
 distance, using three of the minor planets instead Victoria, 
 of Mars, in spite of the fact that they were sen- Sappho. 
 sibly farther away ; and his choice was justified 
 by finding that the results from these three 
 different sets of observations agreed well among 
 themselves, and differed slightly from that given 
 by the observations of Mars. Hence it seems 
 conclusively proved that one of these bodies is 
 a better selection than Mars in any case, and the 
 discovery of Eros, which offered the advantage Eros, 
 of greater proximity in addition, was hailed as 
 a new opportunity of a most welcome kind. It 
 was seen by a little calculation that in the winter 
 
36 ASTRONOMICAL DISCOVERY 
 
 of 1900-1901 the planet would come very near the 
 earth ; not the nearest possible (for it was also 
 realised that a still better opportunity had occurred 
 in 1894, though it was lost because the planet had 
 not yet been discovered), but still the nearest 
 approach which would occur for some thirty years ; 
 and extensive, though somewhat hasty, prepara- 
 tions were made to use it to the fullest advantage. 
 Photography had now become established as an 
 accurate method of making measurements of the 
 kind required ; and all the photographic tele- 
 scopes which could be spared were pressed into 
 the service, and diligently photographed the planet 
 and surrounding stars every fine night during the 
 favourable period. The work of measuring and 
 reducing these photographs involves an enormous 
 amount of labour, and is even yet far from com- 
 pleted, but we know enough to expect a result 
 of the greatest value. More than this we have 
 not time to say here about this great problem, 
 but it will have been made clear that just when 
 astronomers were beginning to wonder whether 
 it was worth while continuing the monotonous 
 discovery of new minor planets by the handful, 
 the 433rd discovery also turned out to be one of 
 the greatest importance. 
 
 To canons for the advantageous prosecution 
 of research, if we care to make them, we may 
 therefore add this that there is no line of re- 
 search, however apparently unimportant or mono- 
 tonous, which we can afford to neglect. Just when 
 
URANUS AND EROS 37 
 
 we are on the point of relinquishing it under the 
 impression that the mine is exhausted, we may 
 be about to find a nugget worth all our previous 
 and future labour. This rule will not, perhaps, 
 help us very much in choosing what to work at ; 
 indeed, it is no rule at all, for it leaves us the 
 whole field of choice unlimited. But this negative 
 result will recur again and again as we examine 
 the lessons taught by discoveries : there seem to 
 be no rules at all. Whenever we seem to be 
 able to deduce one from an experience, some 
 other experience will flatly contradict it. Thus 
 we might think that the discovery of Eros taught 
 us to proceed patiently with a monotonous duty, 
 and not turn aside to more novel and attractive 
 work ; yet it is often by leaving what is in hand 
 and apparently has first claim on our attention 
 that we shall do best, and we shall learn in the 
 next chapter how a failure thus to turn flexibly 
 aside was repented. 
 
CHAPTER II 
 THE DISCOVERY OF NEPTUNE 
 
 Search IN the last chapter we saw that the circumstances 
 finite 6 under which planets were discovered varied con- 
 objects. s iderably. Sometimes the discoveries were not 
 previously expected, occurring during a general 
 examination of the heavens, or a search for other 
 objects ; and, on one occasion at -least, the dis- 
 covery may be said to have been even contrary 
 to expectation, though, as the existence of a 
 number of minor planets began to be realised, 
 there have also been many cases where the dis- 
 covery has been made as the result of a definite 
 and deliberate search. But the search cannot be 
 said to have been inspired by any very clear or 
 certain principle : for the law of Bode, successful 
 though it has been in indicating the possible 
 existence of new planets, cannot, as yet, be said 
 to be founded upon a formulated law of nature. 
 We now come, however, to a discovery made in 
 direct interpretation of Newton's great law of 
 gravitation the discovery of Neptune from its 
 observed disturbance of Uranus. I will first 
 briefly recall the main facts relating to the actual 
 discovery. 
 
THE DISCOVERY OF NEPTUNE 39 
 
 After Uranus had been discovered and observed Disturb- 
 sufficiently long for its orbit to be calculated, it 
 was found that the subsequent position of the 
 planet did not always agree with this orbit ; and, 
 more serious than this, some early observations 
 were found which could not be reconciled with 
 the later ones at all. It is a wonderful testimony 
 to the care and sagacity of Sir William Herschel, 
 as was remarked in the last chapter, that Uranus 
 was found to have been observed, under the mis- 
 taken impression that it was an ordinary star, by 
 Flamsteed, Lemonnier, Bradley, and Mayer, all 
 observers of considerable ability. Flamsteed's 
 five observations dated as far back as 1690, 
 1712, and 1715 ; observations by others were 
 in 1748, 1750, 1753, 1756, and so on up to 1771, 
 and the body of testimony was so considerable 
 that there was no room for doubt as to the 
 irreconcilability of the observations with the 
 orbit, such as might have been the case had 
 there been only one or two, possibly affected 
 with some errors. 
 
 It is difficult to mention an exact date for the 
 conversion into certainty of the suspicion that 
 no single orbit could be found to satisfy all the 
 observations ; but we may certainly regard this 
 fact as established in 1821, when Alexis Bouvard 
 published some tables of the planet, and showed 
 fully in the introduction that when every correc- 
 tion for the disturbing action of other planets had 
 been applied, it was still impossible to reconcile 
 
40 ASTRONOMICAL DISCOVERY 
 
 the old observations with the orbit calculated from 
 Suspicion the new ones. The idea accordingly grew up 
 turtog that there might be some other body or bodies 
 iet ' attracting the planet and causing these dis- 
 crepancies. Here again it is not easy to say 
 exactly when this notion arose, but it was cer- 
 tainly existent in 1834, as the following letter 
 to the Astronomer Royal will show. I take it 
 from his well-known " Account of some Circum- 
 stances historically connected with the Discovery 
 of the Planet exterior to Uranus," which he gave to 
 the Royal Astronomical Society at its first meet- 
 ing after that famous discovery (Monthly Notices 
 of the R.A.S., vol. iii., and Memoirs, vol. xvi.). 
 
 No. i. The REV. T. J. HUSSEY to G. B. AIRY. 
 
 [Extract.] 
 "'HAYES, KENT, ijih November 1834. 
 
 " ' With M. Alexis Bouvard I had some con- 
 versation upon a subject I had often meditated, 
 which will probably interest you, and your opinion 
 may determine mine. Having taken great pains 
 last year with some observations of Uranus, I 
 was led to examine closely Bouvard's tables of 
 that planet. The apparently inexplicable dis- 
 crepancies 4 between the ancient and modern ob- 
 servations suggested to me the possibility of some 
 disturbing body beyond Uranus, not taken into 
 account because unknown. My first idea was 
 to ascertain some approximate place of this sup- 
 
THE DISCOVERY OF NEPTUNE 41 
 
 posed body empirically, and then with my large 
 reflector set to work to examine all the minute 
 stars thereabouts : but I found myself totally 
 inadequate to the former part of the task. If 
 I could have done it formerly, it was beyond 
 me now, even supposing I had the time, which 
 was not the case. I therefore relinquished the 
 matter altogether ; but subsequently, in conversa- 
 tion with Bouvard, I inquired if the above might 
 not be the case : his answer was, that, as might 
 have been expected, it had occurred to him, and 
 some correspondence had taken place between 
 Hansen and himself respecting it. Hansen's 
 opinion was, that one disturbing body would 
 not satisfy the phenomena; but that he conjec- 
 tured there were two planets beyond Uranus 
 Upon my speaking of obtaining the places em- 
 pirically, and then sweeping closely for the bodies, 
 he fully acquiesced in the propriety of it, intimat- 
 ing that the previous calculations would be more 
 laborious than difficult ; that if he had leisure he 
 would undertake them and transmit the results 
 to me, as the basis of a very close and accurate 
 sweep. I have not heard from him since on the 
 subject, and have been too ill to write. What is 
 your opinion on the subject? If you consider the 
 idea as possible, can you give me the limits, 
 roughly, between which this body or those bodies 
 may probably be found during the ensuing winter ? 
 As w r e might expect an eccentricity [inclination ?] 
 approaching rather to that of the old planets than 
 
42 ASTRONOMICAL DISCOVERY 
 
 of the new, the breadth of the zone to be examined 
 will be comparatively inconsiderable. I may be 
 wrong, but I am disposed to think that, such is 
 the perfection of my equatoreal's object-glass, I 
 could distinguish, almost at once, the difference 
 of light of a small planet and a star. My plan 
 of proceeding, however, would be very different : 
 I should accurately map the whole space within 
 the required limits, down to the minutest star I 
 could discern ; the interval of a single week would 
 then enable me to ascertain any change. If the 
 whole of this matter do not appear to you a 
 chimaera, which, until my conversation with 
 Bouvard, I was afraid it might, I shall be very 
 glad of any sort of hint respecting it.' 
 
 " My answer was in the following terms : 
 No. 2. G. B. AIRY to the REV. T. J. HUSSEY. 
 
 [Extract.] 
 
 "'OBSERVATORY, CAMBRIDGE, 1834, Nov. 23. 
 
 Airy's " ' I have often thought of the irregularity of 
 
 cS 1 Uranus, and since the receipt of your letter have 
 looked more carefully to it. It is a puzzling 
 subject, but I give it as my opinion, without 
 hesitation, that it is not yet in such a state as 
 to give .the smallest hope of making out the 
 nature of any external action on the planet . . . 
 if it were certain that there were any extraneous 
 action, I doubt much the possibility of determin- 
 ing the place of a planet which produced it. I 
 am sure it could not be done till the nature of 
 
THE DISCOVERY OF NEPTUNE 43 
 
 the irregularity was well determined from several 
 successive revolutions.' " 
 
 Although only a sentence or two have been 
 selected from Airy's reply (he was not yet Astro- 
 nomer Royal), they are sufficient to show that the 
 problem of finding the place of such a possible 
 disturbing body was regarded at that time as one 
 of extreme difficulty ; and no one appears seriously 
 to have contemplated embarking upon its solu- 
 tion. It was not until many years later that the 
 solution was attempted. Of the first attempt we 
 shall speak presently, putting it aside for the 
 moment because it had no actual bearing on the 
 discovery of the planet, for reasons which form 
 an extraordinary episode of this history. The 
 attempt which led to success dates from Novem- 
 ber 1845. The great French astronomer Le Le 
 Verrier, on November 10, 1845, read to the papers! S 
 French Academy a paper on the Orbit of Uranus, 
 considering specially the disturbances produced 
 by Jupiter and Saturn, and showing clearly that 
 with no possible orbit could the observations be 
 satisfied. On June i, 1846, followed a second 
 paper by the same author, in which he considers 
 all the possible explanations of the discordance, 
 and concludes that none is admissible except that 
 of a disturbing planet exterior to Uranus. And 
 assuming, in accordance with Bode's Law, that 
 the distance of this new planet from the sun 
 would be about double that of Uranus (and it 
 
44 ASTRONOMICAL DISCOVERY 
 
 is important to note this assumption), he proceeds 
 
 to investigate the orbit of such a planet, and to 
 
 calculate the place where it must be looked for 
 
 in the heavens. This was followed by a third 
 
 paper on August 3ist, giving a rather completer 
 
 discussion, and arriving at the conclusion that 
 
 Planet the planet should be recognisable from its disc. 
 
 detected This again is an important point. We remem- 
 
 bydisc. 1^ ^at | n ^ e djgcoyer O f Uranus it needed 
 
 considerable skill on the part of Sir William 
 Herschel to detect the disc, to see in fact any 
 difference between it and surrounding stars ; and 
 that other observers, even when their attention 
 had been called to the planet, found it difficult 
 to see this difference. It might be expected, 
 therefore, that with a planet twice as far away 
 (as had been assumed for the new planet) the 
 disc would be practically unrecognisable, and as 
 we shall presently see, this assumption was made 
 in some searches for the planet which had been 
 commenced even before the publication of this 
 third paper. Le Verrier's courageous announce- 
 ment, which he deduced from a consideration of 
 the mass of the planet, that the disc should be 
 recognisable, led immediately to the discovery of 
 Gaiie's the suspected body. He wrote to a German 
 o?t C hT ry astronomer, Dr. Galle (still, I am glad to say, 
 planet, alive and well, though now a very old man), 
 telling him the spot in the heavens to search, 
 and stating that he might expect to detect the 
 planet by its appearance in this way; and the 
 
THE DISCOVERY OF NEPTUNE 45 
 
 same night Dr. Galle, by comparing a star map 
 with the heavens, found the planet. 
 
 To two points to which I have specially called 
 attention in this brief summary namely, the 
 preliminary assumption that the planet would be, 
 according to Bode's Law, twice as far away as 
 Uranus ; secondly, the confident assertion that it 
 would have a visible disc I will ask you to add, 
 thirdly, that it was found by the aid of a star map, 
 for this map played an important part in the 
 further history to which we shall now proceed. 
 It may naturally be supposed that the announce- 
 ment of the finding of a planet in this way, the 
 calculation of its place from a belief in the uni- 
 versal action of the great Law of Gravitation, the 
 direction to an eminent observer to look in that 
 place for a particular thing, and his immediate 
 success, this extraordinary combination of cir- 
 cumstances caused a profound sensation through- 
 out not only the astronomical, but the whole 
 world ; and this sensation was greatly enhanced 
 by the rumour which had begun to gather strength 
 that, but for some unfortunate circumstances, the 
 discovery might have been made even earlier and as 
 a consequence of totally independent calculations 
 made by a young Cambridge mathematician, 
 J. C. Adams. Some of you are doubtless already Adams' 
 familiar with the story in its abridged form, for it uciy an- 
 has been scattered broadcast through literature. nouced - 
 In England it generally takes the form of em- 
 phasising the wickedness or laziness of the 
 
46 ASTRONOMICAL DISCOVERY 
 
 Astronomer Royal who, when told where to look 
 for a planet, neglected his obvious duty, so that 
 in consequence another astronomer who made the 
 calculation much later and gave a more virtuous 
 observer the same directions where to look, 
 obtained for France the glory of a discovery which 
 ought to have been retained in England. There 
 is no doubt that Airy's conduct received a large 
 amount of what he called " savage abuse." When 
 the facts are clearly stated I think it will be 
 evident that many of the harsh things said of him 
 were scarcely just, though at the same time it is 
 also difficult to understand his conduct at two or 
 three points of the history, even as explained by 
 himself. 
 
 Facts un- There is fortunately no doubt whatever about 
 doubted. anv o f fa e facts. Airy himself gave a very clear 
 and straightforward account of them at the time, 
 for which more credit is due to him than he 
 commonly receives ; and since the death of the 
 chief actors in this sensational drama they have 
 been naturally again ransacked, with the satis- 
 factory result that there is practically no doubt 
 about any of the facts. As to the proper interpre- 
 tations of them there certainly may be wide dif- 
 ferences of opinion, nor does this circumstance 
 detract from their interest. It is almost impossible 
 to make a perfectly colourless recital of them, nor 
 is it perhaps necessary to do so. I will therefore 
 ask you to remember in what I now say that there 
 is almost necessarily an element of personal bias, 
 
THE DISCOVERY OF NEPTUNE 47 
 
 and that another writer would probably give a 
 different colouring. Having said this, I hope I 
 may speak quite freely as the matter appears in 
 my personal estimation. 
 
 Airy's account was, as above stated, given to the A 
 Royal Astronomical Society at their first meeting count." 
 (after the startling announcement of the discovery 
 of the new planet), on November 13, 1846, and 
 I have already quoted an extract from it. He 
 opens with a tribute to the sensational character 
 of the discovery, and then states that although 
 clearly due to two individuals (namely, Le Verrier 
 and Galle), it might also be regarded as to some 
 extent the consequence of a movement of the age. "Amove- 
 .His actual words are these : "The principal steps the age." 
 in the theoretical investigations have been made 
 by one individual, and the published discovery of 
 the planet was necessarily made by one individual. 
 To these persons the public attention has been 
 principally directed ; and well do they deserve the 
 honours which they have received, and which they 
 will continue to receive. Yet we should do wrong 
 if we considered that these two persons alone are 
 to be regarded as the authors of the discovery 
 of this planet. I am confident that it will 
 be found that the discovery is a consequence 
 of what may properly be called a movement of 
 the age ; that it has been urged by the feeling 
 of the scientific world in general, and has been 
 nearly perfected by the collateral, but independent 
 labours, of various persons possessing the talents 
 
48 ASTRONOMICAL DISCOVERY 
 
 or powers best suited to the different parts of the 
 researches." 
 
 I have quoted these words as the first point at 
 which it is difficult to understand Airy's conduct 
 in excluding from them all specific mention of 
 Adams, knowing as he did the special claims 
 which entitled him to such mention ; claims 
 indeed which he proceeded immediately to make 
 Airy clear. It seems almost certain that Airy entirely 
 estimated under-estimated the value of Adams' work through- 
 work 18 ' ou k But this will become clearer as we proceed. 
 The "account" takes the form of the publication 
 of a series of letters with occasional comments. 
 Airy was a most methodical person, and filed all 
 his correspondence with great regularity. It was 
 jestingly said of him once that if he wiped his 
 pen on a piece of blotting-paper, he would date 
 the blotting-paper and file it for reference. The 
 letters reproduced in this "account" are still in 
 the Observatory at Greenwich, pinned together 
 just as Airy left them ; and in preparing his 
 "account" it was necessary to do little else than 
 to have them copied out and interpolate comments. 
 From two of them I have already quoted to show 
 how difficult the enterprise of finding an exterior 
 planet from its action on Uranus was considered 
 in 1834. To these may be added the following 
 sentence from No. 4, dated 1837. "If it be the 
 effect of any unseen body," writes Airy to Bouvard, 
 " it will be nearly impossible ever to find out its 
 place." But the first letter which need concern 
 
THE DISCOVERY OF NEPTUNE 49 
 
 us is No. 6, and it is only necessary to explain that 
 Professor Challis was the Professor of Astronomy 
 at Cambridge, and in charge of the Cambridge 
 Observatory, in which offices he had succeeded 
 Airy himself on his leaving Cambridge for Green- 
 wich some eight years earlier. 
 
 No. 6. PROFESSOR CHALLIS to G. B. AIRY. 
 
 [Extract.} 
 
 "'CAMBRIDGE OBSERVATORY, Feb. 13, 1844. 
 " * A young friend of mine, Mr. Adams of St. chains 
 John's College, is working at the theory of 
 Uranus, and is desirous of obtaining errors of the Airy 
 tabular geocentric longitudes of this planet, when 
 near opposition, in the years 1818-1826, with the 
 factors for reducing them to errors of heliocentric 
 longitude. Are your reductions of the planetary 
 observations so far advanced that you could furnish 
 these data? and is the request one which you 
 have any objection to comply with ? If Mr. 
 Adams may be favoured in this respect, he is 
 further desirous of knowing, whether in the 
 calculation of the tabular errors any alterations 
 have been made in Bouvard's Tables of Uranus 
 besides that of Jupiter s mass/ 
 
 " My answer to him was as follows : 
 No. 7. G. B. AIRY to PROFESSOR CHALLIS. 
 
 [Extract.'] 
 
 "' ROYAL OBSERVATORY, GREENWICH, 1844, Feb. 15. 
 " * I send all the results of the observations of 
 Uranus made with both instruments (that is, the 
 
 D 
 
50 ASTRONOMICAL DISCOVERY 
 
 heliocentric errors of Uranus in longitude and 
 latitude from 1754 to 1830, for all those days 
 on which there were observations, both of right 
 ascension and of polar distance). No alteration 
 is made in Bouvard's Tables of Uranus except 
 in increasing the two equations which depend on 
 Jupiter by -^ part. As constants have been 
 added (in the printed tables) to make the 
 equations positive, and as ^ part of the numbers 
 in the tables has been added, -^ part of the 
 constants has been subtracted from the final 
 results.' 
 
 " Professor Challis in acknowledging the receipt 
 of these, used the following expressions : 
 
 No. 8. PROFESSOR CHALLIS to G. B, AIRY. 
 
 [Extract.} 
 
 "'CAMBRIDGE OBSERVATORY, Feb. 16, 1844. 
 " ' I am exceedingly obliged by your sending so 
 complete a series of tabular errors of Uranus. 
 . . . The list you have sent will give Mr. Adams 
 the means of carrying on in the most effective 
 manner the inquiry in which he is engaged.' 
 
 " The next letter shows that Mr. Adams has 
 derived results from these errors. 
 
 No. 9. PROFESSOR CHALLIS to G. B. AIRY. 
 
 "'CAMBRIDGE OBSERVATORY, Sept. 22, 1845. 
 " ' My friend Mr. Adams (who will probably 
 deliver this note to you) has completed his 
 
THE DISCOVERY OF NEPTUNE 51 
 
 calculations respecting the perturbation of the and 
 orbit of Uranus by a supposed ulterior planet, and Adams' 3 
 has arrived at results which he would be glad to Green- 
 communicate to you personally, if you could spare wlch> 
 him a few moments of your valuable time. His 
 calculations are founded on the observations you 
 were so good as to furnish him with some time 
 ago ; and from his character as a mathematician, 
 and his practice in calculation, I should consider 
 the deductions from his premises to be made in a 
 trustworthy manner. If he should not have the 
 good fortune to see you at Greenwich, he hopes 
 to be allowed to write to you on this subject/ 
 
 " On the day on which this letter was dated, I 
 was present at a meeting of the French Institute. 
 I acknowledged it by the following letter : 
 
 No. 10. G. B. AIRY to PROFESSOR CHALLIS. 
 
 "' ROYAL OBSERVATORY, GREENWICH, 1845, Sept. 29. 
 
 " ' I was, I suppose, on my way from France, 
 when Mr. Adams called here ; at all events, I 
 had not reached home, and therefore, to my 
 regret, I have not seen him. Would you mention 
 to Mr. Adams that I am very much interested 
 with the subject of his investigations, and that 
 I should be delighted to hear of them by letter 
 from him ? ' 
 
 "On one of the last days of October 1845, Mr. 
 Adams called at the Eoyal Observatory, Green- 
 
52 ASTRONOMICAL DISCOVERY 
 
 wich, in my absence and left the following 
 important paper : 
 
 No. ii. J. C. ADAMS, Esq., to G. B. AIRY. 
 
 Adams' "'According to my calculations, the observed 
 irregularities in the motion of Uranus may be 
 accounted for by supposing the existence of an 
 exterior planet, the mass and orbit of which are 
 as follows : 
 
 Mean distance (assumed nearly in accord- 
 ance with Bode's Law) . . . 38.4 
 Mean sidereal motion in 365.25 days . i3o'-9 
 Mean longitude, ist October 1845 3 2 3 34 
 Longitude of perihelion . . . 315 55 
 
 Eccentricity . . . . . .0.1610. 
 
 Mass (that of the sun being unity) . 0.0001656. 
 
 For the modern observations I have used the 
 method of normal places, taking the mean of the 
 tabular errors, as given by observations near three 
 consecutive oppositions, to correspond with the 
 mean of the times ; and the Greenwich observa- 
 tions have been used down to 1830: since which, 
 the Cambridge and Greenwich observations, and 
 those given in the Astronomische Nachrichten, 
 have been made use of. The following are the 
 remaining errors of mean longitude : 
 Observation Theory. 
 
 1780 
 
 + 0.27 
 
 1801 
 
 - 0.04 
 
 l822 
 
 + 0.30 
 
 1783 
 
 -0.23 
 
 1804 
 
 + 1.76 
 
 1825 
 
 + 1.92 
 
 1786 
 
 - 0.96 
 
 1807 
 
 -0.21 
 
 1828 
 
 + 2.25 
 
 1789 
 
 + 1.82 
 
 1810 
 
 + 0.56 
 
 1831 
 
 - 1. 06 
 
 1792 
 
 -0.91 
 
 1813 
 
 -0.94 
 
 1834 
 
 - i-44 
 
 1795 
 
 + 0.09 
 
 1816 
 
 -0. 3 I 
 
 1837 
 
 -1.62 
 
 1798 
 
 -0.99 
 
 1819 
 
 - 2.00 
 
 1840 
 
 + 1-73 
 
THE DISCOVERY OF NEPTUNE 53 
 
 The error for 1780 is concluded from that for 
 1781 given by observation, compared with those 
 of four or five following years, and also with 
 Lemonnier's observations in 1769 and 1771. 
 
 " ' For the ancient observations, the following 
 are the remaining errors : 
 
 Observation Theory. 
 
 1690 +44-4 175 -1-6 1763 - s'.'i 
 1712 + 6.7 1753 +5.7 1769 + 0.6 
 1715 - 6.8 1756 -4.0 1771 +n. 8 
 
 The errors are small, except for Flamsteed's 
 observation of 1690. This being an isolated 
 observation, very distant from the rest, I thought 
 it best not to use it in forming the equations of 
 condition. It is not improbable, however, that 
 this error might be destroyed by a small change 
 in the assumed mean motion of the planet.' 
 
 "I acknowledged the receipt of this paper in 
 the following terms : 
 
 No. 12. G. B. AIRY to J, C. ADAMS, Esq. 
 
 tte EOYAL OBSERVATORY, GREENWICH, 1845, Nov. 5. 
 
 " * I am very much obliged by the paper of 
 results which you left here a few days since, 
 showing the perturbations on the place of 
 Uranus produced by a planet with certain 
 assumed elements. The latter numbers are all 
 extremely satisfactory : I am not enough 
 acquainted with Flamsteed's observations about 
 
54 ASTRONOMICAL DISCOVERY 
 
 1690 to say whether they bear such an error, 
 but I think it extremely probable. 
 
 " * But I should be very glad to know whether 
 this assumed perturbation will explain the error 
 of the radius vector of Uranus. This error is 
 now very considerable, as you will be able to 
 ascertain by comparing the normal equations, 
 given in the Greenwich observations for each 
 year, for the times before opposition with the 
 times after opposition/ 
 
 " I have before stated that I considered the 
 establishment of this error of the radius vector 
 of Uranus to be a very important determination. 
 I therefore considered that the trial, whether 
 the error of radius vector would be explained 
 by the same theory which explained the error of 
 longitude, would be truly an experimentum crucis. 
 And I waited with much anxiety for Mr. Adams' 
 answer to my query. Had it been in the affir- 
 mative, I should at once have exerted all the 
 influence which I might possess, either directly, 
 or indirectly through my friend Professor Challis, 
 to procure the publication of Mr. Adams' theory. 
 
 "From some cause with which I am un- 
 acquainted, probably an accidental one, I received 
 no immediate answer to this inquiry. I regret 
 this deeply, for many reasons." 
 
 Adams' Here we may leave Airy's "account" for a few 
 moments to consider the reason why he received 
 no answer. Adams was a very shy and retiring 
 
THE DISCOVERY OF NEPTUNE 55 
 
 young man, and very sensitive ; though capable of 
 a great resolution, and of enormous perseverance 
 in carrying it out. We know (what is not in- 
 dicated in the above account), how steadily he 
 had kept in view the idea of solving this great 
 problem. It was characteristic of him that as 
 early as 1841 he had formed a resolution to under- 
 take it, although at the time he was not able to 
 enter upon its accomplishment. The following 
 memorandum, which is still in existence, having 
 been found among his papers after his death, 
 records these facts : 
 
 " 1841, July 3. Formed a design, in the 
 beginning of this week, of investigating, as 
 soon as possible after taking my degree, the 
 irregularities in the motion of Uranus, which 
 were as yet unaccounted for : in order to find 
 whether they may be attributed to the action of 
 an undiscovered planet beyond it, and if possible 
 thence to determine the elements of its orbit, 
 &c., approximately, which would probably lead 
 to its discovery." 
 
 Accordingly, " as soon asj possible after taking 
 his degree" he embarked upon the enterprise, and 
 the first solution was made in the long vacation of 
 1843, assuming the orbit of the unknown planet 
 to be a circle with a radius equal to twice the 
 mean distance of Uranus from the sun (an as- 
 sumption which, as we have seen, was also made 
 by Le Verrier). Having satisfied himself that 
 
56 ASTRONOMICAL DISCOVERY 
 
 there was a good general agreement between his 
 results and the observations, Adams began a more 
 complete solution ; indeed from first to last he 
 made no less than six separate solutions, the one 
 which he announced to Airy in the above letter 
 being the fourth. Hence he had already done an 
 enormous amount of work on the problem, and 
 was in his own mind so justly convinced of the 
 correctness and value of his results that he was 
 liable to forget that others had not had the same 
 opportunity of judging of their completeness ; 
 and he was grievously disappointed when his 
 announcement was not received with full 
 confidence. 
 
 But perhaps it should first be stated that by a 
 series of mischances Adams had been already much 
 disappointed at the failure of his attempts to see 
 the Astronomer Royal on his visits to Greenwich. 
 This does not seem to have been exactly Airy's 
 fault ; he was, as may well be supposed, an 
 extremely busy man, and was much occupied at 
 the time on a question of great practical im- 
 portance, at the direct request of the Government, 
 namely, the settling of the proper gauge for rail- 
 ways throughout the country. The first time 
 Adams cajled to see him, he was actually in 
 London sitting on the Committee which dealt 
 with this question, and Adams was asked to call 
 later ; when the visit was repeated, Airy was un- 
 fortunately at dinner (and it may be added that 
 his hours for dinner were somewhat peculiar), and 
 
THE DISCOVERY OF NEPTUNE 57 
 
 the butler, acting somewhat in the manner of his 
 kind, protected his master's dinner by sending 
 away one whom he doubtless regarded as a trouble- 
 some visitor. There is, as I have said, little doubt 
 about any of the facts, and it seems well established 
 that Airy himself did not learn of Adams' visits 
 until afterwards, and it would scarcely be just to 
 blame him for a servant's oversight. But Adams 
 had left the paper above reproduced, and Airy with 
 his business-like habits ultimately proceeded to 
 deal with it ; he wrote the answer given above 
 asking Adams a definite question, filed a copy of 
 it with the original letter, and then dismissed the 
 matter from his thoughts until the reply from 
 Adams, which he confidently expected should 
 again bring it under notice. 
 
 This further disappointment was, however, too and at 
 much for Adams ; he regarded the question put question. 
 by Airy as having so obvious an answer that it 
 was intended as an evasion, though this was far 
 from being the case. Airy was thoroughly in 
 earnest about his question, though it must be 
 admitted that a more careful study of the problem 
 would have shown him that it was unnecessary. 
 Later, when he learnt of Le Verrier's researches, 
 he put the same question to him, and received a 
 polite but very clear answer, showing that the sug- 
 gested test was not an experimentum crucis as he 
 supposed. But Adams did not feel equal to 
 making this reply; he shrank into his shell 
 and solaced himself only by commencing afresh 
 
58 ASTRONOMICAL DISCOVERY 
 
 another solution of the problem which had so 
 engrossed his life at that time. 
 
 The I have heard severe or contemptuous things said 
 
 Afcy? of a bout this question by those who most blame Airy. 
 
 question. S ome of them have no hesitation in accusing him 
 of intellectual incompetence : they say that it was 
 the question of a stupid man. I think that in the 
 first place they forget the difference between a 
 deliberate error of judgement and a mere conse- 
 quence of insufficient attention. But there is 
 even more than this to be said in defence of 
 the question. The " error of radius vector " came 
 before Airy in an entirely independent way, and 
 as an entirely independent phenomenon, from the 
 " error of longitude," and there was nothing un- 
 natural in regarding it as requiring independent 
 explanation. It is true that, as the event proved, 
 a mere readjustment of the orbit of Uranus got rid 
 of this error of radius vector (this was substantially 
 Le Verrier's answer to Airy's question) ; but we 
 must not judge of what was possible before the 
 event in the light of what we now know. The 
 
 The range original possibilities were far wider, though we 
 have forgotten their former extent now that they 
 have been narrowed down by the discovery. If a 
 sentry during war time hears a noise in a certain 
 direction, he may be compelled to make the as- 
 sumption that it is the movement of an enemy ; 
 and if he fires in that direction and kills him, and 
 thus saves his own army from destruction, he is 
 deservedly applauded for the success which attends 
 
THE DISCOVERY OF NEPTUNE 59 
 
 his action. But it does not follow that the as- 
 sumption on which he acted was the only possible 
 one. Or, to take a more peaceful illustration, in 
 playing whist it sometimes becomes apparent that 
 the game can only be won if the cards lie in a 
 certain way ; and a good player will thereupon 
 assume that this is the fact, and play accordingly. 
 Adams and Le Verrier played to win the game on 
 the particular assumption that the disturbance of 
 Uranus was due to an external planet revolving 
 at a distance from the sun about twice that of 
 Uranus ; and won it ; and we applaud them for 
 doing so. But it is easy to imagine a rearrange- 
 ment of the cards with which they would have 
 lost it ; and Airy's question simply meant that he 
 was alive to these wider possibilities, and did not 
 see the need for attempting to win the game in 
 that particular way. 
 
 One such alternative possibility has already been 
 mentioned. " Hansen's opinion was, that one dis- 
 turbing body would not satisfy the phenomena; but 
 he conjectured that there were two planets beyond 
 Uranus" Another conceivable alternative is that 
 there was some change in the law of gravitation at 
 the distance of Uranus, which, it must be remem- 
 bered, is twice as great as that of any planet pre- 
 viously known. Or some wandering body might 
 have passed close enough to Uranus to change 
 its orbit somewhat suddenly. We now know, for 
 instance, that the swarm of meteorites which 
 
60 ASTRONOMICAL DISCOVERY 
 
 gives rise to the well-known " November meteors " 
 must have passed very close to Uranus in A.D. 126, 
 assuming that neither the planet nor the swarm 
 have been disturbed in any unknown manner in 
 the meantime. It is to this encounter that we 
 owe the introduction of this swarm to our solar 
 system : wandering through space, they met 
 Uranus, and were swept by his attraction into an 
 orbit round the sun. Was there no reaction upon 
 Uranus himself? The probabilities are that the 
 total mass of the swarm was so small as to affect 
 the huge planet inappreciably ; but who was to 
 say that some other swarm of larger mass, or other 
 body, might not have approached near Uranus at 
 some date between 1690 and 1845, an( ^ been 
 responsible at any rate in part for the observed 
 errors ? These are two or three suppositions from 
 our familiar experience ; and there are, of course, 
 limitless possibilities beyond. Which is the true 
 scientific attitude, to be alive to them all, or to 
 concentrate attention upon one ? 
 
 But we are perhaps wandering too far from 
 the main theme. It is easy to do so in review- 
 ing this extraordinary piece of history, for at 
 almost every point new possibilities are suggested. 
 
 We must return, however, to Airy's " account." 
 We reached the point where he had written to 
 Adams (on November 5, 1845), asking his ques- 
 tion about the radius vector, and received no 
 reply; and there the matter remained, so far as 
 
III. U. J. LK VERRIER. 
 
 (From a print in the possession of the R-tyal Astronomical Society.) 
 
 IV J. G. GALLE. 
 
 WHO FIRST SAW THE PLANET NEPTUXE. 
 
THE DISCOVERY OF NEPTUNE 61 
 
 he was concerned, until the following June, 
 when Le Verrier's memoir reached him; and Airy re- 
 we will let him give his own version of the 
 result. 
 
 "This memoir reached me about the 23rd or 
 24th of June. I cannot sufficiently express the 
 feeling of delight and satisfaction which I received 
 from it. The place which it assigned to the 
 disturbing planet was the same, to one degree, 
 as that given by Mr. Adams' calculations, which 
 I had perused seven months earlier. To this 
 time I had considered that there was still room 
 for doubt of the accuracy of Mr. Adams' in- 
 vestigations ; for I think that the results of 
 algebraic and numerical computations, so long 
 and so complicated as those of an inverse 
 problem of perturbations, are liable to many 
 risks of error in the details of the process : I 
 know that there are important numerical errors 
 in the Mecanique Celeste of Laplace ; in the 
 Theorie de la Lune of Plan a ; above all, in 
 Bouvard's first tables of Jupiter and Saturn; 
 and to express it in a word, I have always 
 considered the correctness of a distant mathe- 
 matical result to be a subject rather of moral 
 than of mathematical evidence. But now I felt 
 no doubt of the accuracy of both calculations, as 
 applied to the perturbation in longitude. I was, 
 however, still desirous, as before, of learning 
 whether the perturbation in radius vector was 
 
62 ASTRONOMICAL DISCOVERY 
 
 fully explained. I therefore addressed to M. Le 
 Verrier the following letter: 
 
 No. 13. G. B. AIRY to M. LE VERRIER. 
 
 "' ROYAL OBSERVATORY, GREENWICH, 1846, June 26. 
 
 He puts "'I have read, with very great interest, the 
 "radius- account of your investigations on the probable 
 question place of a planet disturbing the motions of 
 Verrier Uranus, which is contained in the Compte 
 Rendu de TAcademie of June i ; and I now 
 beg leave to trouble you with the following 
 question. It appears, from all the later obser- 
 vations of Uranus made at Greenwich (which 
 are most completely reduced in the Greenwich 
 Observations of each year, so as to exhibit the 
 effect of an error either in the tabular helio- 
 centric longitude, or the tabular radius vector), 
 that the tabular radius vector is considerably too 
 small. And I wish to inquire of you whether 
 this would be a consequence of the disturbance 
 produced by an exterior planet, now in the 
 position which you have indicated?" 
 
 There is more of the letter, but this will suffice 
 to show that he wrote to Le Verrier in the same 
 way as to Adams, and, as already stated, received 
 a reply dated three or four days later. But the 
 rest of the letter contains no mention of Adams, 
 and thus arises a second difficulty in understand- 
 ing Airy's conduct. It seems extraordinary that 
 
THE DISCOVERY OF NEPTUNE 63 
 
 when he wrote to Le Verrier he made no mention but makes 
 of the computations which he had previously Sor^o?" 
 received from Adams ; or that he should not Adams - 
 have written to Adams, and made some attempt 
 to understand his long silence, now that, as he 
 himself states, he " felt no doubt of the accuracy 
 of both calculations." The omission may have 
 been, and probably was, mere carelessness or for- 
 getfulness ; but he could hardly be surprised if 
 others mistook it for deliberate action. 
 
 However, attention had now been thoroughly Airy an- 
 attracted to the near possibility of finding the J^e Si- 
 planet. On June 29, 1846, there was a special *^ 
 meeting of the Board of Visitors of Greenwich P lanet 
 Observatory, and Airy incidentally mentioned to 
 them this possibility. The impression produced 
 must have been definite and deep; for Sir John 
 Herschel, who was present, was bold enough to 
 say on September loth following to the British 
 Association assembled at Southampton: "We 
 see it (the probable new planet) as Columbus 
 saw America from the shores of Spain. Its 
 movements have been felt trembling along the 
 far-reaching line of our analysis with a certainty 
 hardly inferior to that of ocular demonstration." 
 Airy discussed the matter with Professor Challis a nd 
 (who, it will be remembered, had originally 
 written to him on behalf of Adams), suggesting 
 that he should immediately commence a search bridge 
 for the supposed planet at Cambridge. It may 
 be asked \vhy Airy did not commence this search 
 
64 ASTRONOMICAL DISCOVERY 
 
 himself at Greenwich, and the answer is that he 
 had no telescope which he regarded as large 
 enough for the purpose. The Royal Observatory 
 at Greenwich has always been, and is now, better 
 equipped in some respects than any other observa- 
 tory, as might be expected from its deservedly 
 great reputation ; but to possess the largest exist- 
 ing telescope has never been one of its ambitions. 
 The instruments in which it takes most pride 
 are remarkable for their steadiness and accuracy 
 rather than for their size ; and at that time the 
 nothav- best telescope possessed by the observatory was 
 abLftete not > i n Airy's opinion, large enough to detect the 
 scope at pl an et with certainty. In this opinion we now 
 wich know that he was mistaken ; but, again, we must 
 not judge his conduct before the event in the 
 light of what we have since discovered. It may 
 be recalled here that it was not until Le Verrier's 
 third paper, published on August 31, that he (Le 
 Verrier) emphatically pointed out that the new 
 planet might be of such a size as to have a 
 sensible disc ; and it was this remark which led 
 immediately to its discovery. Until this was so 
 decisively stated, it must have seemed exception- 
 ally improbable ; for we saw in the last chapter 
 how diligently the Zodiac had been swept in 
 the search for minor planets, how, for instance, 
 Hencke had searched for fifteen years without 
 success ; and it might fairly be considered that 
 if there were a fairly bright object (such as 
 Neptune has since been found to be) it would 
 
THE DISCOVERY OF NEPTUNE 65 
 
 have been discovered earlier. Hence Airy not 
 unreasonably considered it necessary to spread 
 his net for very small objects. On July 9 he 
 wrote to Professor Challis as follows : 
 
 No. 15. G. B. AIRY to PROFESSOR CHALLIS. 
 
 " THE DEANERY, ELY, 1846, July 9. 
 
 " You know that I attach importance to the 
 examination of that part of the heavens in which 
 there is ... reason for suspecting the existence 
 of a planet exterior to Uranus. I have thought 
 about the way of making such examination, but I 
 am convinced that (for various reasons, of declina- 
 tion, latitude of place, feebleness of light, and 
 regularity of superintendence) there is no prospect 
 whatever of its being made with any chance of 
 success, except with the Northumberland tele- 
 scope. 
 
 "Now, I should be glad to ask you, in the 
 first place, whether you could make such an 
 examination ? 
 
 " Presuming that your answer would be in the 
 negative, I would ask, secondly, whether, suppos- 
 ing that an assistant were supplied to you for this 
 purpose, you would superintend the examination? 
 
 " You will readily perceive that all this is in 
 a most unformed state at present, and that I am 
 asking these questions almost at a venture, in the 
 hope of rescuing the matter from a state which is, 
 
 without the assistance that you and your instru- 
 
 E 
 
66 ASTRONOMICAL DISCOVERY 
 
 ments can give, almost desperate. Therefore I 
 should be glad to have your answer, not only 
 responding simply to my questions, but also enter- 
 ing into any other considerations which you think 
 likely to bear on the matter. 
 
 " The time for the said examination is approach- 
 ing near." 
 
 chains Professor Challis did not require an assistant, 
 tairesthe ^ ut determined to undertake the work himself, 
 search. an( j ^ ev ised his own plan of procedure ; but he 
 also set out on the undertaking with the expecta- 
 tion of a long and arduous search. No such idea 
 as that of finding the planet on the first night ever 
 entered his head. For one thing, he had no map 
 of the region to be examined, for although the 
 map used by Galle had been published, no copy 
 of it had as yet reached Cambridge, and Professor 
 Challis had practically to construct a map for 
 himself. In these days of photography to make 
 such a map is a simple matter, but at that time 
 the process was terribly laborious. " I get over 
 the ground very slowly," he wrote on September 
 2nd to Airy, " thinking it right to include all stars 
 to 10-1 1 magnitude ; and I find that to scrutinise 
 thoroughly in this way the proposed portion of 
 the heavens will require many more observations 
 than I can take this year." With such a prospect, 
 it is not surprising that one night's observations 
 were not even compared with the next ; there 
 would be a certain economy in waiting until a 
 
THE DISCOVERY OF NEPTUNE 67 
 
 large amount of material had been accumulated, 
 and then making the comparisons all together, 
 and this was the course adopted. But when Le 
 Verrier's third paper, with the decided opinion 
 that the planet would be bright enough to be seen 
 by its disc, ultimately reached Professor Challis, it 
 naturally gave him an entirely different view of the 
 possibilities ; he immediately began to compare 
 the observations already made, and found that he He finds 
 had observed the planet early in August. But it that ^e 
 was now too late to be first in the field, for Galle 
 had already made his announcement of discovery. P lanet 
 Writing to Airy on October 12, Challis could , 
 only lament that after four days' observing the 
 planet was in his grasp, {/"only he had examined 
 or mapped the observations, and if he had not 
 delayed doing so until he had more observations 
 to reduce, and if he had not been very busy with 
 some comet observations. Oh ! these terrible ifs 
 which come so often between a man and success ! 
 The third of them is a peculiarly distressing one, 
 for it represents that eternal conflict between one 
 duty and another, which is so constantly recurring 
 in scientific work. Shall we finish one piece of 
 work now well under way, or shall we attend to 
 something more novel and more attractive ? 
 Challis thought his duty lay in steadily com- 
 pleting the comet observations already begun. 
 We saw in the last lecture how the steady pursuit 
 of the discovery of minor planets, a duty which 
 had become tedious and apparently led nowhere, 
 
68 ASTRONOMICAL DISCOVERY 
 
 suddenly resulted in the important discovery of 
 Eros. But Challis was not so fortunate in elect- 
 ing to plod along the beaten track ; he would 
 have done better to leave it. There is no golden 
 rule for the answer; we must be guided in 
 each case by the special circumstances, and the 
 dilemma is" consequently a new one on every 
 occasion, and perhaps the more trying with each 
 repetition. 
 
 Such are briefly the events which led to the 
 discovery of Neptune, which was made in Ger- 
 many by direction from France, when it might 
 have been made in Cambridge alone. The in- 
 sensation cidents created a great stir at the time. The 
 byThe "Account" of them, as read by Airy to the Royal 
 discovery. Astronomical Society on November 13, 1846, 
 straightforward and interesting though it was, 
 making clear where he had himself been at fault, 
 nevertheless stirred up angry passions in many 
 quarters, and chiefly directed against Airy himself. 
 Cambridge was furious at Airy's negligence, 
 which it considered responsible for costing the 
 University a great discovery ; and others were 
 equally irate at his attempting to claim for Adams 
 some of that glory which they considered should 
 go wholly to Le Verrier. But it may be remarked 
 Not ail that feeling was not purely national. Some 
 jealousy, foreigners were cordial in their recognition of the 
 work of Adams, while some of those most eager to 
 oppose his claims were found in this country. In 
 their anxiety to show that they were free from 
 
THE DISCOVERY OF NEPTUNE 69 
 
 national jealousy, scientific men went almost too 
 far in the opposite direction. 
 
 Airy's conduct was certainly strange at several 
 points, as has already been remarked. One cannot 
 understand his writing to Le Verrier in June 1846 
 without any mention of Adams. He could not 
 even momentarily have forgotten Adams' work ; 
 for he tells us himself how he noticed the close ., 
 correspondence of his result with that of Le 
 Verrier : and had he even casually mentioned 
 this fact in writing to the latter, it would have 
 prepared the way for his later statement. But we 
 can easily understand the unfavourable impression 
 produced by this statement after the discovery had 
 been made, when there had been no previous hint 
 on the subject at all. Of those who abused him The 
 Cambridge had the least excuse ; for there is no 
 doubt that with a reasonably competent Professor 
 of Astronomy in Cambridge, she need not have matter. 
 referred to Airy at all. It would not seem to 
 require any great amount of intelligence to under- 
 take to look in a certain region for a strange object 
 if one is in possession of a proper instrument. 
 We have seen that Challis had the instrument, 
 and when urged to do so was equal to the task of 
 finding the planet ; but he was a man of no initia- 
 tive, and the idea of doing so unless directed by 
 some authority never entered his head. He had 
 been accustomed for many years to lean rather 
 helplessly upon Airy, who had preceded him in 
 office at Cambridge. For instance, when appointed 
 
70 ASTRONOMICAL DISCOVERY 
 
 to succeed him, and confronted with the necessity 
 of lecturing to students, he was so helpless that 
 he wrote to implore Airy to come back to Cam- 
 bridge and lecture for him ; and this was actually 
 done, Airy obtaining leave from the Government 
 to leave his duties at Greenwich for a time in 
 order to return to Cambridge, and show Challis 
 chains how to lecture. Now it seems to me that this 
 weakest helplessness was the very root of all the mischief 
 of which Cambridge so bitterly complained. I 
 claimed at the outset the privilege of stating my 
 own views, with which others may not agree : and 
 of all the mistakes and omissions made in this 
 little piece of history, the most unpardonable and 
 the one which had most serious consequences 
 seems to me to be this : that Challis never made 
 the most casual inquiry as to the result of the 
 visit to Greenwich which he himself had directed 
 Adams to make. I am judging him to some 
 extent by default ; because I assume the facts 
 from lack of evidence to the contrary : but it 
 seems practically certain that after sending this 
 young man to see Airy on this important topic, 
 Challis thereupon washed his hands of all respon- 
 sibility so completely that he never even took the 
 trouble to inquire on his return, " Well ! how did 
 you get on ? What did the Astronomer Royal 
 say ? " Had he put this simple question, which 
 scarcely required the initiative of a machine, and 
 learnt in consequence, as he must have done, that 
 the sensitive young man thought Airy's question 
 
THE DISCOVERY OF NEPTUNE 71 
 
 trivial, and did not propose to answer it, I think 
 we might have trusted events to right themselves. 
 Even Challis might have been trusted to reply, 
 " Oh ! but you must answer the Astronomer 
 Royal's question : you may think it stupid, but 
 you had better answer it politely, and show him 
 that you know what you are about." It is un- 
 profitable to pursue speculation further ; this did 
 not happen, and something else did. But I have 
 always felt that my old University made a scape- 
 goat of the wrong man in venting its fury upon 
 Airy, when the real culprit was among themselves, 
 and was the man they had themselves chosen to 
 represent astronomy. He was presumably the 
 best they had ; but if they had no one better than 
 this, they should not have been surprised, and 
 must not complain, if things went wrong. If a 
 University is ambitious of doing great things, it 
 must take care to see that there are men of ability 
 and initiative in the right places. This is a most 
 difficult task in any case, and we require all pos- 
 sible incentives towards it. To blink the facts 
 when a weak spot is mercilessly exposed by the 
 loss of a great opportunity is to lose one kind of 
 incentive, and perhaps not the least valuable. 
 
 Let us now turn to some curious circumstances Curious 
 attending this remarkable discovery of a planet between 
 
 by mathematical investigation, of which there are 
 several. The first is, that although Neptune was P lanefc - 
 found so near the place where it was predicted, 
 its orbit, after discovery, proved to be very dif- 
 
72 ASTRONOMICAL DISCOVERY 
 
 ferent from that which Adams and Le Verrier 
 had supposed. You will remember that both 
 calculators assumed the distance from the sun, 
 in accordance with Bode's Law, to be nearly twice 
 that of Uranus. The actual planet was found to 
 have a mean distance less than this by 25 per 
 cent, an enormous quantity in such a case. For 
 instance, if the supposed planet and the real were 
 started round the sun together, the real planet 
 would soon be a long way ahead of the other, 
 and the ultimate disturbing effect of the two on 
 Uranus would be very different. To explain the 
 difference, we must first recall a curious pro- 
 perty of such disturbances. When two planets 
 are revolving, so that one takes just twice or 
 three times, or any exact number of times, as long 
 to revolve round the sun as the other, the usual 
 mathematical expressions for the disturbing action 
 of one planet on the other would assign an infinite 
 disturbance, which, translated into ordinary lan- 
 guage, means that we must start with a fresh 
 assumption, for this state of things cannot persist. 
 If the period of one were a little longer than this 
 critical value, some of the mathematical expres- 
 sions would be of contrary sign from those corre- 
 sponding to a period a little shorter. Now it is 
 curious that the supposed planet and the real had 
 orbits on opposite sides of a critical value of this 
 kind, namely, that which would assign a period 
 of revolution for Neptune exactly half that of 
 Uranus ; and it was pointed out in America by 
 
THE DISCOVERY OF NEPTUNE 73 
 
 Professor Peirce that the effect of the planet Professor 
 imagined by Adams and Le Verrier was thus 
 totally different from that of Neptune. He there- 
 fore declared that the mathematical work had not covei r 
 
 WtlS Si 
 
 really led to the discovery at all ; but that it had mere 
 
 11- accident. 
 
 resulted irom mere coincidence, and this opinion 
 somewhat paradoxical though it was found con- 
 siderable support. It was not replied to by Adams 
 until some thirty years later, when a short reply 
 was printed in Liouville's Journal. The explana- The ex- 
 
 ,. i ,i i -i 1 -r planation. 
 
 tion is this : the expressions considered by Pro- 
 fessor Peirce are those representing the action of 
 the planet throughout an indefinite past, and did 
 not enter into the problem, which would have 
 been precisely the same if Neptune had been 
 suddenly created in 1690; while, on the other 
 hand, if Neptune had existed up till 1690 (the 
 time when Uranus was first observed, although 
 unknowingly), and then had been destroyed, 
 there would have been no means of tracing its 
 previous existence. In past ages it had no doubt 
 been perturbing the orbit of Uranus, and had 
 effected large changes in it ; but if it had then 
 been suddenly destroyed, we should have had no 
 means of identifying these changes. There might 
 have been instead of Neptune another planet, such 
 as .that supposed by Adams and Le Verrier ; and 
 its action in all past time would have been very 
 different from that of Neptune, as is properly re- 
 presented in the mathematical expressions which 
 Professor Peirce considered. In consequence the 
 
74 ASTRONOMICAL DISCOVERY 
 
 orbit of Uranus in 1690 would have been very 
 different from the orbit as it was actually found ; 
 but in either case the mathematicians Adams and 
 Le Verrier would have had to take it as they 
 found it ; and the disturbing action which they 
 considered in their calculations was the compara- 
 tively small disturbance which began in 1690 and 
 ended in 1846. During this limited number of 
 years the disturbance of the planet they imagined, 
 although not precisely the same as that of Nep- 
 tune, was sufficiently like it to give them the 
 approximate place of the planet. 
 
 Still it is somewhat bewildering to look at the 
 mathematical expressions for the disturbances as 
 used by Adams and Le Verrier, when we can now 
 compare with them the actual expressions to which 
 they ought to correspond; and one may say frankly 
 that there seems to be no sort of resemblance. 
 Recently a memorial of Adams' work has been 
 published by the Royal Astronomical Society ; 
 they have reproduced in their Memoirs a facsimile 
 of Adams' MS. containing the " first solution," 
 which he made in 1843 in the Long Vacation 
 after he had taken his degree, and which would 
 have given the place of Neptune at that time with 
 an error of 15. In an introduction describing 
 the whole 'of the MSS., written by Professor R. A. 
 Sampson of Durham, it is shown how different 
 the actual expressions for Neptune's influence are 
 from those used by Adams, and it is one of the 
 curiosities of this remarkable piece of history that 
 
THE DISCOVERY OF NEPTUNE 75 
 
 some of them seem to be actually in the wrong 
 direction; and others are so little alike that it is 
 only by fixing our attention resolutely on the 
 considerations above mentioned that we can 
 realise that the analytical work did indeed lead 
 to the discovery of the planet. 
 
 A second curiosity is that a mistaken idea Suggested 
 should have been held by at least one eminent tary 
 man (Sir J. Herschel), to the effect that it would 
 have been possible to find the place of the planet 
 by a much simpler mathematical calculation than 
 that actually employed by Adams or Le Verrier. 
 In his famous '* Outlines of Astronomy " Sir John 
 Herschel describes a simple graphical method, 
 which he declares would have indicated the place 
 of the planet without much trouble. Concerning 
 it I will here merely quote Professor Sampson's 
 words : 
 
 " The conclusion is drawn that Uranus arrived 
 at a conjunction with the disturbing planet about 
 1822 ; and this was the case. Plausible as this 
 argument may seem, it is entirely baseless. For 
 the maximum of perturbations depending on the 
 eccentricities has no relation to conjunction, and 
 the others which depend upon the differences of 
 the mean motions alone are of the nature of forced 
 oscillations, and conjunction is not their maximum 
 or stationary position, but their position of most 
 rapid change." 
 
 Professor Sampson goes on to show that a more 
 
76 ASTRONOMICAL DISCOVERY 
 
 elaborate discussion seems quite as unpromising ; 
 and he concludes that the refinements employed 
 were not superfluous, although it seems now clear 
 that a different mode of procedure might have led 
 more certainly to the required conclusion. 
 The evil For the third curious point is that both calcula- 
 of^Bode's t rs should have adhered so closely to Bode's Law. 
 If they had not had this guiding principle it seems 
 almost certain that they would have made a better 
 approximation to the place of the planet, for 
 instead of helping them it really led them astray. 
 We have already remarked that if two planets are 
 at different distances from the sun, however slight, 
 and if they are started in their revolution together, 
 they must inevitably separate in course of time, 
 and the amount of separation will ultimately 
 become serious. Thus by assuming a distance for 
 the planet which was in error, however slight, the 
 calculators immediately rendered it impossible for 
 themselves to obtain a place for the planet which 
 should be correct for more than a very brief period. 
 Professor Sampson has given the following inter- 
 esting lists of the dates at which Adams' six solu- 
 tions gave the true place of the planet and the 
 intervals during which the error was within 5 
 either way. 
 
 I. II. III. IV. v. VI. 
 
 Correct .... 1820 1835 1872 1830 1861 1856 
 
 Within + c J l812 I82 7 I86 5 I8l 3 1815 1826 
 (1827 1842 1877 1866 1871 1868 
 
 Now the date at which it was most important to 
 obtain the correct place was 1845 or thereabouts 
 
THE DISCOVERY OF NEPTUNE 77 
 
 when it was proposed to look for the planet ; but 
 no special precaution seems to have been taken by 
 either investigator to secure any advantage for 
 this particular date. Criticising the procedure 
 after the event (and of course this is a very 
 unsatisfactory method of criticism), we should say 
 that it would have been better to make several 
 assumptions as regards the distance instead of 
 relying upon Bode's Law ; but no one, so far as I 
 know, has ever taken the trouble to write out a 
 satisfactory solution of the problem as it might 
 have been conducted. Such a solution would be 
 full of interest, though it could only have a small" 
 weight in forming our estimation of the skill 
 with which the problem was solved in the first 
 instance. 
 
 Fourthly, we may notice a very curious point. Le 
 Le Verrier went to some trouble not only to point erroneous 
 out the most likely place for the planet, but to 
 indicate limits outside which it was not necessary 
 to look. This part of his work is specially com- 
 mented upon with enthusiasm by Airy, and I will 
 reproduce what he says. It is rather technical 
 perhaps, but those who cannot follow the mathe* 
 matics will be able to appreciate the tone of 
 admiration. 
 
 " M. Le Verrier then enters into a most ingenious 
 computation of the limits between which the 
 planet must be sought. The principle is this : 
 assuming a time of revolution, all the other un- 
 
78 ASTRONOMICAL DISCOVERY 
 
 known quantities may be varied in such a manner 
 that though the observations will not be so well 
 represented as before, yet the errors of observation 
 will be tolerable. At last, on continuing the 
 variation of elements, one error of observation will 
 be intolerably great. Then, by varying the ele- 
 ments in another way, we may at length make 
 another error of observation intolerably great ; and 
 so on. If we compute, for all these different 
 varieties of elements, the place of the planetfor 1847, 
 its locus will evidently be a discontinuous curve 
 or curvilinear polygon. If we do the same thing 
 with different periodic times, we shall get different 
 polygons ; and the extreme periodic times that 
 can be allowed will be indicated by the polygons 
 becoming points. These extreme periodic times 
 are 207 and 233 years. If now we draw one 
 grand curve, circumscribing all the polygons, it is 
 certain that the planet must be within that curve. 
 In one direction, M. Le Verrier found no difficulty 
 in assigning a limit ; in the other he was obliged 
 to restrict it, by assuming a limit to the eccentri- 
 city. Thus he found that the longitude of the 
 planet was certainly not less than 321, and not 
 greater than 335 or 345, according as we limit 
 the eccentricity to 0.125 or 0.2. And if we adopt 
 0.125 as the limit, then the mass will be included 
 between the limits 0.00007 an d 0.00021 ; either 
 of which exceeds that of Uranus. From this cir- 
 cumstance, combined with a probable hypothesis 
 as to the density, M. Le Verrier concluded that 
 
THE DISCOVERY OF NEPTUNE 79 
 
 the planet would have a visible disk, and sufficient The 
 light to make it conspicuous in ordinary tele- 
 scopes. 
 
 " M. Le Verrier then remarks, as one of the strong 
 proofs of the correctness of the general theory, 
 that the error of radius vector is explained as 
 accurately as the error of longitude. And finally, 
 he gives his opinion that the latitude of the dis- 
 turbing planet must be small. 
 
 "My analysis of this paper has necessarily been 
 exceedingly imperfect, as regards the astronomical 
 and mathematical parts of it ; but I am sensible 
 that, in regard to another part, it fails totally. I 
 cannot attempt to convey to you the impression 
 which was made on me by the author's undoubt- 
 ing confidence in the general truth of his theory, 
 by the calmness and clearness with which he 
 limited the field of observation, and by the firm- 
 ness with which he proclaimed to observing 
 astronomers, 'Look in the place which I have 
 indicated, and you will see the planet well.' 
 Since Copernicus declared that, when means 
 should be discovered for improving the vision, 
 it would be found that Venus had phases like 
 the moon, nothing (in my opinion) so bold, 
 and so justifiably bold, has been uttered in 
 astronomical prediction. It is here, if I mis- 
 take not, that we see a character far superior 
 to that of the able, or enterprising, or indus- 
 trious mathematician ; it is here that we see the 
 philosopher." 
 
8o ASTRONOMICAL DISCOVERY 
 
 Peirce's But now this process of limitation was faulty 
 the limits, and actually misleading. Let us compare what 
 
 is said about it by Professor Peirce a little 
 
 later. 
 
 " Guided by this principle, well established, 
 and legitimate, if confined within proper limits, 
 M. Le Verrier narrowed with consummate skill the 
 field of research, and arrived at two fundamental 
 propositions, namely : 
 
 " i st. That the mean distance of the planet 
 cannot be less than 35 or more than 37.9. The 
 corresponding, limits of the time of sidereal revolu- 
 tion are about 207 and 233 years. 
 
 " 2nd. * That there is only one region in which 
 the disturbing planet can be placed in order to 
 account for the motions of Uranus ; that the mean 
 longitude of this planet must have been, on 
 January i, 1800, between 243 and 252.' 
 
 " ' Neither of these propositions is of itself 
 necessarily opposed to the observations which 
 have been made upon Neptune, but the two com- 
 bined are decidedly inconsistent with observation. 
 It is impossible to find an orbit, which, satisfying 
 the observed distance and motion, is subject to 
 them. If, for instance, a mean longitude and 
 time of revolution are adopted according with the 
 first, the corresponding mean longitude in 1800 
 must have been at least 40 distant from the 
 limits of the second proposition. And again, if 
 the planet is assumed to have had in 1800 a 
 
THE DISCOVERY OF NEPTUNE 81 
 
 mean longitude near the limits of the second 
 proposition, the corresponding time of revolution 
 with which its motions satisfy the present observa- 
 tions cannot exceed 1 70 years, and must therefore 
 be about 40 years less than the limits of the first 
 proposition.' 
 
 "Neptune cannot, then, be the planet of M. 
 Le Verrier's theory, and cannot account for the 
 observed perturbations of Uranus under the form 
 of the inequalities involved in his analysis" 
 (Proc. Amer. Acad. /., 1846-1848, p. 66). 
 
 At the time when Professor Peirce wrote, the 
 orbit of Neptune was not sufficiently well deter- 
 mined to decide whether one of the two limita- 
 tions might not be correct, though he could see 
 that they could not both be right, and we now 
 know that they are both wrong. The mean dis- 
 tance of Neptune is 30, which does not lie between 
 35 and 37.9; and the longitude in 1800 was 225, 
 which does not lie between 243 and 252. The 
 ingenious process which Airy admired and which 
 Peirce himself calls "consummately skilful" was 
 wrong in principle. As Professor Newcomb has New- 
 said, "the error was the elementary one that, criticism, 
 instead of considering all the elements simultane- 
 ously variable, Le Verrier took them one at 
 a time, considering the others as fixed, and 
 determining the limits between which each could 
 be contained on this hypothesis. No solver of 
 least square equations at the present day ought to 
 
82 ASTRONOMICAL DISCOVERY 
 
 make such a blunder. Of course one trouble in 
 Le Verrier's demonstration, had he attempted a 
 rigorous one, would have been the impossibility 
 of forming the simultaneous equations expressive 
 of possible variations of all the elements." 
 
 The account of Le Verrier's limits by Professor 
 Peirce, though it exhibits the error with special 
 clearness, is a little unfair to Le Verrier in one 
 point. If, instead of taking the limits for the 
 date 1 800, we take them for 1846 (when the 
 search for Neptune was actually made), we shall 
 find that they do include the actual place of 
 the planet, as Airy found. The erroneous mean 
 motion of Le Verrier's planet allowed of his 
 being right at one time and wrong at another; 
 and Airy examined the limits under favourable 
 conditions, which explains his enthusiasm. But 
 we can scarcely wonder that Professor Pairce 
 came to the conclusion that the planet discovered 
 was not the one pointed out by Le Verrier, and 
 had been found by mere accident. And all these 
 circumstances inevitably contribute to a general 
 impression that the calculators had a large 
 Element element of good fortune to thank for their 
 fortune, success. Nor need we hesitate to make this 
 admission, for there is an element of good 
 fortune in all discoveries. To look no further 
 than this if a man had not been doing a par- 
 ticular thing at a particular time, as he might 
 easily not have been, most discoveries would 
 never have been made. If Sir William Herschel 
 
15" * 
 
 V. CORNER OF '/HE BERLIN MAP, BY THE USE OF WHICH GALLE FOUND NEPTUNE 
 
THE DISCOVERY OF NEPTUNE 83 
 
 had not been looking at certain small stars for 
 a totally different purpose he would never have 
 found Uranus ; and no one need hesitate to admit 
 the element of chance in the finding of Neptune. 
 It is well illustrated by a glance at the map The map 
 which, as has been remarked, Galle used to com- ^ 7 
 pare with the sky on the night when he made the 
 actual discovery. The planet was found down 
 near the bottom corner of the map, and since the 
 limits assigned for its place might easily have 
 varied a few degrees one way or the other, it 
 might easily have been off the map ; in which case 
 it is probable that the search would not have been 
 successful, or at any rate that success would have 
 been delayed. 
 
 Thus, it is a most remarkable feature of the Everyone 
 discovery of Neptune that mistakes were made mistakes, 
 by almost every one concerned, however eminent. 
 Airy made a mistake in regarding the question of 
 the Radius Vector as of fundamental importance ; 
 Sir J. Herschel was wrong in describing an ele- 
 mentary method which he considered might have 
 found the planet ; Professor Peirce was wrong 
 in supposing that the actual and the supposed 
 planet were essentially different in their action 
 on Uranus ; Le Verrier was wrong in assigning 
 limits outside which it was not necessary to look 
 when the actual planet was outside them ; Adams 
 was more or less wrong in thinking that the 
 eccentricity of the new planet could be found 
 from the material already at disposal of man. 
 
84 ASTRONOMICAL DISCOVERY 
 
 Both Adams and Le Verrier gave far too much 
 importance to Bode's Law. 
 
 To review a piece of history of this kind and 
 note the mistakes of such men is certainly 
 comforting, and need not in any way lessen our 
 admiration. In the case of the inve'sti-gaftors 
 themselves, much may be set down to 1 excitement 
 in the presence of a possible discovery. Professor 
 Sampson has provided us with a small but typical 
 instance of this fact. When Adams had carried 
 through all his computations for finding Neptune, 
 and was approaching the actual place of the 
 planet, he, "who could carry through fabulous 
 computations without error," for the first time 
 wrote down a wrong figure. The mistake was 
 corrected upon the MS., "probably as soon as 
 made," but no doubt betrays the excitement 
 which the great worker could not repress at this 
 critical moment. There is a tradition that, simi- 
 larly, when the mighty Newton was approaching 
 the completion of his calculations to verify the 
 Law of Gravitation, his excitement was so great 
 that he was compelled to assign to a friend the 
 task of finishing them. 
 
 Finally, we may remark how the history of 
 the discovery of Neptune again illustrates the 
 difficulty of formulating any general principles 
 for guiding scientific work. Sometimes it is 
 well to follow the slightest clue, however im- 
 perfectly understood ; at other times we shall 
 do better to refuse such guidance. Bode's Law 
 
THE DISCOVERY OF NEPTUNE 85 
 
 pointed to the existence of minor planets, and 
 might conceivably have helped in finding Uranus : 
 but by trusting to it in the case of Neptune, the 
 investigators were perilously near going astray. 
 Sometimes it is better to follow resolutely the 
 work in hand whatever it may be, shutting one's 
 ears to other calls ; but Airy and Challis lost their 
 opportunities by just this course of action. The 
 history of science is full of such contradictory 
 experiences ; and the only safe conclusion seems 
 to be that there are no general rules of conduct 
 for discovery. 
 
CHAPTEK III 
 
 BRADLEY'S DISCOVERIES OF THE ABERRATION OF 
 LIGHT AND OF THE NUTATION OF THE EARTH'S 
 AXIS 
 
 Bio- IN examining different types of astronomical dis- 
 meSwcT covery, we shall find certain advantages in varying 
 adopted. j. Q some extent the method of presentation. In the 
 two previous chapters our opportunities for learn- 
 ing anything of the life and character of those 
 who made the discoveries have been slight ; but I 
 propose to adopt a more directly biographical 
 method in dealing with Bradley' s discoveries, 
 which are so bound up with the simple earnest- 
 ness of his character that we could scarcely 
 appreciate their essential features properly with- 
 out some biographical study. But the record of 
 his life apart from his astronomical work is not 
 in any way sensational ; indeed it is singularly 
 devoid of incident. He had not even a scientific 
 quarrel. There was scarcely a man of science of 
 that periool who had not at least one violent 
 quarrel with some one, save only Bradley, whose 
 gentle nature seems to have kept him clear of 
 them all. Judged by ordinary standards his life 
 was uneventful : and yet it may be doubted 
 whether, to him who lived it, that life contained 
 
 86 
 
BRADLEY'S DISCOVERIES 87 
 
 one dull moment. Incident came for him in his 
 scientific work : in the preparation of apparatus, 
 the making of observations, above all in the hard- 
 thinking which he did to get at the clue which 
 would explain them ; and after reviewing his 
 biography, 1 I think we shall be inclined to admit 
 that if ever there was a happy life, albeit one of 
 unremitting toil, it was that of James Bradley. 
 
 He was born at Sherbourn, in Gloucestershire, Bradiey's 
 in 1693. We know little of his boyhood except 
 that he went to the Grammar School at North- 
 leach, and that the memory of this fact was 
 preserved at the school in 1832 when Rigaud was 
 writing his memoir. [The school is at present 
 shut up for want of funds to carry it on ; and all 
 inquiries I have made have failed to elicit any 
 trace of this memory.] Similarly we know little 
 of his undergraduate days at Oxford, except that 
 he entered as a commoner at Balliol in 1710, took 
 his B.A. in the regular course in 1714, and his 
 M.A. in 1717. As a career he chose the Church, 
 being ordained in 1719, and presented to the 
 vicarage of Bridstow in Monmouthshire ; but he 
 only discharged the duties of vicar for a couple of 
 years, for in 1721 he returned to Oxford as Pro- 
 fessor of Astronomy, an appointment which 
 involved the resignation of his livings ; and so 
 slight was this interruption to his career as an 
 
 1 The facts were collected with great care and ability by S. P. 
 Rigaud, and published by the Oxford University Press in 1832 as 
 " Miscellaneous Works and Correspondence of the Rev. James 
 Bradley." 
 
88 ASTRONOMICAL DISCOVERY 
 
 astronomer that we may almost disregard it, and 
 consider him as an astronomer from the first. 
 But to guard against a possible misconception, let 
 Brief me say that Bradley entered on a clerical career 
 career! in a thoroughly earnest spirit ; to do otherwise 
 would have been quite foreign to his nature. When 
 vicar of Bridstow he discharged his duties faith- 
 fully towards that tiny parish, and moreover was 
 so active in his uncle's parish of Wansted that he 
 left the reputation of having been curate there, 
 although he held no actual appointment. And 
 thirty years later, when he was Astronomer Royal 
 and resident at Greenwich, and when the valuable 
 vicarage of Greenwich was offered to him by 
 the Chancellor of the Exchequer, he honourably 
 refused the preferment, "because the duty of a 
 pastor was incompatible with his other studies 
 and necessary engagements." 
 
 Learnt But now let us turn to Bradley' s astronomical 
 nomynoi education. I must admit, with deep regret, that 
 at Oxford, we canno t allow any of the credit of it to 
 Oxford. There was a great astronomer in Oxford 
 when Bradley was an undergraduate, for Edmund 
 Halley had been appointed Savilian Professor of 
 Geometry in 1703, and had immediately set to 
 work to compute the orbits of comets, which led 
 to hisimrn'ortal discovery that some of these bodies 
 return to us again and again, especially the one 
 which bears his name Halley's Comet and 
 returns every seventy-five years, being next ex- 
 pected about 1910. But there is no record that 
 
BRADLEY'S DISCOVERIES 89 
 
 Bradley came under Halley's teaching or influ- 
 ence as an undergraduate. In later years the two 
 men knew each other well, and it was Halley's 
 one desire towards the close of his life that 
 Bradley should succeed him as Astronomer Royal 
 at Greenwich ; a desire which was fulfilled in 
 rather melancholy fashion, for Halley died with- 
 out any assurance that his wish would be gratified. 
 But Bradley got no astronomical teaching at 
 Oxford either from Halley or others. The art 
 of astronomical observation he learnt from his 
 maternal uncle, the Rev. James Pound, Rector of but from 
 Wansted, in Essex. He is the man to whom we Slm^s 016 ' 
 owe Bradley' s training and the great discoveries Pound - 
 which came out of it. He was, I am glad to say, 
 an Oxford man too ; very much an Oxford man ; 
 for he seems to have spent some thirteen years 
 there migrating from one Hall to another. His 
 record indeed was such as good tutors of colleges 
 frown upon ; for it was seven years before he 
 managed to take a degree at all ; and he could 
 not settle to anything. After ten years at Oxford 
 he thought he would try medicine ; after three 
 years more he gave it up and went out in 1700 
 as chaplain to the East Indies. But he seems to 
 have been a thoroughly lovable man, for news 
 was brought of him four years later that he had 
 a mind to come home, but was dissuaded by the 
 Governor saying that " if Dr. Pound goes, I and 
 the rest of the Company will not stay behind." 
 Soon afterwards the settlement was attacked in an 
 
90 ASTRONOMICAL DISCOVERY 
 
 insurrection, and Pound was one of the few who 
 escaped with his life, losing however all the pro- 
 perty he had gradually acquired. He returned to 
 England in 1 706, and was presented to the living 
 of Wansted ; married twice, and ended his days 
 in peace and fair prosperity in 1724. Such are 
 briefly the facts about Bradley's uncle, James 
 Pound ; but the most important of all remains 
 to be told that somehow or other he had learnt 
 Pound a to make first-rate astronomical observations, how 
 
 first-rate 
 
 observer, or when is not recorded; but m 1719 he was 
 already so skilled that Sir Isaac Newton made 
 him a present of fifty guineas for some observa- 
 tions ; and repeated the gift in the following 
 year ; and even three years before this we find 
 Halley writing to ask for certain observations 
 from Mr. Pound. 
 
 With this excellent man Bradley used fre- 
 quently to stay. To his nephew he seems to 
 have been more like a father than an uncle. 
 When his nephew had smallpox in 1717, he 
 nursed him through it ; and he supplemented 
 from his own pocket the scanty allowance which 
 was all that Bradley's own father could afford. 
 But what concerns us most is that he fostered, 
 if he did not actually implant, a love of astro- 
 nomical observation in his nephew. The two 
 workeT wor ^ e( ^ together, entering their observations one 
 with him. after the other on the same paper; and it was 
 to the pair of them together, rather than to the 
 uncle alone, that Newton made his princely pre- 
 
BRADLEY'S DISCOVERIES 91 
 
 sents, and Halley wrote for help in his observa- 
 tions. There seems to be no doubt that the uncle 
 and nephew were about this time the best astro- 
 nomical observers in the world. There was no 
 rivalry between them, and therefore there is no 
 need to discuss whether the partnership was one 
 of equal merit on both sides ; but it is interesting 
 to note that it probably was. The ability of 
 Pound was undoubted; many were keenly de- 
 sirous that he, and not his nephew, should be 
 elected to the Oxford Chair in 1721, but he felt 
 unequal to the duties at his advanced age. On 
 the other hand, when Bradley lost his uncle's 
 help, there was no trace of faltering in his steps 
 to betray previous dependence on a supporting 
 or guiding hand. He walked erect and firm, and 
 trod paths where even his uncle might not have 
 been able to follow. 
 
 A few instances will suffice to show the kind The work 
 of observations made by this notable firm of Pound 7 
 Pound and Bradley. They observed the positions Bradley. 
 of the fixed stars and nebulse : these being gene- 
 rally the results required by Halley and Newton. 
 They also observed the places of the planets 
 among the stars, and especially the planet Mars, 
 and determined its distance from the Earth by 
 the method of parallax, thus anticipating the 
 modern standard method of finding the Sun's 
 distance ; and though with their imperfect instru- 
 ments they did not obtain a greater accuracy than 
 i in 10, still this was a great advance on what 
 
92 ASTRONOMICAL DISCOVERY 
 
 had been done before, and excited the wonder 
 and admiration of Halley. They also paid some 
 attention to double stars, and did a great deal of 
 work on Jupiter's satellites. We might profitably 
 linger over the records of these early years, which 
 are full of interest, but we must press on to the 
 time of the great discoveries, and we will dismiss 
 them with brief illustrations of three points : 
 Bradley's assiduity, his skill in calculation, and 
 his wonderful skill in the management of instru- 
 ments. Of his assiduity an example is afforded 
 by his calculations of the orbits of two comets 
 which are still extant. One of them fills thirty- 
 two pages of foolscap, and the other sixty; and 
 it must be remembered that the calculations them- 
 selves were quite novel at that time. Of his skill 
 in calculation, apart from his assiduity, we have 
 a proof in a paper communicated to the Royal 
 Society rather later (1726), where he determines 
 the longitudes of Lisbon and New York from the 
 eclipses of Jupiter's satellites, using observations 
 which were not simultaneous, and had therefore 
 to be corrected by an ingenious process which 
 Bradley devised expressly for this purpose. And 
 Use of finally, his skill in the management of instruments 
 rexy long - g s ] lown by n i s measuring the diameter of the 
 scopes, planet Venus with a telescope actually 2 1 2 J feet 
 in length. It is difficult for us to realise in these 
 days what this means ; even the longest telescope 
 of modern times does not exceed 100 feet in 
 length, and it is mounted so conveniently with 
 
BRADLEY'S DISCOVERIES 93 
 
 all the resources of modern engineering, in the 
 shape of rising floors, &c., that the management 
 of it is no more difficult than that of a iofoot 
 telescope. But Bradley had no engineering appli- 
 ances beyond a pole to hold up one end of the 
 telescope and his own clever fingers to work the 
 other; and he managed to point the unwieldy 
 weapon accurately to the planet, and measure the 
 diameter with an exactness which would do credit 
 to modern times. A few words of explanation Keason 
 may be given why such long telescopes were used 
 at all. The reason lay in the difficulty of getting 
 rid of coloured images, due to the composite 
 character of white light. Whenever we use a 
 single lens to form an image, coloured fringes 
 appear. Nowadays we know that by making 
 two lenses of different kinds of glass and putting 
 them together, we can practically get rid of these 
 coloured fringes ; but this discovery had not been 
 made in Bradley's time. The only known ways 
 of dealing with the evil then were to use a reflect- 
 ing telescope like Newton and Gregory, or if a 
 lens was used, to make one of very great focal 
 length ; and hence the primary necessity for these 
 very long telescopes. They had another advan- 
 tage in producing a large image, or they would 
 probably have given way to the reflector. This 
 advantage is gradually bringing them back into 
 use, and perhaps in the eclipse of 1905 we may 
 use a telescope as long as Bradley's ; but we shall 
 not use it as he did in any case. It will be laid 
 
94 ASTRONOMICAL DISCOVERY 
 
 comfortably flat on the ground, and the rays of 
 light reflected into it by a coelostat. 
 
 Bradley In 1721 Bradley was appointed to the Savilian 
 at^xford, Professorship of Astronomy at Oxford, vacant by 
 the death of Dr. John Keill. Once it became 
 clear that there was no chance of securing his 
 uncle for this position, Bradley himself was sup- 
 ported enthusiastically by all those whose support 
 was worth having, especially by the Earl of 
 Macclesfield, who was then Lord Chancellor; 
 by Martin Foulkes, who was afterwards the 
 President of the Royal Society ; and by Sir 
 Isaac Newton himself. He was accordingly 
 elected on October 31, 1721, and forthwith re- 
 signed his livings. His resignation of the livings 
 was necessitated by a definite statute of the Uni- 
 versity relating to the Professorship, and not by 
 the existence of any very onerous duties attach- 
 ing to it ; indeed such duties seem to have been 
 conspicuously absent, and after Bradley's election 
 but con- he passed more time than ever with his uncle in 
 work at Wansted, making the astronomical observations 
 Wansted. ^^ both loved ; for there was not the vestige 
 of an observatory in Oxford. His uncle's death 
 in 1724 interrupted the continuity of these joint 
 observations, and by an odd accident prepared the 
 way for Bradley's great discovery. He was fain 
 to seek elsewhere that companionship in his work 
 which had become so essential to him, and his 
 new friend gave a new bent to his observations. 
 Samuel Molyneux was a gentleman of fortune 
 
BRADLEY'S DISCOVERIES 95 
 
 much attached to science, and particularly to Samuel 
 astronomy, who was living about this time at jjux. 
 Kew. He was one of the few, moreover, who 
 are not content merely to amuse themselves 
 with a telescope, but had the ambition to do 
 some real earnest work, and the courage to 
 choose a problem which had baffled the human 
 race for more than a century. The theory 
 of Copernicus, that the earth moved round 
 the sun, necessitated a corresponding apparent 
 change in the places of the stars, one relatively 
 to another; and it was a standing difficulty in 
 the way of accepting this theory that no such 
 change could be detected. In the old days 
 before the telescope it was perhaps easy to 
 understand that the change might be too small 
 to be noticed, but the telescope had made it 
 possible to measure changes of position at least 
 a hundred times as small as before, and still no 
 " parallax/' as the astronomical term goes, could 
 be found for the stars. The observations of 
 Galileo, and the measures of Tycho Brans', as 
 reduced to systematic laws by Kepler, and finally 
 by the great Newton, made it clear that the 
 Copernican theory was true : but no one had 
 succeeded in proving its truth in this particular 
 way. Samuel Molyneux must have been a man 
 of great courage to set himself to try to crack 
 this hard nut ; and we can understand the attrac- Attempts 
 tion which his enterprise must have had for s e iiar 
 Bradley, who had just lost the beloved colleague of P arallax - 
 
9 6 ASTRONOMICAL DISCOVERY 
 
 many courageous astronomical undertakings. His 
 co-operation seems to have been welcomed from 
 the first ; his help was invited and freely given 
 in setting up the instrument, and he fortunately 
 had the leisure to spend considerable time at Kew 
 making the observations with Molyneux, just as 
 he had been wont to observe with his uncle. 
 
 I must now briefly explain what these observa- 
 tions were. There is a bright star y Draconis, 
 which passes almost directly overhead in the lati- 
 tude of London. Its position is slowly changing 
 owing to the precession of the equinoxes, but for 
 two centuries it has been, and is still, under con- 
 stant observation by London astronomers owing 
 to this circumstance, that it passes directly over- 
 head, and so its position is practically undisturbed 
 by the refraction of our atmosphere. 
 
 It was therefore thought at the time that, there 
 being no disturbance from refraction, the disturb- 
 ance from precession being accurately known, and 
 there being nothing else to disturb the position 
 but "parallax" (the apparent shift due to the 
 earth's motion which it was desirable to find), 
 this star ought to be a specially favourable object 
 for the determination of parallax. Indeed it had 
 been announced many years before by Hooke that 
 its parallax had been found ; but his observations 
 were not altogether satisfactory, and it was with 
 a view of either confirming them or seeing what 
 was wrong with them that Molyneux and Bradley 
 started their search. They set up a much more 
 
BRADLEY'S DISCOVERIES 97 
 
 delicate piece of apparatus than Hooke had em- 
 ployed. It was a telescope 24 feet long pointed The m- 
 vertically upwards to the star, and firmly attached 
 to a large stack of brick chimneys within the 
 house. The telescope was not absolutely fixed, for 
 the lower end could be moved by a screw so as to 
 make it point accurately to the star, and a plumb- 
 line showed how far it was from the vertical when 
 so pointing. Hence if the star changed its posi- 
 
 Fio. 2. 
 
 tion, however slightly, the reading of this screw 
 would show the change. Now, before setting out Expected 
 On the observations, the observers knew what to r( 
 expect if the star had a real parallax ; that is to 
 say, they knew that the star would seem to be 
 farthest south in December, farthest north in 
 June, and at intermediate positions in March and 
 September ; though they did not know how much 
 farther south it would appear in December than 
 in June this was exactly the point to be decided. 
 
 G 
 
98 ASTRONOMICAL DISCOVERY 
 
 The reason of this will be clear from Fig. 2. 
 [Remark, however, that this figure and the cor- 
 responding figure 4 do not represent the case of 
 Bradley's star, 7 Draconis : another star has been 
 chosen which simplifies the diagram, though the 
 principle is essentially the same.] Let A B O D 
 represent the earth's orbit, the earth being at 
 A in June, at B in September, and so on, 
 and let K represent the position of the star on 
 the line D B. Then in March and September 
 it will be seen from the earth in the same 
 direction, namely, D B K ; but the directions 
 in which it is seen in June and December, viz. 
 A K and C K, are inclined in opposite ways 
 to this line. The farther away the star is, the 
 less will this inclination or " parallax " be ; and 
 the star is actually so far away that the inclina- 
 tion can only be detected with the utmost diffi- 
 culty : the lines C K and A K are sensibly 
 parallel to D B K. But Bradley did not know 
 this; it was just : this point which he was to 
 examine, and he expected the greatest inclina- 
 tion in one direction to be in December. Accord- 
 ingly when a few observations had been made 
 on December 3, 5, n, and 12 it was thought that 
 the star had been caught at its most southerly 
 apparent' position, and might be expected there- 
 after to move northwards, if at all. But when 
 Unex- Bradley repeated the observation on December 
 resuhl : 7 ne found to his great surprise that the star 
 was still moving southwards. Here was some- 
 
BRADLEY'S DISCOVERIES 99 
 
 thing quite new and unexpected, and such a 
 keen observer as Bradley was at once on the 
 alert. He soon found that the changes in the 
 position of the star were of a totally unex- 
 pected character. Instead of the extreme posi- 
 tions being occupied in June and December, 
 they were occupied in March and September, 
 just midway between these. And the range 
 in position was quite large, about 40" not a 
 quantity which could have been detected in 
 the days before telescopes, but one which was 
 unmistakable with an instrument of the most 
 moderate measuring capacity. 
 
 What, then, was the cause of this quite unfore- 
 seen behaviour on the part of the star ? The first Tentative 
 thought of the observers was that something might tk>n&] na 
 be wrong with their instrument, and it was care- 
 fully examined, but without result The next was 
 that the apparent movement was in the plumb- 
 line, the line of reference. If the whole earth, 
 instead of carrying its axis round the sun in a 
 constant direction, were to be executing an oscil- 
 lation, then all our plumb-lines would oscillate, 
 and when the direction of a star like 7 Draconis 
 was compared with that of the plumb-line it 
 would seem to vary, owing actually to the varia- 
 tion in the plumb-line. The earth might have 
 a motion of this kind in two ways, which it will 
 be necessary for us to distinguish, and the adopted 
 names for them are " nutation of the axis " and 
 " variation of latitude " respectively. In the case 
 
ioo ASTRONOMICAL DISCOVERY 
 
 of nutation the North Pole remains in the same 
 geographical position, but points to a different 
 part of the heavens. The "variation of lati- 
 tude," on the other hand, means that the North 
 Pole wanders about on the earth itself. We 
 shall refer to the second phenomenon more par- 
 ticularly in the sixth chapter. 
 
 Nutation ? But it was the first kind of change, the nutation, 
 which Bradley suspected ; and very early in the 
 series of observations he had already begun to 
 test this hypothesis. If it was not the star, but 
 the earth and the plumb-line, which were in 
 motion, then other stars ought to be affected. 
 The telescope had been deliberately restricted in 
 its position to suit 7 Draconis ; but since the stars 
 circle round the Pole, if we draw a narrow belt in 
 the heavens with the Pole as centre, and includ- 
 ing 7 Draconis, the other stars included would 
 make the same circuit, preceding or following 
 7 Draconis by a constant interval. Most of them 
 would be too faint for observation with Bradley's 
 telescope ; but there was one bright enough to 
 be observed, which also came within its limited 
 range, and it was promptly put under surveillance 
 when a nutation of the earth's axis was suspected. 
 Careful watching showed that it was not affected 
 in the saine way as 7 Draconis, and hence the 
 movement could not be in the plumb-line. Was 
 there, then, after all, some effect of the earth's 
 atmosphere which had been overlooked ? We 
 have already remarked that since the star passes 
 
BRADLEY'S DISCOVERIES 
 
 101 
 
 directly overhead there should be practically no 
 refraction ; and this assumption was made by 
 Molyneux and Bradley in choosing this parti- 
 cular star for observation. It follows at once, if 
 we assume that the atmosphere surrounds the 
 earth in spherical layers. But perhaps this was Anoma- 
 not so ? Perhaps, on the contrary, the atmos- fraction. 
 phere was deformed by the motion of the earth, 
 streaming out behind her like the smoke of a 
 moving engine ? No possibility must be over- 
 
 Atmospfcere/ 
 
 Earth* 
 
 FIG. 3. 
 
 looked if the explanation of this puzzling fact 
 was to be got at. 
 
 The way in which a deformation of the atmos- 
 phere might explain the phenomenon is best seen 
 by a diagram. First, it must be remarked that 
 rays of light are only bent by the earth's atmos- 
 phere, or ''refracted," if they enter it obliquely. 
 
 If the atmosphere were of the same density 
 throughout, like a piece of glass, then a vertical 
 ray of light, A B (see Fig. 3), entering the 
 atmosphere at B would suffer no bending or 
 
102 ASTRONOMICAL DISCOVERY 
 
 refraction, and a star shining from the direction 
 A B would be seen truly in that direction from 
 C. But an oblique ray, D E, would be bent on 
 entering the atmosphere at E along the path 
 EF, and a star shining along D E would appear 
 from F to be shining along the dotted line G E F. 
 The atmosphere is not of the same density 
 throughout, but thins out as we go upwards from 
 
 Apparent 
 
 the earth ; and in consequence there is no clear- 
 cut surface, B E, and no sudden bending of the 
 rays as at E : they are gradually bent at an in- 
 finite succession of imaginary surfaces. But it 
 still remains true that there is no bending at all 
 for vertical rays ; and of oblique rays those most 
 oblique are most bent. 
 
 Now, suppose the atmosphere of the earth took 
 up, owing to its revolution round the sun, an 
 elongated shape like that indicated in diagram 4, 
 
BRADLEY'S DISCOVERIES 103 
 
 and suppose the star to be at a great distance away 
 to the right of the diagram. When the earth is in 
 the position labelled "June," the light would fall 
 vertically on the nose of the atmosphere at A, 
 and there would be no refraction. Similarly in 
 "December" the light would fall at C on the 
 stern, also vertically, and there would be no 
 refraction. [The rays from the distant star in 
 December are to be taken as sensibly parallel to 
 those received in June, notwithstanding that the 
 earth is on the opposite side of the sun, as was 
 remarked on p. 98.] But in March and Sep- 
 tember the rays would strike obliquely on the 
 sides of the supposed figure, and thus be bent in 
 opposite directions, as indicated by the dotted 
 lines ; and the extreme positions would thus 
 occur in March and September, as had been 
 observed. The explanation thus far seems satis- 
 factory enough. 
 
 But we have assumed the star to lie in the 
 plane of the earth's orbit; and the stars under 
 observation by Bradley did not lie in this plane, 
 nor did they lie in directions equally inclined to 
 it. Making the proper allowance for their direc- 
 tions, it was found impossible to fit in the facts 
 with this hypothesis, which had ultimately to be 
 abandoned. 
 
 It is remarkable to find that two or three years Delay m 
 went by before the real explanation of this new reai^f- 
 phenomenon occurred to Bradley, and during this P lanatlon - 
 time he must have done some hard thinking. 
 
io 4 ASTRONOMICAL DISCOVERY 
 
 We have all had experience of the kind of think- 
 ing if only in the guessing of conundrums. We 
 know the apparent hopelessness of the quest at 
 the outset : the racking of our brains for a clue, 
 the too frequent despair and "giving it up," and 
 the simplicity of the answer when once it is 
 declared. But with scientific conundrums the 
 expedient of " giving it up " is not available. We 
 must find the answer for ourselves or remain in 
 ignorance ; and though we may feel sure that the 
 answer when found will be as simple as that to 
 the best conundrum, this expected simplicity does 
 not seem to aid us in the search. Bradley was 
 not content with sitting down to think : he set to 
 work to accumulate more facts. Molyneux's 
 instrument only allowed of the observation of 
 two stars, 7 Draconis and the small star above 
 Bradley mentioned. Bradley determined to have an 
 another instrument of his own which should command 
 menTat a w ^ er range of stars ; and by this time he was 
 Wansted. a ble to return to his uncle's house at Wansted for 
 this purpose. His uncle had been dead for two 
 or three years, and the memory of the loss was 
 becoming mellowed with time. His uncle's widow 
 was only too glad to welcome back her nephew, 
 though no longer to the old rectory, and she 
 allowed him to set up a long telescope, even 
 though he cut holes in her floor to pass it through. 
 The object-glass end was out on the roof and the 
 eye end down in the coal cellar ; and accordingly in 
 this coal cellar Bradley made the observations which 
 
BRADLEY'S DISCOVERIES 105 
 
 led to his immortal discovery. He had a list of 
 seventy stars to observe, fifty of which he observed 
 pretty regularly. It may seem odd that he did 
 not set up this new instrument at Oxford, but 
 we find from an old memorandum that his pro- 
 fessorship was not bringing him in quite ^140 a 
 year, and probably he was glad to accept his 
 aunt's hospitality for reasons of economy. By 
 watching these different stars he gradually got 
 a clear conception of the laws of aberration. 
 The real solution of the problem, according to 
 a well-authenticated account, occurred to him 
 almost accidentally. We all know the story of the Finds the 
 apple falling and setting Newton to think about " 
 the causes of gravitation. It was a similarly 
 trivial circumstance which suggested to Bradley 
 the explanation which he had been seeking for 
 two or three years in vain. In his own words, 
 " at last, when he despaired of being able to 
 account for the phenomena which he had observed, 
 a satisfactory explanation of them occurred to 
 him all at once when he was not in search of 
 it." He accompanied a pleasure party in a sail 
 upon the river Thames. The boat in which they 
 were was provided with a mast which had a vane 
 at the top of it. It blew a moderate wind, and 
 the party sailed up and down the river for a con- 
 siderable time. Dr. Bradley remarked that every 
 time the boat put about the vane at the top of the A wind- 
 boat's mast shifted a little, as if there had been a 
 slight change in the direction of the wind. He 
 
io6 ASTRONOMICAL DISCOVERY 
 
 observed this three or four times without speak- 
 ing ; at last he mentioned it to the sailors, and 
 expressed his surprise that the wind should shift 
 so regularly every time they put about. The 
 sailors told him that the wind had not shifted, but 
 that the apparent change was owing to the change 
 in the direction of the boat, and assured him that 
 the same thing invariably happened in all cases. 
 This accidental observation led him to conclude 
 
 iE 
 
 ->0 
 
 FIG. 5. 
 
 that the phenomenon which had puzzled him so 
 much was owing to the combined motion of light 
 and of the earth. To explain exactly what is 
 meant we must again have recourse to a diagram ; 
 and we may also make use of an illustration which 
 has become classical. 
 
 If rain is falling vertically, as represented by 
 the direction A B ; and if a pedestrian is walking 
 horizontally, in the direction C D, the rain will 
 appear to him to be coming in an inclined 
 direction, E F, and he will find it better to tilt his 
 umbrella forwards. The quicker his pace the 
 more he will find it advisable to tilt the umbrella. 
 This analogy was stated by Lalande before the 
 
BRADLEY'S DISCOVERIES 107 
 
 days of umbrellas in the following words: "Je 
 suppose que, dans un temps calme, la pluie tombe 
 perpend iculairement, et qu'on soit dans une 
 voiture ouverte sur le devant ; si la voiture est en 
 repos, on ne reoit pas la moindre goutte de 
 pluie ; si la voiture avance avec rapidite, la pluie 
 entre sensiblement, comme si elle avoit pris une 
 direction oblique." Lalande's example, modified 
 to suit modern conditions, has been generally 
 adopted by teachers, and in examinations candi- 
 dates produce graphic pictures of the stationary, 
 the moderate-paced, and the flying, possessors of 
 umbrellas. 
 
 Applying it to the phenomenon which it is 
 intended to illustrate, if light is being received 
 from a star by an earth, travelling across the 
 direction of the ray, the telescope (which in this 
 case represents the umbrella) must be tilted for- 
 ward to catch the light. Now on reference to 
 Fig. 4 it will be seen that the earth is travelling 
 across the direction of rays from the star in 
 March and September ; and in opposite directions 
 in the two cases. Hence the telescope must be 
 tilted a little, in opposite directions, to catch the 
 light ; or, in other words, the star will appear to 
 be farthest south in March, farthest north in 
 September. And so at last the puzzle was solved, 
 and the solution was found, as so often happens, 
 to be of the simplest kind ; so simple when once 
 we know, and so terribly hard to imagine when 
 we don't ! It may comfort us in our struggles 
 
1 08 ASTRONOMICAL DISCOVERY 
 
 with minor problems to reflect that Bradley man- 
 fully stuck to his problem for two or three years. 
 It was probably never out of his thoughts, 
 waking or sleeping ; when at work it was the 
 chief object of his labours, and when on a 
 pleasure party he was ready to catch at the 
 slightest clue, in the motion of a wind-vane on 
 a boat, which might help him to the solution. 
 Results of The discovery of aberration made Bradley 
 ' famous at a bound. Oxford might well be proud 
 of her two Savilian Professors at this time, for 
 they had both made world-famous discoveries 
 Halley that of the periodicity of comets, and 
 Bradley of the aberration of light. How dif- 
 ferent their tastes were and how difficult it 
 would have been for either to do the work of 
 the other ! Bradley was no great mathematician, 
 and though he was quite able to calculate the 
 orbit of a comet, and carried on such work when 
 Halley left it, it was probably not congenial to 
 him. Halley, on the other hand, almost despised 
 accurate observations as finicking. " Be sure you 
 are correct to a minute," he was wont to say, 
 " and the fractions do not so much matter." 
 With such a precept Bradley would never have 
 made his discoveries. No quantity was too small 
 in his eyes, and no sooner was the explanation 
 of aberration satisfactorily established than he 
 perceived that though it would account for the 
 main facts, it would not explain all. There was 
 something left. This is often the case in the 
 
BRADLEY'S DISCOVERIES 109 
 
 history of science. A few years ago it was thought 
 that we knew the constitution of our air com- 
 pletely oxygen, nitrogen, water vapour, and 
 carbonic acid gas; but a great physicist, Lord 
 Rayleigh, found that after extracting all the 
 water and carbonic acid gas, all the oxygen and 
 all the nitrogen, there was something left a 
 very minute residuum, which a careless experi- 
 menter would have overlooked or neglected, but 
 which a true investigator like Lord Rayleigh saw 
 the immense importance of. He kept his eye on 
 that something left, and presently discovered a 
 new gas which we now know as argon. Had he 
 repeated the process, extracting all the argon 
 after the nitrogen, he might have found by a 
 scrutiny much more accurate still yet another gas, 
 helium, which we now know to exist in extremely 
 minute quantities in the air. But meantime this 
 discovery was made in another way. 
 
 When Bradley had extracted all the aberration stiiisome- 
 from his observations he found that there was be JL 
 something left, another problem to be solved and pla 
 some more thinking to be done to solve it. But 
 he was now able to profit by his previous labours, 
 and the second step was made more easily than 
 the first. The residuum was not the parallax 
 of which he had originally been in search, for 
 it did not complete a cycle within the year; it 
 was rather a progressive change from year to 
 year. But there was an important clue of another 
 kind. He saw that the apparent movements of 
 
no ASTRONOMICAL DISCOVERY 
 
 all stars were in this case the same ; and he 
 knew that a movement of this kind can be 
 referred, not to the stars themselves, but to 
 the plumb-line from which their directions are 
 measured. He had thought out the possible 
 causes of such a movement of the plumb-line or 
 of the earth itself, and had realised that there 
 Probably might be a nutation which would go through a 
 cycle in about nineteen years, the period in which 
 the moon's nodes revolve. He was not mathe- 
 matician enough to work out the cause completely, 
 but he saw clearly that to trace the whole effect 
 he must continue the observations for nineteen 
 years ; and accordingly he entered on this long 
 campaign without any hesitation. His instru- 
 ment was still that in his aunt's house atWansted, 
 where he continued to live and make the obser- 
 vations for a few years, but in 1732 he removed 
 to Oxford, as we shall see, and he must have 
 made many journeys between Wansted and 
 Oxford in the course of the remaining fifteen 
 years during which he continued to trace out the 
 effects of nutation. His aunt too left Wansted 
 to accompany Bradley to Oxford, and the house 
 His nine- passed into other hands. It is to the lasting 
 campaign, credit of the new occupant, Mrs. Elizabeth 
 Williams,- that the great astronomer was allowed 
 to go on and complete the valuable series of 
 observations which he had commenced. Bradley 
 was not lodged in her house ; he stayed with a 
 friend close by on his visits to Wansted, but 
 
BRADLEY'S DISCOVERIES in 
 
 came freely in and out of his aunt's old home 
 to make his observations. How many of us are 
 there who would cheerfully allow an astronomer 
 to enter our house at any hour of the night to 
 make observations in the coal-cellar ! It says much, 
 not only for Bradley's fame, but for his personal 
 attractiveness, that he should have secured this 
 permission, and that there should be no record 
 of any friction during these fifteen years. At the 
 end of the whole series of nineteen years his 
 conclusions were abundantly verified, and his 
 second great discovery of nutation was established. 
 Honours were showered upon him, and no doubt 
 the gentle heart of Mrs. Elizabeth Williams was 
 uplifted at the glorious outcome of her long for- 
 bearance. 
 
 But we may now turn for a few moments from 
 Bradley's scientific work to his daily life. We 
 have said that in 1732, after holding his profes- 
 sorship for eleven years, he first went definitely to 
 reside in Oxford. He actually had not been able Residence 
 
 re , . i IT- i at Oxford. 
 
 to anord it previously. His income was only 
 ^140 a year, and the statutes prevented him from 
 holding a living : so that he was fain to accept 
 Mrs. Pound's hospitable shelter. But in 1729 an 
 opportunity of adding to his income presented 
 itself, by giving lectures in " experimental philo- 
 sophy." The observations on nutation were not 
 like those on aberration : he was not occupied 
 day and night trying to find the solution : he had 
 practically made up his mind about the solution, 
 
ii2 ASTRONOMICAL DISCOVERY 
 
 and the actual observations were to go on in a 
 quiet methodical manner for nineteen years, so 
 that he now had leisure to look about him for 
 
 
 
 other employment. Dr. Keill, who had been Pro- 
 fessor of Astronomy before Bradley, had attracted 
 large classes to lectures, not on astronomy, but on 
 experimental philosophy : but had sold his ap- 
 paratus and goodwill to Mr. Whiteside, of Christ 
 Church, one of the candidates who were disap- 
 pointed by Bradley's election. In 1729 Bradley 
 purchased the apparatus from Whiteside, and 
 began to give lectures in experimental philosophy. 
 His discovery of aberration had made him famous, 
 so that his classes were large from the first, and 
 paid him considerable fees. Suddenly therefore 
 he changed his poverty for a comfortable income, 
 and he was able to live in Oxford in one of two 
 red brick houses in New College Lane, which 
 were in those days assigned to the Savilian Pro- 
 fessors (now inhabited by New College under- 
 graduates). His aunt, Mrs. Pound, to whom he 
 was devotedly attached, came with him, and two 
 of her nephews. In his time of prosperity Bradley 
 was thus able to return the hospitality which had 
 been so generously afforded him in times of stress. 
 Astro- Before he completed his observations for nuta- 
 
 tion another great change in his fortunes took 
 place. In 1742 he was elected to succeed Halley 
 as Astronomer Royal. It was Halley's dying 
 wish that Bradley should succeed him, and it is 
 said that he was even willing to resign in his 
 
 
BRADLEY'S DISCOVERIES 113 
 
 * 
 
 favour, for his right hand had been attacked by 
 paralysis, and the disease was gradually spreading. 
 But he died without any positive assurance that 
 his wish would be fulfilled. The chief difficulty 
 in securing the appointment of Bradley seems to 
 have been that he was the obvious man for the 
 post in universal opinion. " It is not only my 
 friendship for Mr. Bradley that makes me so Letter 
 ardently wish to see him possessed of the posi- Earl of 
 tion," wrote the Earl of Macclesfield to the Lord 
 Chancellor ; " it is my real concern for the 
 honour of the nation with regard to science. For 
 as our credit and reputation have hitherto not 
 been inconsiderable amongst the astronomical 
 part of the world, I should be extremely sorry we 
 should forfeit it all at once by bestowing upon a 
 man of inferior skill and abilities the most hon- 
 ourable, though not the most lucrative, post in 
 the profession (a post so well filled by Dr. Halley 
 and his predecessor), when at the same time we 
 have amongst us a man known by all the foreign, 
 as well as our own astronomers, not to be inferior 
 to either of them, and one whom Sir Isaac Newton 
 was pleased to call the best astronomer in Europe." 
 And again, " As Mr. Bradley' s abilities in astro- 
 nomical learning are allowed and confessed by 
 all, so his character in every respect is so well 
 established, and so unblemished, that I may defy 
 the worst of his enemies (if so good and worthy a 
 man have any) to make even the lowest or most 
 
 trifling objection to it." 
 
 H 
 
ii4 ASTRONOMICAL DISCOVERY 
 
 " After all," the letter goes on, " it may be said 
 if Mr. Bradley's skill is so universally acknow- 
 ledged, and his character so established, there is 
 little danger of opposition, since no competitor 
 can entertain the least hope of success against 
 him. But, my lord, we live in an age when most 
 men how little soever their merit may be, seem to 
 think themselves fit for whatever they can get, 
 and often meet with some people, who by their 
 recommendations of them appear to entertain the 
 same opinion of them, and it is for this reason 
 that I am so pressing with your lordship not to 
 lose any time." 
 
 Such recommendations had, however, their 
 effect : the dreaded possibility of a miscarriage of 
 justice was averted, and Bradley became the third 
 Astronomer Royal, though he did not resign his 
 professorship at Oxford. Halley, Bradley, and 
 Bliss, who were Astronomers Royal in succession, 
 all held the appointment along with one of the 
 Savilian professorships at Oxford ; but since the 
 death of Bliss in 1761, the appointment has 
 always gone to a Cambridge man. 
 
 instru- When Bradley went to Greenwich, in June 
 
 veryde- 1 74 2 > ne was at first unable to do much from the 
 
 3tive. wretched, state in which he found the instruments. 
 
 Halley was not a good observer : his heart was 
 
 not in the work, and he had not taken the trouble 
 
 to set the instruments right when they went wrong. 
 
 The counterpoises of that instrument which ought 
 
 to have been the best in the world at the time 
 
BRADLEY'S DISCOVERIES 115 
 
 rubbed against the roof so that the telescope could 
 scarcely be moved in some positions : and some 
 of the screws were broken. There was no proper 
 means of illuminating the cross-wires, and so on. 
 With care and patience Bradley set all this right, 
 and began observations. He had the good fortune 
 to secure the help of his nephew, John Bradley, 
 as assistant, and the companionship seems to have 
 been as happy as that previous one of James Bradley 
 and his uncle Pound. John Bradley was able to 
 carry on the observations when his uncle was absent 
 in Oxford, and the work the two got through 
 together in the first year is (in the words of 
 Bradley's biographer Rigaud) " scarcely to be 
 credited." The transit observations occupy 177 
 folio pages, and no less than 255 observations 
 were taken on one night. And at the same time, 
 it must be remembered, Bradley was still carrying 
 on his nutation observations at Wansted, still 
 lecturing at Oxford, and not content with all this, 
 began a course of experiments on the length of 
 the seconds' pendulum. Truly a giant for hard 
 work! 
 
 But, in spite of his care in setting them right, 
 the instruments in the Observatory were found to 
 be hopelessly defective. The history of the in- 
 struments at the Royal Observatory is a curious 
 one. When Flamsteed was appointed the first 
 Astronomer Royal he was given the magnificent 
 salary of 100 a year, and no instruments to 
 observe with. He purchased some instruments 
 
n6 ASTRONOMICAL DISCOVERY 
 
 with his own money, and at his death they were 
 claimed by his executors. Hence Halley, the 
 second Astronomer Royal, found the Observatory 
 totally unprovided in this respect. He managed 
 to persuade the nation to furnish the funds for an 
 equipment ; but Halley, though a man of great 
 ability in other ways, did not know a good instru- 
 ment from a bad one ; so that Bradley's first few 
 years at the Observatory were wasted owing to the 
 imperfection of the equipment. When this was 
 New in- fully realised he asked for funds to buy new 
 
 struments. . -i i i n i c t 
 
 instruments, and such was the confidence felt 
 in him that he got what he asked for without 
 much difficulty. More than ^1000, a large sum 
 for those days, was spent under his direction, 
 the principal purchases being two quadrants for 
 observation of the position of the stars, one to 
 the north and the other to the south. With 
 these quadrants, which represented the perfection 
 of such apparatus at that time, Bradley made 
 that long and wonderful series of observations 
 which is the starting-point of our knowledge of 
 the movements of the stars. The instruments 
 are still in the Royal Observatory, the more 
 important of the two in its original position as 
 Bradley mounted it and left it. 
 
 Work at It seems needless to mention his work as 
 Astronomer Royal, but I will give quite briefly 
 a summary of what he accomplished, and then 
 recall a particular incident, which shows how far 
 ahead of his generation his genius for observa- 
 
BRADLEY'S DISCOVERIES 117 
 
 tion placed him. The summary may be given as 
 follows. We owe to Bradley 
 
 1. A better knowledge of the movements of 
 Jupiter's satellites. 
 
 2. The orbits of several comets calculated 
 directly from his own observations, when such 
 work was new and difficult. 
 
 3. Experiments on the length of the pendulum. 
 
 4. The foundation of our knowledge of the 
 refraction of our atmosphere. 
 
 5. Considerable improvements in the tables of 
 the moon, and the promotion of the method for 
 finding the longitude by lunar distances. 
 
 6. The proper equipment of our national Ob- 
 servatory with instruments, and the use of these 
 to form the basis of our present knowledge of 
 the positions and motions of the stars. 
 
 Many men would consider any one of these six 
 achievements by itself a sufficient title to fame. 
 Bradley accomplished them all in addition to his 
 great discoveries of aberration and nutation. 
 
 And with a little more opportunity he might Might 
 have added another great discovery which has found 
 
 shed lustre on the work of the last decade. We 
 said earlier in this chapter that the axis of the tude> 
 earth may move in one or two ways. Either it 
 may point to a different star, remaining fixed 
 relatively to the earth, as in the nutation which 
 Bradley discovered ; or it may actually change its 
 position in the earth. This second kind of move- 
 ment was believed until twenty years ago not to 
 
nS ASTRONOMICAL DISCOVERY 
 
 exist appreciably ; but the work of Kiistner and 
 Chandler led to the discovery that it did exist, 
 and its complexities have been unravelled, and 
 will be considered in the sixth chapter. Now a 
 century and a half ago Bradley was on the track 
 of this "variation of latitude." His careful obser- 
 vations actually showed the motion of the pole, 
 as Mr. Chandler has recently demonstrated ; and, 
 moreover, Bradley himself noticed that there was 
 something unexplained. Once again there was 
 a residuum after (first) aberration and (next) 
 nutation had been extracted from the observa- 
 tions ; and with longer life he might have ex- 
 plained this residuum, and added a third great 
 discovery to the previous two. Or another coming 
 after him might have found it ; but after the giant 
 came men who could not tread in his footsteps, 
 and the world waited 150 years before the dis- 
 crepancy was explained. 
 
 The attitude of our leading universities towards 
 science and scientific men is of sufficient import- 
 ance to justify another glance at the relations 
 Oxford's between Bradley and Oxford. We have seen that 
 recogni- Oxford's treatment of Bradley was not altogether 
 Bradley, satisfactory. She left him to learn astronomy as 
 he best cQuld, and he owes no teaching to her. 
 She made him Professor of Astronomy, but gave 
 him no observatory and a beggarly income which 
 he had to supplement by giving lectures on a 
 different subject. But when he had disregarded 
 these discouragements and made a name for him- 
 
BRADLEY'S DISCOVERIES 119 
 
 self, Oxford took her share in recognition. He 
 was created D.D. by diploma in 1742 ; and when 
 his discovery of nutation was announced in 1 748, 
 and produced distinctions and honours of all kinds 
 from over the world, we are are told that " amidst 
 all these distinctions, wide as the range of modern 
 science, and permanent as its history, there was 
 one which probably came nearer his heart, and 
 was still more gratifying to his feeling than all. 
 Lowth (afterwards Bishop of London), a popular 
 man, an elegant scholar, and possessed of con- 
 siderable eloquence, had in 1751 to make his last 
 speech in the Sheldonian Theatre at Oxford as 
 Professor of Poetry. In recording the benefits 
 for which the University was indebted to its 
 benefactors, he mentioned the names of those 
 whom Sir Henry Savile's foundation had estab- 
 lished there : * What men of learning ! what 
 mathematicians ! we owe to Savile, Briggs, 
 Wallis, Halley ; to Savile we owe Greaves, 
 Ward, Wren, Gregory, Keill, and one whom I 
 will not name, for posterity will ever have his 
 name on its lips.' Bradley was himself present ; 
 there was no one in the crowded assembly on 
 whom the allusion was lost, or who did not feel 
 the truth and justice of it ; all eyes were turned 
 to him, while the walls rung with shouts of 
 heartfelt affection and admiration ; it was like the 
 triumph of Themistocles at the Olympic games." 
 
 These words of Eigaud indicate the fame 
 deservedly acquired by an earnest and simple- 
 
120 ASTRONOMICAL DISCOVERY 
 
 minded devotion to science : but can we learn 
 The study anything from the study of Bradley's work to 
 siduai guide us in further research ? The chief lessons 
 mena" would seem to be that if we make a series of 
 careful observations, we shall probably find some 
 deviation from expectation : that we must follow 
 up this clue until we have found some explana- 
 tion which fits the facts, not being discouraged if 
 we cannot hit upon the explanation at once, since 
 Bradley himself was puzzled for several years : 
 that after finding one vera causa, and allowing 
 for the effect of it, the observations may show 
 traces of another which must again be patiently 
 hunted, even though we spend nineteen years in 
 the chase : and that again we may have to leave 
 the complete rectification of the observations to 
 posterity. But though we may admit the general 
 helpfulness of these directions, and that this 
 patient dealing with residual phenomena seems 
 to be a method capable of frequent application, 
 we cannot deduce any universal principle of pro- 
 cedure from them : witness the difficulty of deal- 
 ing with meteorological observations, for instance. 
 It is not always possible to find any orderly 
 arrangement of the residuals which will give us 
 a clue to start with. When such an arrangement 
 is manifested, we must certainly follow up the 
 clue ; it would almost seem that no expense 
 should be prohibitive, since it is impossible to 
 foresee the importance of the result. 
 
CHAPTER IV 
 ACCIDENTAL DISCOVERIES 
 
 IN reviewing various types of astronomical dis- 
 covery I have laid some stress upon the fact that 
 they are, generally speaking, far from being acci- 
 dental in character. A new planet does not 
 " swim into our ken," at any rate not usually, but 
 is found only after diligent search, and then only 
 by an investigator of acute vision, or other special 
 qualifications. But this is, of course, not always 
 the case. Some discoveries are made by the 
 merest accident, as we have had occasion to 
 remark incidentally in the case of the minor 
 planets ; and for the sake of completeness it is 
 desirable to include among our types at least one 
 case of such accidental discovery. As, however, 
 the selection is a little invidious, I may perhaps 
 be pardoned for taking the instance from my own 
 experience, which happens to include a case where 
 one of those remarkable objects called " new stars " 
 walked deliberately into a net spread for totally The Ox- 
 different objects. There is the further reason for star D 
 choosing this instance : that it will afford me the 
 opportunity of saying something about the special 
 research in which we were actually engaged, the 
 work of mapping out the heavens by photography, 
 
122 ASTRONOMICAL DISCOVERY 
 
 found or, as it has been called, the Astrographic Chart 
 workon a great scheme of international co-operation by 
 graphic which it is hoped to leave as a legacy for future 
 chart. centuries a record of the state of the sky in our 
 age. Such a record cannot be complete ; for how- 
 ever faint the stars included, we know that there 
 are fainter stars, which might have been included 
 had we given longer exposures to the plates. Nor 
 can it be in other ways final or perfect ; however 
 large the scale, for instance, on which the map is 
 made, we can imagine the scale doubled or increased 
 many-fold. But the map will be a great advance 
 on anything that has hitherto been made, and some 
 account of it will therefore no doubt be of interest. 
 Origin of We may perhaps begin with a brief historical 
 lrt< account of the enterprise. Photographs of the 
 stars were taken many years ago, but only by a few 
 enthusiasts, and with no serious hope of competing 
 with eye observations of the sky. The old wet- 
 plate photography was, in fact, somewhat unsuited 
 to astronomical purposes ; to photograph faint 
 objects a long exposure is necessary, and the wet 
 plate may dry up before the exposure is concluded 
 nay, even before it is commenced, if the observer 
 has to wait for passing clouds and therefore it 
 may be said that the successful application of 
 photography to astronomy dates ,from the time 
 when the dry plate was invented ; when it became 
 possible to expose a plate in the telescope for 
 hours, or by accumulation even for days. The 
 dry plate remains sensitive for a long period, and 
 if it is desired to extend an exposure beyond the 
 
VII. GREAT COMET OF Nov. 7TH, 1882. 
 
 (From a photograph taken at the Royal Observatory ; Cape of Good Hope.) 
 
ACCIDENTAL DISCOVERIES 123 
 
 limits of one night, it is quite easy to close up the 
 telescope and return to the operations again on 
 the next fine night ; and so on, if not perhaps 
 indefinitely, at any rate so long as to transcend 
 the limits of human patience up to the present. 
 
 But to consider our particular project. We 
 may assign, perhaps, the date 1882 as that incometof 
 which it first began to take shape. In that year I( 
 there was a magnificent bright comet, the last 
 really large comet which we, in the Northern 
 Hemisphere, have had the good fortune to see. 
 Some of us, of course, were not born at that time, 
 and perhaps others who were alive may neverthe- 
 less not have seen that comet ; for it kept somewhat 
 uncomfortably early morning hours, and I can well 
 remember myself feeling rather more resentment 
 than gratitude to the man who waked me up 
 about four o'clock to see it. Many observations 
 were of course made of this interesting visitor, 
 and what specially concerns us is that at the Cape 
 of Good Hope some enterprising photographers 
 tried to photograph it. They tried in the first 
 instance with ordinary cameras, and soon found 
 what any astronomer could have told them that 
 the movement of the earth, causing an apparent 
 movement of the comet and the stars in the 
 opposite direction, frustrated their efforts. The 
 difficulties of obtaining pictures of moving objects 
 are familiar to all photographers. A " snap-shot" 
 might have met the difficulty, but the comet was 
 scarcely bright enough to affect the plate with a 
 short exposure. Ultimately Dr. David Gill, the 
 
i2 4 ASTRONOMICAL DISCOVERY 
 
 astronomer at the Cape Observatory, invited one 
 of the photographers to strap his camera to one 
 of the telescopes at the Observatory, a telescope 
 which could be carried round by clockwork in the 
 usual way, so as to counteract the earth's motion, 
 and in effect to keep the comet steadily in view, 
 as though it were at rest. As a consequence, 
 stars some very beautiful and successful pictures of the 
 thepie- n cornet were obtained, and on them a large number 
 tures. o s ^ ars were also shown. They were, as I have 
 said, not by any means the first pictures of stars 
 obtained by photography, but they represented in 
 facility and in success so great an advance upon 
 what had been formerly obtained that they at- 
 tracted considerable attention. They were sent to 
 Europe and stimulated various workers to further 
 experiments. 
 
 The late Dr. Common in England, an amateur 
 astronomer, began that magnificent pioneer work 
 in astronomical photography which soon brought 
 him the Gold Medal of the Eoyal Astronomical 
 Society for his photographs of nebulae. But the 
 most important result for our purpose was pro- 
 duced in France. There had been started many 
 years before by the French astronomer Chacornac 
 a series 9f star maps round the Zodiac similar 
 in intention to the Berlin maps which figured in 
 the history of the discovery of Neptune. Chacornac 
 died before his enterprise was very far advanced, 
 and the work was taken up by two brothers, 
 Paul and Prosper Henry, who followed Cha- 
 cornac in adopting for the work the laborious 
 
ACCIDENTAL DISCOVERIES 
 
 125 
 
 method of eye observation of each individual 
 star. They proceeded patiently with the work on 
 these lines ; but when they came to the region 
 where the Zodiac is crossed by the Milky Way, 
 and the number of stars in a given area increases 
 enormously, they found the labour so great as to 
 be practically prohibitive, and were in doubt how 
 to deal with the difficulty. It was at this critical The 
 moment that these comet photographs, showing 
 the stars so beautifully, suggested the alternative gin work - 
 of mapping the stars photographically. They 
 immediately set to work with a trial lens, and 
 obtained such encouraging results that they pro- 
 ceeded themselves to make a larger lens of the 
 same type ; this again was satisfactory, and the 
 idea naturally arose of extending to the whole 
 heavens the scheme which they had hitherto 
 intended only for the Zodiac, a mere belt of the 
 heavens. But this rendered the enterprise too 
 large for a single observatory. It became 
 necessary to obtain the co-operation of other 
 observatories, and with this end in view an confer- 
 International Conference was summoned to meet 1887. J 
 in Paris in 1887 to consider the whole project. 
 There were delegates from, if not all nations, at 
 any rate a considerable number : 
 
 France . 20 
 British Em- 
 
 U.S. America 
 
 Austria . 
 
 3 
 
 2 
 
 Spain . . i 
 Switzerland . i 
 
 pire . 8 
 Germany . 6 
 Russia . 3 
 Holland . 3 
 
 Sweden . 
 Denmark 
 Belgium . 
 Italy 
 
 2 
 2 
 I 
 I 
 
 Portugal . i 
 Brazil . . i 
 Argentine Re- 
 public . i 
 
i'26 ASTRONOMICAL DISCOVERY 
 
 The Conference had a number of very impor- 
 tant questions to discuss, for knowledge of the 
 photographic method and its possibilities was at 
 that time in its infancy. There was, for instance, 
 the question whether all the instruments need be 
 of the same pattern, and if so what that pattern 
 should be. The first of these questions was 
 settled in the affirmative, as we might expect ; 
 in the interests of uniformity it was desirable 
 that the maps should be as nearly similar as 
 
 choice of possible. The second question was not so easy; 
 
 ment. there were at least three different types of instru- 
 ments which might be used. First of all, there 
 was the photographic lens, such as is familiar to 
 all who have used an ordinary camera, consisting 
 of two lenses with a space between ; though since 
 each of these lenses is itself made up of two, we 
 should more correctly say four lenses in all. It 
 was with a lens of this kind that the comet 
 pictures had been taken at the Cape of Good 
 Hope, and it might seem the safest plan to adopt 
 what had been shown to be capable of such good 
 work. But there was this difficulty ; the pictures 
 of the comet were on a very small scale, and taken 
 with a small lens ; a much larger lens was re- 
 quired for the scheme now under contemplation, 
 and when' there are four separate lenses to be 
 made, each with two surfaces to polish, and each 
 requiring a perfectly sound clear piece of glass, 
 it will be obvious that the difficulties of making 
 a large compound lens of this kind are much 
 
ACCIDENTAL DISCOVERIES 127 
 
 greater, and the expense much more serious than Expense 
 
 ,1 /. . , i f. of'doub- 
 
 in the case of a single lens, or even a pair. It i e t." 
 was this question of expense which had led the 
 brothers Henry to experiment with a different 
 kind of instrument, in which only one pair of 
 lenses was used instead of two. Their instru- 
 ment was, in fact, very similar to the ordinary 
 telescope, excepting that they were bound to make 
 their lenses somewhat different in shape in order 
 to bring to focus the rays of light suitable for 
 photography, which are not the same as those 
 suitable for eye observation with the ordinary 
 telescope. Dr. Common, again, had used a third 
 kind of instrument, mainly with the view of re- 
 ducing the necessary expense still further, or, 
 perhaps, of increasing the size of the instrument 
 for the same expense. His telescope had no lens 
 at all, but a curved mirror instead, the mirror 
 being made of glass silvered on the face (not on 
 the back as in the ordinary looking-glass). In Advan- 
 this case there is only one surface to polish reflector. 
 instead of four, as in the Henrys' telescope, or 
 eight, as in the case of the photographic doublet ; 
 and, moreover, since the rays of light are reflected 
 from the surface of the glass, and do not pass 
 through it at all, the internal structure of the 
 glass is not so strictly important as in the other 
 cases. Hence the reflector is a very cheap instru- 
 ment, and it is, moreover, quite free from some 
 difficulties attached to the other instruments. No 
 correction for rays of light of different colours is 
 
128 ASTRONOMICAL DISCOVERY 
 
 required, since all rays of whatever colour come 
 to the same focus automatically. These advan- 
 tages of the reflector were so considerable as to 
 almost outweigh one well-known disadvantage, 
 which is, however, not very easily expressed in 
 words. The reflector might be described as an 
 instrument with a temper ; sometimes it gives 
 excellent results, but at others something seems 
 to be wrong, though the worried observer does 
 not exactly know what. Long experience and 
 patience are requisite to humour the instrument 
 and get the best results from it, and it was felt 
 that this uncertainty was sufficient to disqualify 
 the instrument for the serious piece of routine 
 work contemplated in mapping the heavens. 
 Refractor Accordingly the handier and more amiable in- 
 strument with which the brothers Henry had 
 done such good work was selected as the pattern 
 to be adopted. 
 
 It is curious that at the Conference of 1887 
 nothing at all was said about the type of in- 
 strument first mentioned (the "doublet lens"), 
 although a letter was written in its favour by 
 Professor Pickering of Harvard College Observa- 
 tory. Since that time we have learnt much of its 
 advantages, and it is probable that if the Con- 
 ference we're to meet now they might arrive at a 
 different decision ; but at that time they were, to 
 put it briefly, somewhat afraid of an instrument 
 which seemed to promise, if anything, too well, 
 especially in one respect. With the reflector and 
 
ACCIDENTAL DISCOVERIES 129 
 
 the refractor it had been found that the field of 
 good images was strictly limited. The Henrys' 
 telescope would not photograph an area of the 
 sky greater in extent than 2 in diameter at any 
 one time, and the reflector was more limited still ; 
 within this area the images of the stars were 
 good, and it had been found that their places 
 were accurately represented. Now the "doublet" Doublet 
 seemed to be able to show much larger areas than ha 
 this with accuracy, but no one had been able to better - 
 test the accuracy to see whether it was sufficient 
 for astronomical purposes ; and although no such 
 feeling was openly expressed or is on record, I 
 think there is no doubt that a feeling existed of 
 general mistrust of an instrument which seemed 
 to offer such specious promises. Whatever the 
 reason, its claims were passed over in silence at 
 the Conference, and the safer line (as it was then 
 thought) of adopting as the type the Henrys' 
 instrument, was taken. 
 
 This was perhaps the most important question 
 settled at the Conference, and the answers to 
 many of the others naturally followed. The size 
 of the plates, for instance, was settled automatic- 
 ally. The question down to what degree of faint- 
 ness should stars be included, resolved itself 
 into the equivalent question, What should be 
 the length of time during which the plates 
 were exposed ? Then, again, the question, What 
 observatories should take part in the work ? be- 
 came simply this: What observatories could afford 
 
 i 
 
130 
 
 ASTRONOMICAL DISCOVERY 
 
 The 
 otServ 
 
 Sky 
 
 covered 
 
 twice. 
 
 to acquire the instruments of this new pattern 
 and get other funds for carrying out the work 
 specified ? It was ultimately found that eighteen 
 observatories were able to obtain the apparatus 
 and funds, though unfortunately three of the 
 eighteen have since found it impossible to pro- 
 ceed. The following is the original list, and 
 in brackets are added the names of three other 
 observatories which in 1900 undertook to fill the 
 places of the defaulters. ' ,. ru 
 
 OBSERVATORIES CO-OPERATING FOR THE ASTROGRAPHIC 
 CHART. 
 
 Observatory. 
 
 Zones of 
 Declination. 
 
 Number 
 of Plates. 
 
 Greenwich 
 
 + 90 to + 65 
 
 1149 
 
 Rome 
 
 + 64 +55 
 
 1140 
 
 Catania 
 
 + 54 +47 
 
 1008 
 
 Helsingfors 
 
 + 46 +40 
 
 100$ 
 
 Potsdam 
 
 + 39 +32 
 
 1232 
 
 Oxford 
 
 + 3i +25 
 
 1180 
 
 Paris . 
 
 + 24 +18 
 
 1260 
 
 Bordeaux 
 
 + 17 +11 
 
 1260 
 
 Toulouse 
 
 + 10 + 5 
 
 1080 
 
 Algiers 
 
 + 4 - 2 
 
 1260 
 
 San Fernando 
 
 - 3 - 9 
 
 1260 
 
 Tacubaya . 
 
 - 10 - 16 
 
 1260 
 
 Santiago (Monte Video) 
 La Plata (Cordoba) . 
 
 -17 -23 
 -24 -31 
 
 1260 
 1360 
 
 Rio (Perth, Australia) 
 
 -32 -40 
 
 1376 
 
 Cape of Good Hope , 
 Sydney 
 Melbourne . 
 
 -4i,, -5i 
 -52,, -64 
 -65 -90 
 
 1512 
 1400 
 1149 
 
 In the list is also shown the total number of 
 plates that were to be taken by each observatory. 
 -When, once the size of the plates had been settled, 
 
ACCIDENTAL DISCOVERIES 131 
 
 it was a straightforward matter to divide up the 
 sky into the proper number of regions necessary 
 to cover it completely, not only without gaps be- 
 tween the plates, but with actually a small over- 
 lap of contiguous plates. And more than this, 
 it was decided that the whole sky should be com- 
 pletely covered twice over. It was conceivable 
 that a question might arise whether an apparent 
 star image on a plate was, on the one hand, a 
 dust speck, or, on the other hand, a planet, or 
 perhaps a variable or new star. By taking two 
 different plates at slightly different times, ques- 
 tions of this kind could be settled ; and to make 
 the check more independent it was decided that 
 the plates should not be exactly repeated on the 
 same portion of sky, but that in the second series 
 the centre of a plate should occupy the point 
 assigned to the corner of a plate in the first 
 series. 
 
 Then there came the important question of Times of 
 time of exposure, which involved a long debate ^ 
 between those who desired the most modest pro- 
 gramme possible consistent with efficiency, and 
 those enthusiasts who were anxious to strain the 
 programme to the utmost limits attainable. Ulti- 
 mately it was resolved to take two series of 
 plates ; one series of long exposure which was 
 set in the first instance at 10 minutes, then 
 became 15, then 30, then 40, and has by some 
 enterprising observers been extended to i J hours ; 
 the other a series of short exposures which have 
 
132 ASTRONOMICAL DISCOVERY 
 
 been generally fixed at 6 minutes. Thus instead 
 of covering the sky twice, it was decided to cover 
 it in all four times, and the number of plates 
 assigned to each observatory in the above list 
 must be regarded as doubled by this new deci- 
 sion. And further still, on the series of short- 
 exposure plates it was decided to add to the ex- 
 posure of six minutes another one of three minutes, 
 having slightly shifted the telescope between the 
 two so that they should not be superimposed ; 
 and later still, a third exposure of twenty seconds 
 was added to these. It would take too long to 
 explain here the reasons for these details, but it 
 will be clear that the general result of the discus- 
 sion was to extend the original programme con- 
 siderably, and render the work even more laborious 
 than it had appeared at the outset. 
 
 When all these plates have been taken, the 
 work is by no means finished ; indeed, it is only 
 Measure- just commencing. There remains the task of mea- 
 suring accurately on each of the short-exposure 
 plates the positions of the stars which it represents, 
 numbering on the average some 300 or 400 ; so that 
 for instance at Oxford the total number of stars 
 measured on the twelve hundred plates is nearly 
 half a million. These are not all separate stars ; 
 for the sky is represented twice over, and there is 
 also the slight overlap of contiguous plates ; but 
 the number of actual separate stars measured at 
 this one observatory is not far short of a quarter 
 of a million, and it has taken nearly ten years to 
 
ACCIDENTAL DISCOVERIES 133 
 
 make the measurements, with the help of three 
 or four measurers trained for the purpose. To The 
 render the measures easy, a network or rseau of n 
 cross lines is photographed on each plate by 
 artificial light after it has been exposed to the 
 stars, so that on development these cross lines 
 and the stars both appear. We can see at a 
 glance the approximate position of a star by 
 counting the number of the space from left to 
 right and from top to bottom in which it occurs ; 
 and we can also estimate the fraction of a space 
 in addition to the whole number ; but it is neces- 
 sary for astronomical purposes to estimate this 
 fraction with the greatest exactness. The whole 
 numbers are already given with great exactness 
 by the careful ruling of the cross lines<> which 
 can be spaced with extraordinary perfection. To 
 measure the fraction, we place the plate under a 
 microscope in the eye-piece of which there is a The 
 finally divided cross scale ; the centre of the cross 
 is placed over a star image, and then it is noted 
 where the lines of the rdseau cut the cross scale. 
 In this way the position of the image of a star is 
 read off with accuracy, and after a little practice 
 with considerable rapidity. It has been found 
 at Oxford that under favourable conditions the 
 places of nearly 200 stars* can be recorded in 
 this way by a single measurer, if he has some 
 one to write down for him the numbers he calls 
 out. This is only one form of measuring appa- 
 ratus ; there are others in which, instead of a 
 
134 ASTRONOMICAL DISCOVERY 
 
 scale in the eye-piece, micrometer screws are used 
 to measure the fractions ; but the general principle 
 in all these instruments is much the same, and 
 the rate of work is not very different; while to 
 the minor advantages and disadvantages of the 
 different types there seems no need here to 
 refer. One particular point, however, is worth 
 noting. After a plate has been measured, it is 
 
 Keversai turned round completely, so that left is now right, 
 and top is now bottom, and the measurements 
 are repeated. This repetition has the advantage 
 first of all of checking any mistakes. When a 
 long piece of measuring or numerical work of any 
 kind is undertaken there are invariably moments 
 when the attention seems to wander, and some 
 small error is the result. But there are also 
 certain errors of a systematic character similar to 
 those denoted by the term "personal equation," 
 which has found its way into other walks of life. 
 
 Personal In the operation of placing a cross exactly over 
 on ' the image of a star, different observers would 
 show slight differences of habit ; one might place 
 it a little more to the right than another. But 
 when the plate is turned round the effect of this 
 habit on the measure is exactly reversed, and 
 hence if we take the mean of the two measures 
 any personal habit of this kind is eliminated. It 
 has been found by experience that such personal 
 habits are much smaller for measures of this kind 
 than for those to which we have long been accus- 
 tomed in observations made by eye on the stars 
 
ACCIDENTAL DISCOVERIES ' fa 5 
 
 themselves. The troubles from " personal equa- 
 tion " have been much diminished by the photo- 
 graphic method, and certain peculiarities of the 
 former method have been clearly exhibited by the 
 comparison. For instance, it has gradually become 
 clear that with eye observations personal equation 
 is not a constant quantity, but is different for 
 stars of different brightness. When observing 
 the transit of a bright star the observer apparently 
 records an instant definitely earlier than in re- 
 cording the transit of a faint one ; and this pecu- 
 liarity seems to be common to the large majority 
 of observers, which is perhaps the reason why it 
 was not noticed earlier. But when positions of 
 the stars determined in this way are compared 
 with their positions measured on the photographic 
 plates, the peculiarity is made clearly manifest. 
 For example, at Oxford, our first business after 
 making measurements is to compare them with 
 visual observations on a limited number of the 
 brighter stars made at Cambridge about twenty 
 years ago. (About 14,000 stars were observed at 
 Cambridge, and we are dealing with ten times 
 that number.) The comparison shows that the 
 Cambridge observations are affected with the 
 following systematic errors : 
 
 If stars of magnitude 10 are observed correctly, 
 then 9 o.io sees, too early 
 
 ,, 8 0.16 
 
 11 11 7 - T 9 . 
 
 ,, 6 0.21 
 
 11 11 5 11 - 2 3 
 
136 ASTRONOMICAL DISCOVERY 
 
 This may serve as an illustration of various 
 incidental results which are already flowing from 
 the enormous and laborious piece of work which, 
 as far as the University Observatory at Oxford is 
 concerned, we have just completed, though some 
 of the other colleagues are not so far advanced. 
 - But the main results will not appear just yet. 
 work? * e The work must be repeated, and the positions of 
 the stars just obtained must be compared with 
 those which they will be found to occupy at some 
 future date, in order to see what kind of changes 
 are going on in the heavens. Whether this future 
 date shall be one hundred years hence, or fifty, or 
 ten, or whether we should begin immediately to 
 repeat what has been done, is a matter not yet 
 decided, and one which requires some little con- 
 sideration. 
 
 I have said perhaps enough to give you a 
 general idea of the work on which we have been 
 engaged at Oxford for the last ten years. Ten 
 years ago it seemed to stretch out in front of us 
 rather hopelessly ; the pace we were able to 
 make seemed so slow in view of the distance to 
 be covered. We felt rather like the schoolboy 
 who has just returned to school and sees the 
 next holidays as a very remote prospect, and we 
 solaced ourselves much in the same way as he 
 does, by making a diagram representing the total 
 number of plates to be dealt with and crossing off 
 each one as it was finished, just as he sometimes 
 crosses off the days still remaining between him 
 
ACCIDENTAL DISCOVERIES 137 
 
 and the prospective holidays. It was pleasant to 
 watch the growth of the number of crosses on this 
 diagram, and by the end of the year 1902 we had 
 the satisfaction of seeing very little blank space The con- 
 remaining. Now, up to this point it had not 
 much mattered whether any particular plate was 
 secured in any particular year, or in a subsequent 
 year, so long as there were always sufficient plates 
 to keep us occupied in measuring them. But it 
 then became a matter of importance to secure each 
 plate at the proper time of year; for the sun, as 
 we know, travels round the Zodiac among the 
 stars, obliterating by his radiance a large section 
 of the sky for a period of some months, and in 
 this way a particular region of the heavens is apt 
 to "run into daylight," as the observatory phrase 
 goes, and ceases to be available for photography 
 during several months, until the sun is again far 
 enough away to allow of the particular region 
 being seen at night. 
 
 Roughly speaking then, if a plate which should 
 be taken in February is not secured in this month 
 owing to bad weather, the proper time for taking 
 it will not occur again until the following 
 February ; and when there was a fair prospect of 
 finishing our work in 1903, it became important 
 to secure each plate at the proper time in that 
 year. Hence we were making special efforts to 
 utilise to the full any fine night that Providence 
 sent in our way, and on such occasions it is clearly 
 an economy, if not exactly to "make hay while 
 the sun shines," at any rate to take plates 
 
1 3 8 ASTRONOMICAL DISCOVERY 
 
 vigorously while the sun is not shining and the 
 night is fine ; leaving the development of them 
 until the daytime. There is, of course, the risk 
 that the whole night's work may in this way be 
 lost owing to some fault in the plates, which 
 might have been detected if some of them were 
 immediately developed. Perhaps in the early days 
 of our work it would have been reckless or foolish 
 to neglect this little precaution ; but we had for 
 years been accustomed to rely upon the excellence 
 of the plates without finding our trust betrayed ; 
 and the sensitiveness of the plates had increased 
 A disap- rather than diminished as time went on. Hence 
 it will be readily understood that when one fatal 
 morning we developed a series of some thirty 
 plates, and found that owing to some unexplained 
 lack of sensitiveness they were all unsuitable for 
 our purpose, it came as a most unwelcome and 
 startling surprise. It was, of course, necessary to 
 make certain that there was no oversight, that the 
 developer was not at fault, and that the weather 
 had not been treacherous. All such possibilities 
 were carefully considered before communication 
 with the makers of the plates, but it ultimately 
 became clear that there had been some unfortunate 
 failure in sensitiveness, and that it would be 
 necessary to repeat the work with opportunities 
 restricted by the intervening lapse of time. How- 
 ever, disappointments from this or similar causes 
 are not unknown in astronomical work ; and we set 
 about this repetition with as little loss of time and 
 cheerfulness as was possible. Under the circum- 
 
ACCIDENTAL DISCOVERIES 139 
 
 stances, however, it seemed desirable to examine 
 carefully whether anything could be saved from 
 the wreck whether any of the plates could be 
 admitted as just coming up to the minimum 
 requirements. And I devoted a morning to this 
 inquiry. In the course of it I came across one A curious 
 plate which certainly seemed worth an inclusion p a 
 among our series from the point of view of the 
 number of stars shown upon it. It seemed quite 
 rich in stars, perhaps even a little richer than 
 might have been expected. On inquiry I was 
 told that this was not one of the originally con- 
 demned plates, but one which had been taken 
 since the failure in sensitiveness of the plates 
 had been detected ; was from a new and specially 
 sensitive batch with which the courteous makers 
 had supplied us ; but though there were cer- 
 tainly a sufficient number of stars upon the plate, 
 owing to some unexplained cause the telescope 
 had been erroneously pointed, and the region 
 taken did not correspond to the region required. 
 To investigate the cause of the discrepancy I 
 thereupon took down from our store of plates the 
 other one of the same region which had been 
 rejected for insufficiency of stars, and on comparing 
 the two it was at once evident that there was a 
 strange object on the plate taken later of the two, Astrauge 
 a bright star or other heavenly body, which was ]ec 
 not on the former plate. I have explained that 
 by repeating the exposure more than once, it is 
 easily possible to recognise whether a mark upon 
 the plate is really a celestial body or is an ac- 
 
i 4 o ASTRONOMICAL DISCOVERY 
 
 cidental blot or dust speck, and there was no 
 doubt that this was the image of some strange 
 celestial body. It might, of course, be a new 
 planet, or even an old one which had wandered 
 into the region ; but a few measures soon showed 
 that it was not in movement. The measures con- 
 sisted in comparing the separation of the three 
 exposures with the separation of the corresponding 
 exposures of obvious stars, for the exposures were 
 not, of course, simultaneous, and if the body were 
 a planet and had moved in the interval between 
 them, this would be made manifest on measuring 
 the separations. No such movements could be 
 detected ; and the possibilities were thus restricted 
 to two. So far as we knew the object was a star, 
 but might be either a star of the class known as 
 variable or of that known as new. In the former 
 case it would become bright and faint at more or 
 less regular intervals, and might possibly have 
 been already catalogued ; for the number of these 
 bodies already known amounts to some hundreds. 
 Search being made in the catalogues, no entry of it 
 was found, though it still might be one of this 
 class which had hitherto escaped detection. Or it 
 A new might be a " new star," one of those curious bodies 
 which blaze up quite suddenly to brightness and 
 then die away gradually until they become practi- 
 cally invisible. The most famous perhaps of these 
 is the star which appeared in 1572, and was so 
 carefully observed by Tycho Brahe ; but such 
 apparitions are rare, and altogether we have not 
 records as yet of a score altogether; so that in 
 
ACCIDENTAL DISCOVERIES 141 
 
 this latter case the discovery would be of much 
 greater interest than in the former. In either 
 event it was desirable to inform other observers as 
 soon as possible of the existence of a strange 
 body ; already some time had elapsed since the 
 plate had been taken, March i6th, for the ex- 
 amination of which I have spoken was not made 
 until March 24th. Accordingly, a telegram was at 
 once despatched to the Central Office at Kiel, 
 which undertakes to distribute such information 
 all over the world, and a few post-cards were sent 
 to observers close at hand who might be able to 
 observe the star the same night. Certain observa- 
 tions with the spectroscope soon made it clear that 
 the object was really a " new star." 
 
 This, therefore, is the discovery wjiich we 
 made at Oxford : as you will see, in an entirely 
 accidental manner, during the course of a piece of 
 work in which it was certainly never contemplated. 
 Its purely accidental nature is sufficiently illus- The 
 trated by the fact that if the plates originally 
 supplied by the makers had been of the proper 
 quality, the plate which led to the discovery would 
 never have been taken. If the plates exposed in 
 February had been satisfactory, we should have 
 been content, and should not have repeated the 
 exposure on March i6th. Again I can testify per- 
 sonally how purely accidental it was that the ex- 
 amination was made on March 24th to see whether 
 anything could be saved, as I have said, from the 
 wreck. The idea came casually into my mind as 
 I was walking through the room and saw the neat 
 
142 
 
 ASTRONOMICAL DISCOVERY 
 
 pile of rejected plates ; and one may fairly call it 
 an accidental impulse. This new star is not, how- 
 ever, the first of such objects to have been dis- 
 covered "accidentally"; many of the others were 
 found just as much by chance, though a notable 
 exception must be made of those discovered at the 
 Harvard Observatory, which are the result of a 
 deliberate search for such bodies by the careful 
 Mrs. examination of photographic plates. Mrs. Fleming, 
 di s emmg s who spends her life in such work, has had the 
 ies> good fortune to detect no less than six of these 
 wonderful objects as the reward of her laborious 
 scrutiny ; and she is the only person who has thus 
 found new stars by photography until this ac- 
 cidental discovery at Oxford. The following is a 
 complete list of new stars discovered to date : 
 
 LIST OF NEW STARS. 
 
 Ref. No. 
 
 Constellation. 
 
 Year. 
 
 Discoverer. 
 
 ! 
 
 Cassiopeia 
 
 1572 
 
 Tycho Brahe. 
 
 2 
 
 3 
 
 Cygnus . 
 Ophiuchus 
 
 1600 
 1604 
 
 Janson. 
 Kepler. 
 
 4 
 
 Vulpecula 
 
 1670 
 
 Anthelm. 
 
 5 
 
 Ophiuchus 
 
 1848 
 
 Hind. 
 
 6 
 
 Scorpio . 
 
 1860 
 
 Auwers. 
 
 7 
 
 Corona Boreal s 
 
 1866 
 
 Birmingham. 
 
 8 
 
 Cygnus . 
 
 1876 
 
 Schmidt. 
 
 9 
 
 Andromeda 
 
 1885 
 
 Hartwig. 
 
 10 
 
 Perseus . 
 
 1887 . 
 
 Fleming. 
 
 ii 
 
 Auriga . 
 
 1891 
 
 Anderson. 
 
 12 
 
 Norma , 
 
 1893 
 
 Fleming. 
 
 *3 
 
 Carina 
 
 1895 
 
 Fleming. 
 
 14 
 
 Centaurus 
 
 1895 
 
 Fleming. 
 
 15 
 
 Sagittarius 
 
 1898 
 
 Fleming. 
 
 16 
 
 Aquila . 
 
 1899 
 
 Fleming. 
 
 17 
 
 Perseus . 
 
 1901 
 
 Anderson. 
 
 18 
 
 Gemini . 
 
 1903 
 
 At Oxford. 
 
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ACCIDENTAL DISCOVERIES 143 
 
 Generally these stars have been noted by eye Dr. An- 
 observation, as in the case of the two found by d 
 Dr. Anderson of Edinburgh. In these cases also 
 we may say that deliberate search was rewarded ; 
 for Dr. Anderson is probably the most assiduous 
 " watcher of the skies " living, though he seldom 
 uses a telescope ; sometimes he uses an opera- 
 glass, but usually the naked eye. He describes 
 himself as an "Astrophil" rather than as an 
 astronomer. "I love the stars," he says; "and 
 whenever they are shining, I must be looking." 
 And so on every fine night he stands or sits at 
 his open study window gazing at the heavens. I 
 believe he was just about to leave them for his 
 bed, near 3 A.M. on the night of February 21, 
 1901, when, throwing a last glance upward, he 
 suddenly saw a brilliant star in the constellation 
 Perseus. His first feeling was actually one of Nova 
 disappointment, for he felt sure that this object P 
 must have been there for some time past without 
 his knowing of it, and he grudged the time lost 
 when he might have been regarding it. More in 
 a spirit of complaint than of inquiry, he made his 
 way to the Royal Observatory at Edinburgh next 
 day to hear what they had to say about it, 
 though he found it difficult to approach the sub- 
 ject. He first talked about the weather, and the 
 crops, and similar topics of general interest ; and 
 only after some time dared he venture a casual 
 reference to the " new portent in the heavens." 
 Seeing his interlocutor look somewhat blank, he 
 
144 ASTRONOMICAL DISCOVERY 
 
 ventured a little farther, and made a direct refer- 
 ence to the nevv star in Perseus ; and then found 
 to his astonishment, as also to his great delight, 
 that he was the first to bring news of it. The 
 news was soon communicated to other observers ; 
 all the telescopes of the world were soon trained 
 upon it; and this wonderful "new star of the 
 new century " has taught us more of the nature 
 of these extraordinary bodies than all we knew 
 before. 
 Records Perhaps I may add a few remarks on one or 
 
 previous /i IT -n i i 
 
 to dis- two features 01 these bodies. r irstly, let us note 
 that Professor Pickering of Harvard is now able 
 to make a most important contribution to the 
 former history of these objects that is to say, 
 their history preceding their actual detection. 
 We remember that, after Uranus had been dis- 
 covered, it was found that several observers had 
 long before recorded its place unknowingly ; and 
 similarly Professor Pickering and his staff have 
 usually photographed other new objects unknow- 
 ingly. There are on the shelves at Harvard vast 
 stores of photographs, so many that they are 
 unable to examine them when they have been 
 taken; but once any object of interest has been 
 discovered, it is easy to turn over the store and 
 examine the particular plates which may possibly 
 show it at an earlier date. In this way it was 
 found that Dr. Anderson's new star had been 
 visible only for a few days before its discovery, 
 there being no trace of it on earlier plates, Simi- 
 
ACCIDENTAL DISCOVERIES 145 
 
 larly, in the case of the new star found at Oxford, 
 plates taken on March ist and 6th, fifteen days 
 and ten days respectively before the discovery- 
 plate of March i6th, showed the star. But, in 
 this particular instance, greater interest attaches 
 to two still earlier plates taken elsewhere, and 
 with exposures much longer than any available 
 at Harvard. One had been obtained at Heidel- 
 berg by Dr. Max Wolf, and another at the 
 Yerkes Observatory of Chicago University, by Mr. 
 Parkhurst ; and on both there appeared to be a 
 faint star of about the fourteenth or fifteenth mag- 
 nitude, in the place subsequently occupied by the 
 Nova; and the question naturally arose, Was Was Nova 
 this the object which ultimately blazed up and 
 became the new star? To settle this point, it 
 was necessary to measure its position, with refer- faintl y* 
 ence to neighbouring stars, with extreme preci- 
 sion ; and here it was unfortunate that the 
 photographs did not help us as much as they 
 might, for they were scarcely capable of being 
 measured with the requisite precision. The point 
 was an important one, because if the identity of 
 the Nova with this faint star could be established, 
 it would be the second instance of the kind ; but so 
 far as they went, measurements of the photographs 
 were distinctly against the identity. Such was 
 the conclusion of Mr. Parkhurst from his photo- 
 graph alone ; and it was confirmed by measures 
 made at Oxford on copies of both plates, which 
 
 K 
 
146 ASTRONOMICAL DISCOVERY 
 
 were kindly sent there for the purpose. The con- 
 clusion seemed to be that there was a faint star 
 very near, but not at, the place of the new star ; 
 and it was therefore probable that, although this 
 faint star was temporarily invisible from the bright- 
 ness of the adjacent Nova, as the latter became 
 fainter (in the way with which we have become 
 familiar in the case of new stars), it might be 
 The possible to see the two stars alongside each other. 
 
 suspicion :_ . . . 
 
 negatived. Ihis critical observation was ultimately made by 
 the sharp eyes of Professor Barnard, aided by the 
 giant telescope of the Yerkes Observatory ; and it 
 seems clear therefore that the object which blazed 
 up to become the Nova of 1903 could not have 
 previously been so bright as a faint star of the 
 fourteenth magnitude. Although this is merely a 
 negative conclusion, it is an important one in 
 the history of these bodies. 
 
 The second point to which I will draw your 
 
 attention is from the history of the other Nova 
 
 just mentioned Dr. Anderson's New Star of 
 
 1901. In this instance it is not the history pre- 
 
 vious to discovery, but what followed many months 
 
 after discovery, that was of engrossing interest ; 
 
 and again Yerkes Observatory, with its magnificent 
 
 equipment, played an important part in the drama. 
 
 When, with its giant reflecting telescope, photo- 
 
 Nebula graphs were taken of the region of Nova Persei 
 
 Nova after it had become comparatively faint, it was 
 
 sel< found that there was an extraordinarily faint nebu- 
 
 losity surrounding the star.* Repeating the photo- 
 
SEPT. 20, IQ01. 
 
 NOV. 13, 1901. 
 
 IX. NEBULOSITY ROUND NOVA PERSEI. 
 
 (From photographs taken at the Yerkcs Observatory by G. W. Ritchey 
 
>' or THE 
 
 UNIVERSITY 
 
ACCIDENTAL DISCOVERIES 147 
 
 graphs at intervals, it was found that this nebulosity its 
 was rapidly changing in shape. " Rapidly" is, of changes< 
 course, a relative term, and a casual inspection 
 of two of the photographs might not convey any 
 impression of rapidity ; it is only when we come 
 to consider the enormous distance at which the 
 movements, or apparent movements, of the nebulae 
 must be taking place that it becomes clear how 
 rapid the changes must be. It was not possible 
 to determine this distance with any exactness, 
 but limits to it could be set, and it seemed pro- 
 bable that the velocity of the movement was 
 comparable with that of light. The conclusion 
 suggested itself that the velocity might actually 
 be identical with that of light, in which case what 
 we saw was not the movement of actual matter, Due to 
 but merely that of illumination, travelling from 
 point to point of matter already existing. tion - 
 
 An analogy from the familiar case of sound 
 may make clearer what is meant. If a loud noise 
 is made in a large hall, we hear echoes from the 
 walls. The sound travels with a velocity of about 
 1 100 feet per second, reaches the walls, is re- 
 flected back from them, and returns to us with 
 the same velocity. From the interval occupied 
 in going and returning we could calculate the 
 distance of the walls. The velocity of light is so 
 enormous compared with that of sound that we 
 are usually quite unable to observe any similar 
 phenomenon in the case of light. If we strike a 
 match in the largest hall, all parts of it are 
 
148 ASTRONOMICAL DISCOVERY 
 
 illuminated so immediately that we cannot pos- 
 sibly realise that there was really an interval 
 between the striking of the match, the travelling 
 of the light to the walls, and its return to our 
 eyes. The scale of our terrestrial phenomenon is 
 far too small to render this interval perceptible. 
 But those who accept the theory above mentioned 
 regarding the appearances round Nova Persei 
 (although there are some who discredit it) believe 
 that we have in this case an illustration of just 
 this phenomenon of light echoes, on a scale large 
 enough to be easily visible. They think that, 
 surrounding the central star which blazed up so 
 brightly in February 1901, there was a vast dark 
 nebula, of which we had no previous knowledge, 
 because it was not shining with any light of its 
 own. When the star blazed up, the illumination 
 travelled from point to point of this dark nebula 
 and lighted it up ; but the size of the nebula was 
 so vast that, although the light was travelling with 
 the enormous velocity of 200,000 miles per second, 
 it was not until months afterwards that it reached 
 different portions of this nebula ; and we accord- 
 ingly got news of the existence of this nebula 
 some months after the news reached us of the 
 When did central conflagration, whatever it was. Eemark 
 happen? that all we can say is that the news of the nebula 
 reached us some months later than that of the 
 outburst. The actual date when either of the 
 actual things happened, we have as yet no means 
 of knowing ; it may have been hundreds or even 
 
ACCIDENTAL DISCOVERIES 149 
 
 thousands of years ago that the conflagration 
 actually occurred of which we got news in Feb- 
 ruary 1901, the light having taken all that time 
 to reach us from that distant part of space ; and 
 the light reflected from the nebula was following 
 it on its way to us all these years at that same 
 interval of a few months. 
 
 Now, let me refer before leaving this point to An objec- 
 the chief objection which has been urged against ta ' 
 this theory. It has been maintained that the 
 illumination would necessarily appear to travel 
 outwards from the centre with an approach to 
 uniformity, whereas the observed rate of travel is 
 not uniform, and has been even towards the 
 centre instead of away from it ; which would 
 seem as though portions of the nebula more 
 distant from the centre were lighted up sooner 
 than those closer to it. By a simple illustration 
 from our solar system, we shall see that these 
 curious anomalies may easily be explained. Let 
 us consider for simplicity two planets only, say 
 the Earth and Saturn. We know that Saturn 
 travels round the sun in an orbit which is ten 
 times larger than the orbit of the earth. Suppose 
 now that the sun were suddenly to be extinguished ; 
 light takes about eight minutes to travel from 
 the sun to the earth, and consequently we should 
 not get news of the extinction for some eight 
 minutes ; the sun would appear to us to still go on 
 shining for eight minutes after he had really been 
 extinguished. Saturn being about ten times as 
 
ISO ASTRONOMICAL DISCOVERY 
 
 far away from the sun, the news would take 
 eighty minutes to reach Saturn ; and from the earth 
 we should see Saturn shining more 1 than eighty 
 minutes after the sun had been extinguished, 
 although we ourselves should have lost the sun's 
 light after eight minutes. I think we already 
 begin to see possibilities of curious anomalies ; 
 but they can be made clearer than this. Instead 
 of imagining an observer on the earth, let us 
 suppose him removed to a great distance away 
 in the plane of the two orbits ; and let us sup- 
 pose that the sun is now lighted up again as 
 suddenly as the new star blazed up in February 
 1901. Then such an observer would first see this 
 blaze in the centre ; eight minutes afterwards the 
 illumination would reach the earth, a little speck 
 of light near the sun would be illuminated, just 
 as we saw a portion of the dark nebula round 
 Nova Persei illuminated ; eighty minutes later 
 another speck, namely, Saturn, would begin to 
 shine. But now, would Saturn necessarily appear 
 to the distant observer to be farther away from 
 the sun than the earth was? Looking at the 
 diagram, we can see that if Saturn were at S 1? then 
 it would present this natural appearance of being 
 farther away from the sun than the earth ; but it 
 might be at S 2 or S 3 , in which case it would seem 
 to be nearer the sun, and the illumination would 
 seem to travel inwards towards the central body 
 
 1 Since the light must travel from the sun to Saturn and back 
 again to the earth, the interval would be more nearly 150 minutes. 
 
ACCIDENTAL DISCOVERIES 151 
 
 instead of outwards. Without considering other 
 cases in detail, it will be tolerably clear that almost 
 any anomalous appearance might be explained by 
 choosing a suitable arrangement of the nebulous 
 matter which we suppose lighted up by the ex- 
 plosion of Nova Persei. Another objection urged 
 against the theory I have sketched is that the 
 light reflected from such a nebula would be so 
 feeble that it would not affect our photographic 
 
 plates. This depends upon various assumptions 
 which we have no time to notice here ; but I 
 think we may say that there is certainly room for 
 the acceptance of the theory. 
 
 Now, if this dark nebula was previously existing Did the 
 in this way all round the star which blazed up, cause^he 
 the question naturally arises whether the nebula outbl 
 had anything to do with the conflagration. Was 
 there previously a star, either so cold or so distant 
 as not to be shining with appreciable light, which, 
 travelling through space, encountered this vast 
 
152 ASTRONOMICAL DISCOVERY 
 
 nebula, and by the friction of the encounter was 
 suddenly rendered so luminous as to outshine a 
 star of the first magnitude ? The case of meteoric 
 stones striking our own atmosphere seems to 
 suggest such a possibility. These little stones 
 are previously quite cold and invisible, and are 
 travelling in some way through space, many of 
 them probably circling round our sun. If they 
 happen in their journey to encounter our earth, 
 even the extremely tenuous atmosphere, so thin 
 that it will scarcely bend the rays of light 
 appreciably, even this is sufficient by its friction 
 to raise the stones to a white heat, so that they 
 blaze up into the falling stars with which we are 
 familiar. This analogy is suggested, but we 
 must be cautious in accepting it; for we know 
 so very little of the nature of nebula? such as 
 that of which we have been speaking. But in 
 any case, a totally new series of phenomena have 
 been laid open to our study by those wonderful 
 photographs taken at the Yerkes Observatory and 
 the Lick Observatory in the few years which the 
 present century has as yet run. 
 
 impor- One thing is quite certain : we must lose no 
 opportunity of studying such stars as may appear, 
 and no diligence spent in discovering them at the 
 earliest possible moment is thrown away. We 
 have only known up to the present, as already 
 stated, less than a score of them, and of these 
 many have told us but little ; partly because they 
 were only discovered too late (after they had 
 
ACCIDENTAL DISCOVERIES 153 
 
 become faint), and partly because the earlier ones 
 could not be observed with the spectroscope, 
 which had not then been invented. It seems 
 clear that in the future we must not allow acci- 
 dent to play so large a part in the discovery of 
 these objects ; more must be done in the way of 
 deliberate search. Although we know beforehand 
 that this will involve a vast amount of apparently 
 useless labour, that months and years must be 
 spent in comparing photographic plates, or por- 
 tions of the sky itself, with one another without 
 detecting anything remarkable, it will not be the 
 first time that years have been cheerfully spent in 
 such searches without result. We need only 
 recall Hencke's fifteen years of fruitless search, 
 before finding a minor planet, to realise this fact. 
 
 One thing of importance may be done ; we 
 may improve our methods of making the search, 
 so as to economise labour, and several suc- 
 cessful attempts have already been made in this 
 direction. The simplest plan is to superpose two Super- 
 photographs taken at different dates, so that the of plates, 
 stars on one lie very close to those on the other ; 
 then if an image is seen to be unpaired we may 
 have found a new star, though of course the object 
 may be merely a planet or a variable. The super- 
 position of the plates may be either actual or 
 virtual. A beautiful instrument has been devised 
 on the principle of the stereoscope for examining 
 two plates placed side by side, one with each eye. 
 We know that in this way two photographs of 
 
154 ASTRONOMICAL DISCOVERY 
 
 the same object from different points of view will 
 appear to coalesce, and at the same time to give 
 an appearance of solidity to the object or land- 
 scape, portions of which will seem to stand out in 
 The front of the background. Applying this principle 
 compara- to two photographs of stars, what happens is this : 
 if the stars have all remained in the same posi- 
 tions exactly, the two pictures will seem to us to 
 coalesce, and the images all to lie on a flat 
 background ; but if in the interval between the 
 exposures of the two plates one of the stars has 
 appreciably moved or disappeared, it will seem, 
 when looked at with this instrument, to stand out 
 in front of this background, and is accordingly 
 detected with comparatively little trouble. This 
 new instrument, to which the name Stereo-com- 
 parator has been given, promises to be of immense 
 value in dredging the sky for strange bodies in 
 the future. I am glad to say that a generous 
 friend has kindly presented the University Obser- 
 vatory at Oxford with one of these beautiful instru- 
 ments, which have been constructed by Messrs. 
 Zeiss of Jena after the skilful designs of Dr. 
 Pulfrich. Whether we shall be able to repeat by 
 deliberate search the success which mere accident 
 threw in our way remains to be seen. 
 
CHAPTER V 
 
 SCHWABE AND THE SUN-SPOT PERIOD 
 
 IN preceding chapters we have reviewed dis- Discove- 
 coveries, some of which have been made as a tiary to 
 
 result of a deliberate search, and others acciden- l 
 tally in the course of work directed to a totally 
 different end ; but so far we have not considered 
 a case in which the discoverer entered upon an 
 enterprise from which he was positively dissuaded. 
 In the next chapter we shall come across a 
 very striking instance of this type ; but even 
 in the discovery that there was a periodicity in 
 the solar spots, with which I propose to deal 
 now, Herr Schwabe began his work in the face 
 of deterrent opinions from eminent men. His 
 definite announcement was first made in 1843, 
 though he had himself been convinced some years 
 earlier. In 1857 the Royal Astronomical Society 
 awarded him their gold medal for the discovery ; 
 and in the address delivered on the occasion 
 the President commenced by drawing atten- 
 tion to this very fact, that astronomers who had 
 expressed any opinions on the subject had been 
 uniformly and decidedly against the likelihood Nothing 
 of there being anything profitable in the study f^ 
 of the solar spots. I will quote the exact words spots ' 
 
 155 
 
156 ASTRONOMICAL DISCOVERY 
 
 of the President, Mr. Manuel Johnson, then Rad- 
 clifie Observer at Oxford. 
 
 "It was in 1826 that Heinrich Schwabe, a 
 gentleman resident in Dessau, entered upon those 
 researches which are now to engage our attention. 
 I am not aware of the motive that induced him 
 whether any particular views had suggested them- 
 selves to his own mind or whether it was a 
 general desire of investigating, more thoroughly 
 than his predecessors had done, the laws of a 
 remarkable phenomenon, which it had long been 
 the fashion to neglect. He could hardly have 
 anticipated the kind of result at which he has 
 arrived ; at the same time we cannot imagine a 
 course of proceeding better calculated for its 
 detection, even if his mind had been prepared 
 for it, than that which he has pursued from the 
 very commencement of his career. Assuredly 
 if he entertained such an idea, it was not borrowed 
 from the authorities of the last century, to whom 
 the solar spots were objects of more attention 
 than they have been of late years. 
 
 " ' Nulla constanti temporum lege apparent aut 
 evanescunt,' says Keill in 1739. Introduct. ad 
 Physic. Astronom., p. 253. 
 
 " ' II est manifest par ce que nous venons de 
 rapporter qu'il n'y a point de regie certaine de 
 leur formation, ni de leur nombre et de leur 
 figure,' says Cassini II. in 1740. EUm d'Astron., 
 vol. i. p. 82. 
 
SCHWABE AND THE SUN-SPOT PERIOD 157 
 
 " ' II semble qu'elles ne suivent aucune loi dans 
 leur apparitions/ says Le Monnier in 1746. 
 Instit. Astron., p. 83. 
 
 " ' Solar spots observe no regularity in their shape, 
 magnitude, number, or in the time of their appear- 
 ance or continuance/ says Long in 1 764. Astron., 
 vol. ii. p. 472. 
 
 " ' Les apparitions des taches du soleil n'ont 
 rien de regulier/ says Lalande in 1771. Astron., 
 vol. iii. 3131, 2nd edit. 
 
 " And Delambre's opinion may be inferred from 
 a well-known passage in the third volume of his 
 'Astronomy' (p. 20), published in 1814, where treat- 
 ing of the solar spots he says, * II est vrai qu'elles 
 sont plus curieuses que vraiment utiles.'" 1 
 
 I* 
 
 It will thus be evident that Herr Schwabe had 
 the courage to enter upon a line of investigation 
 which others had practically condemned as likely 
 to lead nowhere, and that his discovery was quite 
 contrary to expectation. It is a lesson to us that 
 not even the most unlikely line of work is to be 
 despised ; for the outcome of Schwabe's work was 
 the first step in the whole series of discoveries 
 which have gradually built up the modern science 
 of Solar Physics, which occupies so deservedly large 
 a part of the energies of, for instance, the great 
 observatory attached to the University of Chicago. 
 
 It has been our practice to recall the actual 
 
 1 Monthly Notices of the Royal Astronomical Society, vol. xvii. 
 p. 126. 
 
158 ASTRONOMICAL DISCOVERY 
 
 Schwabe's words in which the discoverer himself stated his 
 discovery, and I will give the original modest 
 announcement of Schwabe, though for convenience 
 of those who do not read German I will attempt 
 a rough translation. He had communicated year 
 by year the results of his daily counting of the 
 solar spots to the Astronomische Nachrichten, 
 and after he had given ten years' results in this 
 way he collected them together, but he made 
 no remark on the curious sequence which they 
 undoubtedly showed at that time. Waiting 
 patiently six years for further material, in 1843 
 he ventured to make his definite announcement 
 as follows : " From my earlier observations, 
 which I have communicated annually to this 
 journal, there was manifest already a certain 
 periodicity of sun-spots ; and the probability of 
 this being really the case is confirmed by this 
 year's results. Although I gave in volume 15 
 the total numbers of groups for the years 
 1826-1837, nevertheless I will repeat here a 
 complete series of all my observations of sun- 
 spots, giving not only the number of groups, 
 but also the number of days of observation, and 
 further the days when the sun was free from 
 spots. The number of groups alone will not in 
 itself give 'sufficient accuracy for determination of 
 a period, since 1 have convinced myself that when 
 there are a large number of sun-spots the number 
 will be reckoned somewhat too small, and when 
 few sun- spots, the number somewhat too large ; 
 
FEB. l8, 1894. 
 
 FEB. IQ, 1094. 
 
 X. PHOTOGRAPHS OF THE Sux TAKEN AT THE ROYAL OBSERVATORY 
 GREENWICH, SHEWING SUNSPOTS, 
 
f UNIVERSITY 
 
FEB. 20, 1894. 
 
 FEB. 21, 1894. 
 
 XI. PHOTOGRAPHS ov THE SUN TAKEN AT THE ROYAL OBSERVATORY, 
 GREENWICH, SHEWING SUNSPOTS 
 
Jf THF \ 
 
 UNIVERSITY 
 
 OF 
 
SCHWABE AND THE SUN-SPOT PERIOD 159 
 
 in the first case several groups are often counted 
 together in one, and in the second it is easy to 
 divide a group made up of two component parts 
 into two separate groups. This must be my 
 excuse for repeating the early catalogue, as 
 follows : 
 
 Year. 
 
 Number of 
 Groups. 
 
 Days free 
 from Spots. 
 
 Days of 
 Observation. 
 
 1826 
 1827 
 1828 
 1829 
 1830 
 
 118 
 161 
 225 
 199 
 190 
 
 22 
 2 
 
 
 I 
 
 277 
 
 273 
 282 
 244 
 217 
 
 1831 
 1832 
 1833 
 1834 
 1835 
 
 149 
 84 
 
 33 
 51 
 173 
 
 3 
 49 
 139 
 
 120 
 
 18 
 
 239 
 
 270 
 267 
 
 273 
 244 
 
 1836 
 
 1837 
 1838 
 
 1839 
 1840 
 
 272 
 
 333 
 282 
 162 
 152 
 
 o 
 
 
 
 
 o 
 3 
 
 200 
 
 1 68 
 
 202 
 20 5 
 263 
 
 1841 
 1842 
 
 1843 
 (1844) 
 
 1 02 
 
 68 
 
 ft 
 
 15 
 
 64 
 149 
 (in) 
 
 283 
 307 
 
 324 
 (320) 
 
 " If we now compare together the number of 
 groups, and the days free from spots, we find that 
 the sun-spots have a period of about ten years, 
 and that for about five years they are so numerous 
 that during this period few days, if any, are free 
 from spots. The sequel must show whether this 
 period is constant, whether the minimum activity 
 
160 ASTRONOMICAL DISCOVERY 
 
 of the sun in producing spots lasts for one or two 
 years, and whether this activity increases more 
 quickly than it decreases." 
 
 Attracted This brief announcement is all that the dis- 
 tention, coverer says upon the subject ; and it is perhaps 
 not remarkable that it attracted very little atten- 
 tion, especially when we remember that it related 
 to a matter which the astronomical world had 
 agreed to put aside as unprofitable and not worth 
 attention. Next year, in giving his usual paper 
 on the spots for 1844 he recurs to the subject in 
 the following sentence : " The periodicity of spots 
 of about ten years which was indicated in my 
 summary published last year, is confirmed by this 
 year's observations." I have added in brackets 
 to the table above reproduced the numbers for 
 1 844 subsequently given, and it will be seen how 
 nearly they might have been predicted. 
 
 Still the subject attracted little attention. 
 Turning over the leaves of the journal at random, 
 I came across the annual report of the Astronomer 
 Royal of England, printed at length. But in it 
 there is no reference to this discovery, which 
 opened up a line of work now strongly repre- 
 sented in the annual programme of the Royal 
 Observatory at Greenwich. Mr. Johnson remarks 
 that the only person who had taken it up was 
 Julius Schmidt, who then resided near Hamburg. 
 
 until 
 
 eight But Schwabe went on patiently accumulating 
 
 fater! facts ; and in 1851 the great Von Humboldt in 
 
 the third volume of his Cosmos, drew attention to 
 
SCHWABE AND THE SUN-SPOT PERIOD 161 
 
 the discovery, which was accordiDgly for the first 
 time brought into general notice. It will be seen 
 that there are not many facts of general interest 
 relating to the actual discovery beyond the courage 
 with which the work was commenced in a totally 
 unpromising direction, and the scant attention it 
 received after being made for us. We may admit 
 that interest centres chiefly in the tremendous 
 consequences which flowed from it. We now 
 recognise that many other phenomena are bound 
 up with this waxing and waning of the solar spots. 
 We might be prepared for a sympathy in pheno- other 
 mena obviously connected with the sun itself; but 
 it was an unexpected and startling discovery that 
 magnetic phenomena on the earth had also a 
 sympathetic relation with the changes in, sun- 
 spots, and it is perhaps not surprising that when 
 once this connection of solar and terrestrial pheno- 
 mena was realised, various attempts have been 
 made to extend it into regions where we cannot as 
 yet allow that it has earned a legitimate right of 
 entry. We have heard of the weather and of 
 Indian famines occurring in cycles identical with 
 the sun-spot cycle ; and it is obvious how tremend- 
 ously important it would be for us if this were 
 found to be actually the case. For we might in 
 this way predict years of possible famine and 
 guard against them ; or if we could even partially 
 foretell the kind of weather likely to occur some 
 years hence, we might take agricultural measures 
 accordingly. The importance of the connection, 
 
1 62 ASTRONOMICAL DISCOVERY 
 
 if only it could be established, is no doubt the 
 reason which has misled investigators into laying 
 undue stress on evidence which will not bear 
 and others close scrutiny. For the present we must say 
 decidedly that no case has been made out for 
 paying serious attention to the influence of sun- 
 spots on weather. Nevertheless, putting all this 
 aside, there is quite enough of first-rate import- 
 ance in the sequel to Schwabe's discovery. 
 
 Let us review the facts in order. Most of us, 
 though we may not have had the advantage of 
 seeing an actual sun-spot through a telescope, 
 have seen drawings or photographs of spots. 
 There is a famous drawing made by James 
 Nasmyth (of steam-hammer fame), in July, 1864, 
 which is of particular interest, because at that 
 time Nasmyth was convinced and he convinced 
 many others with him that the solar surface was 
 made up of a miscellaneous heap of solid bodies,, 
 in shape like willow leaves, or grains of rice, 
 thrown together almost at random, and the draw- 
 ing was made by him to illustrate this idea. Com- 
 paring a modern photograph with it, we see that 
 there is something to be said for Nasmyth's view, 
 which attracted much attention at the time and 
 occasioned a somewhat heated controversy. But 
 since the invention of the spectroscope it has 
 become quite obsolete; it probably does not 
 Green- correspond in any way to the real facts. But 
 records, instead of looking at pictures which have been 
 enlarged to show the detailed structure in and 
 
SCHWABE AND THE SUN-SPOT PERIOD 163 
 
 near a spot, we will look at a series of pictures 
 of the whole sun taken on successive days at 
 Greenwich in which the spots are necessarily much 
 smaller, but which show the behaviour of the spots 
 from day to day. (See Plates X. and XI.) From 
 the date at the foot of each it will be seen that 
 they gradually cross the disc of the sun (a fact 
 first discovered by Galileo in 1610), showing that 
 the sun rotates on an axis once in about every The sun's 
 
 r> i mi , , rotation. 
 
 twenty-five days. ihere are many interesting 
 facts connected with this rotation ; especially 
 that the sun does not rotate as a solid body, 
 the parts near the (Sun's) Equator flowing quicker 
 than those nearer the Poles ; but for the present 
 we cannot stop to dwell upon them. What 
 interests us particularly is the history, not from 
 day to day, but from year to year, as Schwabe has 
 already given it for a series of years. 
 
 When it became generally established that this Wolfs 
 periodicity existed, Rudolf Wolf of Zurich col- n 
 lected the facts about sun-spots from the earliest 
 possible date, and represented this history by a 
 series of numbers which are still called Wolf's 
 Sun-Spot Numbers. You will see from the dia- 
 gram the obvious rise and fall for eleven years, 
 not ten years, as Schwabe thought, but just a little 
 over eleven years. The facts are, however, given 
 more completely by the work done at the Eoyal 
 Observatory at Greenwich. It is part of the 
 regular daily work of that Observatory to photo- 
 graph the sun at least twice. Many days are of 
 
1 6 4 ASTRONOMICAL DISCOVERY 
 
 course cloudy or wet, so that photographs cannot 
 be obtained ; but there are available photographs 
 similarly taken in India or in Mauritius, where 
 the weather is more favourable, and from these 
 the gaps are so well filled up that very few days, 
 if any, during the whole year are left without 
 Green- some photograph of the sun's surface. On these 
 areas. photographs the positions and the areas of the 
 spots are carefully measured under a microscope, 
 and the results when submitted to certain neces-t 
 sary calculations are published year by year. It is 
 clearly a more accurate estimate of the spottedness 
 of the sun to take the total area of all the spots 
 rather than their mere number, for in the latter case- 
 a large spot and a small one count equally. Hence 
 the Greenwich records will perhaps give us an 
 even better idea of the periodicity than Wolfs 
 numbers. Now, at the same observatory magnetic 
 observations are also made continuously. If a 
 magnet be suspended freely we are accustomed to 1 
 say that it will point to the North Pole ; but this 
 is only very roughly true. In the first placej it is' 
 probably well known to you that there is a con- 
 siderable deviation from due north owing to the 
 fact that the magnetic North Pole is not the same 
 as the geographical North Pole ; but this for the 
 Magnetic present need not concern us. What does concern 
 Sons 1 *" us i g > that if the nee dle is hung up and left long 
 enough to come to rest, it does not then remain 
 steadily at rest, but executes slow and small 
 oscillations backwards and forwards, up and down, 
 
NUMBER OF SUNSPOTS (Wolf) 
 
 DAILY RANGE <f MAGNETIC DECLINATION 
 
 DAILY RANCXE of MAGNET/ C HOR2.L, FORCE 
 observed at QreenuricK) 
 
 1840 50 60 yp 80 90 
 
 |l"' r flfirffrffff|ffii|iiii|t,|,| im | llvl | lll| | ||M 
 
 HF 
 
 X 
 100 
 
 6'- 
 
 LS& 
 
 IOO 
 
 HORIZONTAL FORCE 
 
 PLATE XII. 
 
GREENWICH MAGNETIC CURVES 
 
 1859-60 
 
 CRENWICH MAGNETIC CURVES FOR APRIL 
 
SCHWABE AND THE SUN-SPOT PERIOD 165 
 
 throughout the day ; repeating nearly the same 
 oscillations on the following day, but at the same 
 time gradually changing its behaviour so as to 
 oscillate differently in summer and winter. These 
 changes are very small, and would pass unnoticed 
 by the naked eye ; but when carefully watched 
 through a telescope, or better still, when photo- 
 graphed by some apparatus which will at the same 
 time magnify them, they can be rendered easily 
 visible. When the history of these changes is 
 traced it is seen at once that there is a manifest 
 connection with the cycle of sun-spot changes ; for 
 instance, if we measure the range of swing back- 
 wards and forwards during the day and take 
 the average for all the days in the year, and then 
 compare this with the average number of sun- 
 spots, we shall see that the averages rise and fall 
 together. Similarly we may take the up and 
 down swing, find the average amount of it 
 throughout the year, and again we shall find that 
 this corresponds very closely with the average 
 number of sun-spots. 
 
 But perhaps the most striking way to exhibit 
 the sympathy is to combine different variations of 
 the needle into one picture. And first we must 
 remark that there is another important variation 
 of the earth's magnetic action which we have not 
 yet considered. We have mentioned the swing of 
 the needle to and fro, and the swing up and down, 
 and these correspond to changes in the direction 
 of the force of attraction on the needle. But 
 
1 66 ASTRONOMICAL DISCOVERY 
 
 there may be also changes in intensity of this 
 action ; the pull may be a little stronger or a 
 little weaker than before, and these variations are 
 not represented by any actual movement of the 
 needle, though they can be measured by proper 
 experiments. We can, however, imagine them 
 represented by a movement of the end of the 
 needle if we suppose it made of elastic material, so 
 that it would lengthen when the force was greater 
 Daily and contract slightly when the force was less. If 
 a pencil were attached to the end of such an 
 elastic needle so as to make a mark on a sheet of 
 paper, and if for a moment we exclude the up 
 and down movements, the pencil would describe 
 during the day a curve on the paper, as the end 
 of the needle swung backwards and forwards with 
 the change in direction, and moved across the 
 direction of swing with the change in intensity. 
 Now when curves of this kind are described for a 
 day in each month of the year, there is a striking 
 difference between the forms of them. During 
 Difference the summer months they are, generally speaking, 
 summer comparatively large and open, and during the 
 and win- w j n t e r months they are small and close. This 
 change in form is seen by a glance at Plate XIII. , 
 which gives the curves throughout the whole of 
 one year. Let us now, instead of forming a curve 
 of this kind for each month, form a single average 
 curve for the whole year ; and let us further do 
 this for a series of years. (Plate XIV.) We 
 see that the curves change from year to year in a 
 
SCHWABE AND THE SUN-SPOT PERIOD 167 
 
 manner very similar to that in which they change 
 from month to month in any particular year, and 
 the law of change is such that in years when there andbe- 
 
 J tween 
 
 are many sun-spots we get a large open curve sun-spot 
 similar to those found in the summer, while for and mini 
 years when there are few sun-spots we get small mum ' 
 close curves very like those in the winter. Hence 
 we have two definite conclusions suggested : firstly, 
 that the changes of force are sympathetic with 
 the changes in the sun-spots ; and secondly, that 
 times of maximum sun-spots correspond to summer, 
 and times of minimum to winter. And here I must 
 admit that this is about as far as we have got at 
 present in the investigation of this relationship. Cause un- 
 Why the needle behaves in this way we have as 
 yet only the very vaguest ideas ; suggestions of 
 different kinds have certainly been put forward, 
 but none of them as yet can be said to have much 
 evidence in favour of its being the true one. For 
 our present purpose, however, it is sufficient to 
 note that there is this very real connection, and 
 that consequently Schwabe's sun-spot period may 
 have a very real importance with regard to changes 
 in our earth itself. 
 
 But I may perhaps repeat the word of caution 
 already uttered against extending without suf- 
 ficient evidence this notion of the influence of 
 sun-spots to other phenomena, such as weather. 
 A simple illustration will perhaps serve better 
 than a long argument to show both the way in 
 which mistakes have been made and the way in 
 
1 68 ASTRONOMICAL DISCOVERY 
 
 which they can be seen to be mistakes. There 
 
 is at the Royal Observatory at Greenwich an in- 
 
 strument for noting the direction of the wind, 
 
 the essential part being an ordinary wind-vane, 
 
 the movements of which are automatically re- 
 
 niustra- corded on a sheet of paper. As the wind shifts 
 
 spurious from north to east the pencil moves in one 
 
 160 direction, and when it shifts back aain towards 
 
 
 the north the pencil moves in the reverse way. 
 But sometimes the wind shifts continuously from 
 north to east, south, west, v and back to north 
 again, the vane making a complete revolution ; 
 and this causes the pencil to move continuously 
 in one direction, until when the vane has come to 
 north again, the pencil is far away from the con- 
 venient place of record ; on such occasions it is 
 often necessary to replace it by hand. Then 
 again, the vane may turn in the opposite direction, 
 sending the pencil inconveniently to the other 
 side of the record. During the year it is easy to 
 count the number of complete changes of wind in 
 either direction, and subtracting one number from 
 the other, we get the excess of complete revolutions 
 of the vane in one direction over that in the other. 
 Now if these rather arbitrary numbers are set 
 down year by year, or plotted in the shape of a 
 diagram, we get a curve which may be compared 
 with the sun-spot curve, and during a period of 
 no less than sixteen years from 1858 to 1874 
 there was a remarkable similarity between the two 
 diagrams. From this evidence alone it might 
 
SCHWABE AND THE SUN-SPOT PERIOD 169 
 
 fairly be inferred that the sun-spots had some 
 curious effect upon the weather at Greenwich, 
 traceable in this extraordinary way in the changes 
 of the wind. But the particular way in which 
 these changes are recorded is so arbitrary that we 
 should naturally feel surprise if there was a real 
 connection between the two phenomena ; and 
 fortunately there were other records preceding 
 these years and following them which enabled us 
 to test the connection further, and it was found, as 
 we might naturally expect, that it was not con- 
 firmed. On looking at diagrams (Plate XV.) for the 
 periods before and after, no similarity can be traced 
 between the sun-spot curve and the wind-vane 
 curve, and we infer that the similarity during the 
 period first mentioned was entirely accidental. 
 This shows that we must be cautious in accepting, 
 from a limited amount of evidence, a connection 
 between two phenomena as real and established ; 
 for it may be purely fortuitous. We may 
 particularly remark that it is desirable to have 
 repetitions through several complete periods in- 
 stead of one alone. It is possible to reduce to 
 mathematical laws the rules for caution in this 
 matter ; and much useful work has already been 
 done in this direction by Professor Schuster of 
 Manchester and others, though as yet too little 
 attention has been paid to their rules by investi- 
 gators naturally eager to discover some hitherto 
 unthought-of connection between phenomena. 
 With this example of the need for caution, we 
 
1 70 ASTRONOMICAL DISCOVERY 
 
 may return to phenomena of which we can cer- 
 tainly say that they vary sympathetically with the 
 sun-spots. Roughly speaking, the whole history 
 of the sun seems to be bound up with them. 
 Besides these dark patches which we call spots 
 (which, by the way, are not really dark but only 
 less bright than the surrounding part of the disc), 
 there are patches brighter than the rest which 
 have been called faculse. With ordinary tele- 
 scopes, either visual or photographic, these can 
 generally only be detected near the edge of the 
 sun's disc ; but even with this limitation it can 
 easily be established that the faculae vary in 
 number and size from year to year much in the 
 same way as the spots, and this conclusion is 
 amply confirmed by the beautiful method of 
 observing the, faculse with the new instrument 
 designed by Professor Hale of the Yerkes Obser- 
 vatory. With this instrument, called a spectro- 
 heliograph, it is possible to photograph the 
 facula3 in all parts of the sun's disc, and thus to 
 obtain a much more complete history of them, 
 and there is no doubt whatever of their variation 
 sympathetically with the spots. Nor is there any 
 doubt about similar variations in other parts of 
 and the the sun which we cannot see at all with ordinary 
 sphere telescopes, except on the occasions when the, sun 
 is totally eclipsed. Roughly speaking, these out- 
 lying portions of the sun consist of two kinds, the 
 chromosphere and the corona, the former looking 
 like an irregular close coating of the ordinary sun, 
 
SMOOTHED SUNSPOT CURVE (WOLF) COMPARED WITH THE NUMBER 
 OF TURNS MADE IN EACH YEAR BY THE Osi.ER ANEMOMETER VANE 
 OF THE ROYAL OBSERVATORY, GREENWICH (THE EXCESS OF THE 
 
 DIRECT TURNS (D) OVER THE RETROGRADE TURNS (R) OR VICE VERSA.) 
 
 1842 
 
 1846 
 
 36 D H 
 
 THE UPPER CURVE IS IN' EACH CASE THE SuNSPOT CURVE, THE LOWER THE VANE CURVE. 
 
 THE BREAK IN l832 IN THE VANE CURVE IS DUE TO THE OMISSION OK EVIDENTLY 
 
 ACCIDENTAL TURNS FROM THAT DATE. 
 
 PLATE XV. 
 
SCHWABE AND THE SUN-SPOT PERIOD 171 
 
 and the latter like a pearly halo of light extending 
 to many diameters of the sun's disc, but not with 
 any very regular form. 
 
 The chromosphere, from which shoot out the 
 prominences or " red flames," can now be observed 
 without an eclipse if we employ the beautiful 
 instrument above-mentioned, the spectrohelio- 
 graph ; and Professor Hale has succeeded in pho- 
 tographing spots, faculae, and prominences all on 
 the same plate. But although many have made 
 the attempt (and Professor Hale, perhaps, a more 
 determined attempt than any man living), no one 
 has yet succeeded in obtaining any picture or 
 evidence of the existence of the corona excepting 
 on the occasion of a total solar eclipse. 
 
 Now these occasions are very rare. There are Eclipses 
 two or three eclipses of the sun every year, but 
 they are generally of the kind known as partial ; 
 when the moon does indeed come between us and 
 the sun to some extent, but only cuts off a portion 
 of his light a clean-cut black disc is seen to en- 
 croach more or less on the surface of the sun. 
 Most of us have had an opportunity of seeing a 
 partial eclipse, probably more than once ; but few 
 have seen a total eclipse. For this the moon 
 must come with great exactness centrally be- 
 tween us and the sun ; and the spot where this 
 condition is fulfilled completely only covers a few 
 hundred miles of the earth's surface at one moment. 
 As the earth turns round, and as the moon revolves 
 in its orbit, this patch from which the sun is totally 
 
ASTRONOMICAL DISCOVERY 
 
 eclipsed travels over the earth's surface, marking 
 out a track some thousands of miles in length 
 possibly, but still not more than 200 miles wide ; 
 Total and in order to see the sun totally eclipsed even 
 rare. on the rare occasions when it is possible at all 
 (for, as already remarked, in the majority of cases 
 the eclipse is only partial), we must occupy some 
 station in this narrow belt or track, which often 
 tantalisingly passes over either the ocean or some 
 regions not easily accessible to civilised man. 
 Moreover, if we travel to such favoured spots 
 the whole time during which the sun is totally 
 eclipsed cannot exceed a few minutes, and hence 
 observations are made under rather hurried and 
 trying conditions. In these modern days of pho- 
 tography it is easier to take advantage of these 
 precious moments than it used to be when there 
 was only the eye and memory of an excited 
 observer to rely upon. It is perhaps not sur- 
 prising that some of the evidence collected on 
 these earlier occasions was conflicting ; but nowa- 
 days the observers, generally speaking, direct 
 their energies in the first place to mounting 
 accurately in position photographic apparatus of 
 different kinds, each item of it specially designed 
 to settle t some particular problem in the most 
 feasible way ; secondly, to rehearsing very care- 
 fully the exact programme of exposures necessary 
 during the critical few minutes ; and finally, to 
 securing these photographs with as few mistakes 
 as possible when the precious moments actually 
 
SCHWABE AND THE SUN-SPOT PERIOD 173 
 
 arrive. Even then the whole of their efforts are 
 quite likely to be rendered unavailing by a passing 
 cloud; and bitter is the disappointment when, 
 after travelling thousands of miles, and spending 
 months in preparation, the whole enterprise ends 
 in nothing owing to some caprice of the weather. 
 Hence it will easily be imagined that our know- 
 ledge of the corona, the part of the sun which we 
 can still only study on occasions of a total solar 
 eclipse, advances but slowly. During the last 
 twenty years there has been altogether scarcely 
 half-an-hour available for this research, though it 
 may fairly be said that the very best possible use 
 has been made of that half-hour. And, what is 
 of importance for our immediate purpose, it has 
 gradually been established by comparing the pho- 
 tographs of one eclipse with those of another, that 
 the corona itself undergoes distinct changes in Corona 
 form in the same period which governs the changes 
 of sun-spots. When there are many sun-spots 
 the corona spreads out in all directions from the 
 edge of the sun's disc ; when there are few sun- 
 spots the corona extends very much further in 
 the direction of the sun's equator, so that at sun- 
 spot minimum there is an appearance of two huge 
 wings. Although the evidence is necessarily 
 collected in a scrappy manner, by this time there 
 is sufficient to remove this relationship out of the 
 region of mere suspicion, and to give it a well- 
 established place in our knowledge of the sun's 
 surroundings. 
 
174 ASTRONOMICAL DISCOVERY 
 
 Now the corona of the sun may be compared 
 to some rare animal which we only see by pay- 
 ing a visit to some distant land, and may con- 
 sider ourselves even then fortunate to get a 
 glimpse of; and it might be thought that the 
 habits of such an animal are not likely to be of 
 any great importance in our everyday life. But 
 so far from this being the case in regard to the 
 corona, it is more than possible that the know- 
 ledge of its changes may be of vital interest to 
 us. I have already said that, as yet, we have no 
 satisfactory account of the reason why changes 
 in sun-spots seem to influence changes in our 
 magnets on the earth ; but one of the theories 
 put forward in explanation, and one by no means 
 the least plausible, is that this influence may come, 
 not from the sun-spots themselves, but from some 
 other solar phenomenon which varies in sympathy 
 with them ; and in particular that it may come 
 Corona from the corona. These wings which reach out 
 fuTence" at sun-spot minimum can be seen to extend a 
 magnets, considerable distance, and there is no reason to 
 suppose that they actually cease at the point 
 where they become too faint for us to detect 
 them further; they may extend quite as far as 
 the earth itself and even beyond ; and they may 
 be of such' a nature as to influence our magnets. 
 As the earth revolves round the sun it may some- 
 time plunge into them, to emerge later and pass 
 above or below them ; as again the wings spread 
 themselves at sun-spot minimum and seem to 
 
SCHWABE AND THE SUN-SPOT PERIOD 175 
 
 shrink at maximum, so our magnets may respond 
 by sympathetic though very small vibrations. 
 Hence it is quite possible that the corona is 
 directly influencing the magnetic changes on the 
 earth. 
 
 But it may be urged that these changes are Possible 
 so slight as to be merely of scientific interest, 
 That may be true to-day, but who will be bold 
 enough to say that it will be true to-morrow ? If 
 we are thinking of practical utility alone, we may 
 remember that two great forces of Nature which 
 we have chained into the service of man, steam 
 and electricity, put forth originally the most 
 feeble manifestations, which might readily have 
 been despised as valueless ; but by careful atten- 
 tion to proper conditions results of overwhelming 
 practical importance have been obtained from 
 these forces, which might have been, and for 
 many centuries were, neglected as too trivial to be 
 worth attention. Recently the world has been 
 startled by the discovery of new elements, such as 
 radium, whose very existence was only detected 
 by a triumph of scientific acuteness in investiga- 
 tion, and yet which promise to yield influences 
 on our lives which may overwhelm in importance 
 all that has gone before. And similarly it may 
 be that these magnetic changes, when properly 
 interpreted or developed, may become of an im- 
 portance in the future out of all proportion to 
 the attention which they have hitherto attracted. 
 Hence, although perhaps sufficient has already 
 
i;6 ASTRONOMICAL DISCOVERY 
 
 been established to show the immense con- 
 sequences which flow from Schwabe's remarkable 
 discovery of the periodicity in solar spots, we 
 may be as yet only on the threshold of its real 
 value. 
 
 From what little causes great events spring ! 
 How little can Schwabe have realised, when he 
 began to point his modest little telescope at the 
 sun, and to count the number of spots the 
 despised spots which he had been assured were 
 of no interest and exhibited no laws, and were 
 generally unprofitable that he was taking the 
 first step in the invention of the great science of 
 Solar Physics ! a science which is, I am glad to 
 say, occupying at the present moment so much 
 of the attention, not only of the great Yerkes 
 Observatory, but of many other observatories 
 scattered over the globe. 
 
CHAPTER VI 
 THE VARIATION OF LATITUDE 
 
 IF we should desire to classify discoveries in 
 order of merit, we must undoubtedly give a high 
 place to those which are made under direct dis- 
 couragements. In the last chapter we saw that 
 Schwabe entered upon his work under conditions 
 of this kind, it being the opinion of experienced 
 astronomers who had looked at the facts that 
 there was nothing of interest to be got by watch- 
 ing sun-spots. In the present chapter I propose 
 to deal with a discovery made in the very teeth 
 of the unanimous opinion of the astronomical 
 world by an American amateur, Mr. S. C. Chandler 
 of Cambridge (Massachusetts). It is my purpose 
 to allow him to himself explain the steps of this 
 discovery by giving extracts from the magni- 
 ficent series of papers which he contributed to 
 the Astronomical Journal on the subject in the 
 years 1891-94, but it may help in the under- 
 standing of these extracts if I give a brief 
 summary of the facts. And I will first explain 
 what is meant by the "Variation of Latitude." 
 
 We are all familiar with the existence of a 
 certain star in the heavens called the Pole' Star, 
 and we know that at any particular place it is 
 
 177 M 
 
1 78 ASTRONOMICAL DISCOVERY 
 
 seen constantly in the north at a definite height 
 Latitude, above the horizon, which is the latitude of the 
 place. When watched carefully with a telescope 
 it is found to be not absolutely stationary, but 
 to describe a small circle in the heavens day 
 by day, or rather night by night. These simple 
 facts are bound up with the phenomenon of the 
 earth's rotation in this way : the axis about which 
 it is rotating points to the centre of that little 
 circle, and any change in the position of the axis 
 can therefore be determined by observing these 
 motions of the Pole Star. Such changes may be 
 of two kinds : firstly, we might find that the size 
 of the circle increased or diminished, and this 
 would mean that the earth's axis was pointing 
 farther away from the Pole Star or nearer to it 
 pointing, that is to say, in a different direction 
 in space. This actually happens (as has been 
 known for some thousands of years) owing to 
 Preces- the phenomenon called " precession " ; the circle 
 described by our Pole Star is at present getting a 
 little smaller, but it will ultimately increase in 
 size, and after thousands of years become so large 
 that the Pole Star will entirely lose its character 
 as a steady guide to the North. 
 
 Secondly (and this is what more immediately 
 change of concerns us), the centre of the circle may alter 
 its position and be no longer at the same height 
 above the horizon of any given place. This would 
 mean that the earth's axis was shifting in the earth 
 itself that the North Pole which our explorers 
 
THE VARIATION OF LATITUDE 179 
 
 go to seek is not remaining in the same place. 
 That it does not change appreciably in position 
 we know from familiar experience ; our climates, 
 for instance, would suffer considerably if there 
 were any large changes. But astronomers are 
 concerned with minute changes which would not 
 have any appreciable effect on climate, and the 
 question has long been before them whether, put- 
 ting aside large movements, there were any minute 
 variations in position of the North Pole. Twenty Twenty 
 years ago the answer to this question would have aS* 8 
 been given decidedly in the negative ; it was b< 
 considered as certain that the North Pole did 
 not move at all within the limits of our most 
 refined astronomical observations. Accepted 
 theory seemed to indicate that any movements 
 must in any case recur after a period of ten 
 months, and careful discussion of the observa- 
 tions showed that there was no oscillation in 
 such a period. Now we know that the theory 
 itself was wrong, or rather was founded upon a 
 mistaken assumption ; and that the facts when 
 properly examined show clearly a distinct move- 
 ment of the North Pole, not a very large one, for 
 all its movements take place within the area 
 occupied by a moderate-sized room, but still a 
 movement easily measurable by astronomical ob- 
 servations, and Mr. Chandler was the first to 
 point out the law of these movements, and very 
 possibly the first to suspect them. 
 
 With these few words of explanation I will 
 
i8o ASTRONOMICAL DISCOVERY 
 
 chand- let Mr. Chandler tell his own story. His first 
 papers, paper appeared in the Astronomical Journal in 
 November 1891, and is courageously headed, " On 
 the Variation of Latitude " I say courageously, 
 because at that time it was believed that the 
 latitude did not vary, and Mr. Chandler him- 
 self was only in possession of a small portion 
 of the facts. They unravelled themselves as 
 he went forward ; but he felt that he had firm 
 hold of the end of the thread, and he faced the 
 world confidently in that belief. He begins 
 thus : 
 
 First "In the determination of the latitude of Cam- 
 
 change, bridge * with the Almucantar, about six years and 
 a half ago, it was shown that the observed 
 values, arranged according to nights of observa- 
 tion, exhibited a decided and curious progression 
 throughout the series, the earlier values being 
 small, the later ones large, and the range from 
 November 1884 to April 1885 being about 
 four-tenths of a second. There was no known 
 or imaginable instrumental or personal cause 
 for this phenomenon, yet the only alternative 
 seemed to be an inference that the latitude had 
 actually changed. This seemed at the time too 
 bold an inference to place upon record, and I 
 therefore left the results to speak for themselves. 
 The subsequent continuation of the series of 
 observations to the end of June 1885 gave a 
 
 1 This should be Cambridge, Mass. 
 
THE VARIATION OF LATITUDE 181 
 
 maximum about May i, while the discussion 
 of the previous observations from May to 
 November 1884 gave a minimum about Sep- 
 tember i, indicating a range of 0^.7 within a 
 half-period of about seven months." 
 
 Mr. Chandler then gives some figures in support 
 of these statements, presenting them with the 
 clearness which is so well marked a feature of 
 the whole series of papers, and concludes this 
 introductory paper as follows : 
 
 "It thus appears that the apparent change in 
 the latitude of Cambridge is verified by this 
 discussion of more abundant material. The 
 presumption that it is real, on this determina- 
 tion alone, would justify further inquiry. 
 
 " Curiously enough Dr. Klistner, in his deter- Confirmed 
 mination of the observation from a series of" 
 observations coincident in time with those of 
 the Almucantar, came upon similar anomalies, 
 and his results, published in 1888, furnish a 
 counterpart to those which I had pointed out 
 in 1885. The verification afforded by the recent 
 parallel determinations at Berlin, Prague, Pots- 
 dam, and Pulkowa, which show a most surprising 
 and satisfactory accordance, as to the character 
 of the change, in range and periodicity, with 
 the Almucantar results, has led me to make 
 further investigations on the subject. They 
 seem to establish the nature of the law of those 
 
1 82 ASTRONOMICAL DISCOVERY 
 
 changes, and I will proceed to present them in 
 due order." 
 
 The second paper appeared on November 23, 
 and opens with the following brief statement of 
 his general results at that time : 
 
 "Before entering upon the details of the 
 investigations spoken of in the preceding 
 number, it is convenient to say that the general 
 result of a preliminary discussion is to show a 
 427 days' revolution of the earth's pole in a period of 427 
 od ' days, from west to east, with a radius of thirty 
 feet, measured at the earth's surface. Assuming 
 provisionally, for the purpose of statement, that 
 this is a motion of the north pole of the principal 
 axis of inertia about that of the axis of rotation, 
 the direction of the former from the latter lay 
 towards the Greenwich meridian about the 
 beginning of the year 1890. This, with the 
 period of 427 days, will serve to fix ap- 
 proximately the relative positions of these axes 
 at any other time, for any given meridian. It 
 is not possible at this stage of the investigation 
 to be more precise, as there are facts which 
 appear to show that the rotation is not a 
 perfectly uniform one, but is subject to secular 
 change, and perhaps irregularities within brief 
 spaces of time." 
 
 It is almost impossible, now that we have 
 become familiar with the ideas conveyed in this 
 
THE VARIATION OF LATITUDE 183 
 
 paragraph, to understand, or even fully to re- 
 member, the impression produced by them at the 
 time ; the sensation caused in some quarters, and 
 the ridicule excited in others. They were in flat Contrary 
 contradiction to all accepted views ; and it was ceived 
 believed that these views were not only theoreti- v] 
 cally sound, but had been matured by a thorough 
 examination of observational evidence. The only 
 period in which the earth's pole could revolve was 
 believed to be ten mouths ; and here was Mr. 
 Chandler proclaiming, apparently without any 
 idea that he was contradicting the laws of 
 dynamics, that it was revolving in fourteen 
 months ! The radius of its path had been found 
 to be insensible by careful discussion of observa- 
 tions, and now he proclaimed a sensible radius of 
 thirty feet. Finally, he had the audacity to 
 announce a variable period, to which there was 
 nothing at all corresponding in the mathematical 
 possibilities. This was the bitterest pill of all. 
 Even after Professor Newcomb had shown us how 
 to swallow the other two, he could not recommend 
 any attempt at the third, as we shall presently 
 see ; and Mr. Chandler was fain ultimately to gild 
 it a little before it could be gulped. 
 
 But this is anticipating, and it is our intention 
 to follow patiently the evidence adduced in support 
 of the above statements, made with such splendid 
 confidence to a totally disbelieving world. Mr. 
 Chandler first examines the observations of Dr. 
 Kiistner of Berlin, quoted at the end of his last 
 
i84 ASTRONOMICAL DISCOVERY 
 
 paper, and shows how well they are suited by the 
 existence of a variation in the latitude of 427 
 days ; and that this new fact is added when the 
 Cambridge (U.S.A.) latitudes were the smallest 
 those of Berlin were the largest, and vice versa, as 
 would clearly be the case if the phenomenon was 
 due to a motion of the earth's pole ; for if it moved 
 nearer America it must move further from Europe. 
 Puikowa He then examines a long series of observations 
 solved, made in the years 1864-1873 at Puikowa, near 
 St. Petersburg, and again finds satisfactory con- 
 firmation of his law of variation. Now it had long 
 been known that there was something curious 
 about these observations, but no one could tell 
 what it was. The key offered by Mr. Chandler 
 fitted the lock exactly, and the anomalies which 
 had been a puzzle were removed. This was in 
 itself a great triumph ; but there was another to 
 come, which we may let Mr. Chandler describe in 
 his own words : 
 
 also "In 1862 Professor Hubbard began a series of 
 
 ton. observations of Lyree at the Washington Obser- 
 vatory with the prime vertical transit instrument, 
 for the purpose of determining the constants of 
 aberration and nutation and the parallax of the 
 star. The 'methods of observation and reduction 
 were conformed to those used with such success 
 by W. Struve. After Hubbard's death the series 
 was continued by Professors Newcomb, Hall, and 
 Harkness until the beginning of 1867. Professor 
 
THE VARIATION OF LATITUDE 185 
 
 Hall describes these observations as the most 
 accurate determinations of declination ever made 
 at the Naval Observatory. The probable error of 
 a declination from a single transit was +o".i4i, 
 and judging from the accidental errors, the series 
 ought to give trustworthy results. Upon reduc- 
 ing them, however, it was found that some ab- 
 normal source of error existed, which resulted in 
 anomalous values of the aberration-constant in 
 the different years, and a negative parallax in 
 all. A careful verification of the processes of 
 reduction failed to discover the cause of the 
 trouble, and Professor Hall says that the results 
 must stand as printed, and that probably some 
 annual disturbance in the observations or the 
 instrument occurred, which will never be ex- 
 plained, and which renders all deductions from 
 them uncertain. The trouble could not be con- 
 nected with personal equation, the anomalies 
 remaining when the observations of the four 
 observers who took part were separately treated. 
 Nor, as Professor Hall points out. will the theo- 
 retical ten-month period in the latitude furnish 
 the explanation. 
 
 "It is manifest, however, that if the 427-day 
 period exists, its effect ought to appear distinctly 
 in declination-measurements of such high degree 
 of excellence as these presumably were, and, as I 
 hope satisfactorily to show, actually are. When 
 this variation is taken into account the observa- 
 tions will unquestionably vindicate the high ex- 
 
1 86 ASTRONOMICAL DISCOVERY 
 
 pectations entertained with regard to them by 
 the accomplished and skilful astronomers who 
 designed and carried them out." 
 
 From this general account I am excluding 
 technical details and figures, and unfortunately 
 a great deal is thereby lost. We lose the sense 
 of conviction which the long rows of accordant 
 figures force upon us, and we lose the oppor- 
 tunities of admiring both the astonishing amount 
 of work done and the beautiful way in which the 
 material is handled by a master. But I am 
 tempted to give one very small illustration of 
 the numerical results from near the end of the 
 paper. After discussing the Washington results, 
 and amply fulfilling the promise made in the pre- 
 Direction ceding extract, Mr. Chandler compares them with 
 tionof the Pulkowa results, and shows that the Earth's 
 Pole must be revolving from west to east, and not 
 from east to west. And then he writes down a 
 simple formula representing this motion, and com- 
 pares his formula with the observations. He 
 gives the results in seconds of arc, but for the 
 benefit of those not familiar with astronomical 
 measurements we may readily convert these into 
 feet ; and in the following tables are shown the 
 Example distances of the Earth's Pole in feet from its 
 
 of results. ... i , n , TT . . , 
 
 average position/ as observed at Washington and 
 
 1 The distances do not represent the total displacement, but only 
 the displacement towards Washington in one case and towards 
 Pulkowa in the other. 
 
THE VARIATION OF LATITUDE 187 
 
 at Pulkowa, and the same distances calculated 
 according to the formula which Mr. Chandler 
 was able to write down at this early stage. The 
 signs 4- and of course indicate opposite direc- 
 tions of displacement : 
 
 WASHINGTON. 
 
 Deviation of Pole. 
 
 Date. 
 
 Observed. 
 
 Formula. 
 
 1864, Dec. 28 
 
 - 28 feet 
 
 - 23 feet 
 
 1865, Mar. 19 
 
 - I 
 
 -12 
 
 June i 
 
 + 15 
 
 + 12 
 
 Aug. ii 
 
 + 22 
 
 + 23 
 
 Oct. 9 
 
 + H 
 
 + 15 
 
 Dec. 13 
 
 -17 
 
 - 6 
 
 PULKOWA. 
 
 Deviation of Pole. 
 
 Date. 
 
 Observed. 
 
 Formula. 
 
 1865, July 25 . . 
 
 -18 feet 
 
 -12 feet 
 
 Sept. 9. 
 
 + 3 
 
 + 3 , 
 
 NOV. 22 . 
 
 + 26 
 
 + 22 
 
 1 866, Feb. 22 . 
 
 + 18 
 
 + 13 
 
 June 4. 
 
 -ii 
 
 -18 
 
 July 17. 
 
 -16 
 
 -23 
 
 Of course the figures are not exact in every case, 
 but they are never many feet wrong ; and it may 
 
1 88 ASTRONOMICAL DISCOVERY 
 
 well be imagined that it is a difficult thing to 
 deduce, even from the most refined observations, 
 the position of the earth's pole to within a foot. 
 The difficulty is exactly the same as that of 
 measuring the length of an object 300 miles 
 away to within an inch ! 
 
 Mr. Chandler winds up his second paper 
 thus : 
 
 "We thus find that the comparison of the 
 simultaneous series at Pulkowa and Washington, 
 1863-1867, leads to the same conclusion as that 
 already drawn from the simultaneous series at 
 Berlin and Cambridge, 1884-1885. The direc- 
 tion of the polar motion may therefore be looked 
 upon as established with a large degree of pro- 
 bability. 
 
 " In the next paper I will present the results 
 derived from PETERS, STRUVE, BRADLEY, and 
 various other series of observations, after which 
 the results of all will be brought to bear upon 
 the determination of the best numerical values 
 of the constants involved." 
 
 Bradiey's The results were not, however, presented in 
 tk>ns. va this order. In the next paper, which appeared 
 on December 23, 1891, Mr. Chandler begins, with 
 the work of Bradley, the very series of observa- 
 tions at Kew and Wansted which led to the 
 discoveries of aberration and nutation, and which 
 we considered in the third chapter. He first 
 
THE VARIATION OF LATITUDE 189 
 
 shows that, notwithstanding the obvious accuracy 
 of the observations, there is some unexplained 
 discordance. The very constant of aberration 
 which Bradley discovered from them differs by 
 half-a-second of arc from our best modern deter- 
 minations. Attempts have been made to ascribe 
 the discordance to changes in the instrument, but 
 Mr. Chandler shows that such changes, setting 
 aside the fact that Bradley would almost certainly 
 have discovered them, will not fit in with the 
 facts. The facts, when analysed with the skill 
 to which we have become accustomed, are that 
 there is a periodic swing in the results with a Latitude 
 period of about a year, and not fourteen months, twelve 
 as before, "a result so curious," as he admits, 
 that "if we found no further support, it might 
 lead us to distrust the above reasoning, and throw 
 us back to the possibility that, after all, BRADLEY'S 
 observations may have been vitiated by some kind 
 of annual instrumental error. But it will abun- 
 dantly appear, when I have had the opportunity 
 to print the deductions from all the other series 
 of observations down to the present time, that the 
 inference of an increase in the period of polar 
 revolution is firmly established by their concur- 
 rent testimony." We shall presently return to 
 this curious result, which might well have dis- 
 mayed a less determined researcher than Mr. 
 Chandler, but which only led him on to re- 
 newed exertions. 
 
 The results obtained from Bradley's obser- 
 
1 90 ASTRONOMICAL DISCOVERY 
 
 vations may be put in the form of a diagram 
 thus : 
 
 VARIATION OF LATITUDE 
 Bradley 's Observations, 
 <A*2**Hf.a*) 
 
 V V 
 
 
 -t'-O APRIL, APRIL, APRIL, -1"O 
 
 1728. 1729. 1730. 
 
 FIG. 7. 
 
 It will be seen that the maxima and minima 
 fall in the spring and autumn, and this fact alone 
 seemed to show that the effect could not be due 
 to temperature, for we should expect the greatest 
 effect in that case in winter and summer. It 
 could not be due to the parallax of the stars 
 for which Bradley began his search, for stars in 
 different quarters of the heavens would then be 
 differently affected, and this was not the case. 
 "There remains," concluded Mr. Chandler after 
 full discussion, " the only natural conclusion of 
 an actual displacement of the zenith, in other 
 words, a change of latitude." And he concludes 
 this paper with the following fine passage : 
 
 u So far, then, as the results of this incompar- 
 able series of observations at Kew and Wansted, 
 
THE VARIATION OF LATITUDE 191 
 
 considered by themselves alone, can now be 
 stated, the period of the polar rotation at that 
 epoch appears to have been probably somewhat 
 over a year, and certainly shorter by about two 
 months than it is at the present time. The 
 range of the variation was apparently in the 
 neighbourhood of a second of arc, or consider- 
 ably larger than that shown by the best modern 
 observations. 
 
 " Before taking leave of these observations for Bradiey's 
 the present I cannot forbear to speak of the pro- gre< 
 found impression which a study of them leaves 
 upon the mind, and the satisfaction which all 
 astronomers must feel in recognising that, besides 
 its first fruits of the phenomena of aberration and 
 nutation, we now owe also our first knowledge of 
 the polar motion to this same immortal work of 
 Bradley. Its excellence, highly appreciated as it 
 has been, has still been hitherto obscured by the 
 presence of this unsuspected phenomenon. When 
 divested of its effects, the wonderful accuracy of 
 this work must appear in a finer light, and our 
 admiration must be raised to higher pitch. Going 
 back to it after one hundred and sixty years seems 
 indeed like advancing into an era of practical 
 astronomy more refined than that from which we 
 pass. And this leads to a suggestion worthy of 
 serious practical consideration whether we can 
 do better in the future study of the polar rotation, 
 than again to avail ourselves of Bradiey's method, 
 
1 92 ASTRONOMICAL DISCOVERY 
 
 without endangering its elegant simplicity and 
 effectiveness by attempts at improvement, other 
 than supplying certain means of instrumental 
 control which would without doubt commend 
 themselves to his sagacious mind. 
 
 " In the next article Bradley's later observations 
 at Greenwich, the results of which are not so 
 distinct, will be discussed ; and also those of 
 Brinkley at Dublin, 1808-13 and 1818-22. 
 This will bring again to the surface one of the 
 most interesting episodes in astronomical history, 
 other the spirited and almost acrimonious dispute 
 explained, between Brinkley and Pond with regard to stellar 
 parallaxes. I hope to show that the hitherto 
 unsolved enigma of Brinkley's singular results 
 finds its easy solution in the fact of the polar 
 motion. The period of his epoch appears to have 
 been about a year, and, its range more than a 
 second. Afterwards will follow various dis- 
 cussions already more or less advanced towards 
 completion. These include Bessel's observations 
 at Konigsberg, 1820-24, with the Reichenbach 
 circle, and in 1842-44 with the Repsold circle; 
 the latitudes derived from the polar-point deter- 
 minations of Struve and Madler with the Dorpat 
 circle, 1822-38; Struve's observations for the 
 determination of the aberration ; Peters' observa- 
 tions of Polaris, 1841-43, with the vertical-circle ; 
 the results obtained from the reflex zenith-tube 
 at Greenwich, 1837-75, whose singular anomalies 
 
THE VARIATION OF LATITUDE 193 
 
 can be referred in large part to our present 
 phenomenon, complicated with instrumental 
 error, to which until now they have been ex- 
 clusively attributed ; the Greenwich transit-circle 
 results, 1851-65, in which case, however, a similar 
 complication and the large accidental errors of 
 observation seem to frustrate efforts to get any 
 pertinent results ; the Berlin prime-vertical obser- 
 vations ofWeyerand Brtinnow, 1845-46, in which 
 I hope to show that the parallax of /3 Draconis 
 derived from them is simply a record of the 
 change of latitude ; the conflicting latitude deter- 
 minations at Cambridge, England ; the Washing- 
 ton observation of Polaris and other close Polars, 
 1866-87, with the transit-circle; also those at 
 Melbourne, 1863-84, a portion of which have 
 already been drawn upon in the last number of 
 the Journal, and some others. While the list is 
 a considerable one, I shall be able to compress 
 the statement of results for many of the series 
 into a short space. 
 
 " In connection with this synopsis of the scope 
 of the investigations, one or two particulars may 
 be of interest, which at the present writing seem 
 to foreshadow the probable outcome. I beg, how- P 
 
 ever, that the statement will be regarded merely nature of 
 as a provisional one. First, while the period is results * 
 manifestly subject to change, as has already once 
 or twice been intimated, I have hitherto failed in 
 tracing the variations to any regular law, expres- 
 sible in a numerical formula. Indeed, the general 
 
 N 
 
194 ASTRONOMICAL DISCOVERY 
 
 impression produced by a study of these changes 
 in the length of the period is that the cause which 
 produces them operates capriciously to a certain 
 degree, although the average; effect for a century 
 has been to diminish the velocity of the revolution 
 of the pole. How far this impression is due to 
 the uncertainty of the observations, and to the 
 .complication of the phenomenon with other 
 periodical changes of a purely instrumental kind, 
 I cannot say. , Almost all of the series of any 
 extent which have been examined, have the 
 peculiarity that ' they manifest the periodicity 
 quite uniformly and distinctly for a number of 
 years, then for a while obscurely. In some cases, 
 however, what at first .appears to be an objective 
 irregularity proves - not to be so by comparison 
 with overlapping series at other observatories. ' 
 
 "Another characteristic which has struck my 
 attention, although somewhat vaguely, is that the 
 variations in, the length of the period seem to go 
 hand in hand with simultaneous alterations in the 
 amplitude of the rotation ; the shorter periods 
 being apparently associated with the larger cor 
 efficients for the latter. The verification of these 
 surmises awaits a closer comparative scrutiny, the 
 opportunity for which will come when the com- 
 putations are in a more forward state. If con- 
 firmed, these observations will afford a valuable 
 touchstone, in seeking for the cause of a pheno- 
 menon which now seems to be at variance with 
 the accepted laws of terrestrial rotation." 
 
THE VARIATION OF LATITUDE 195 
 
 Let us now for a few moments turn aside from Reception 
 the actual research to see how the announcement 
 was received. It would be ungracious to reprint 
 here any of the early statements of incredulity 
 which found their way into print, especially in 
 Germany. But the first note of welcome came 
 from Simon Newcomb, in the same number of 
 the Astronomical Journal as the paper just dealt 
 with, and the following extract will indicate both 
 the difficulties felt in receiving Mr. Chandler's 
 results and the way in which Newcomb struck at 
 the root of them. 
 
 " Mr. Chandler's remarkable discovery, that the 
 apparent variations in terrestrial latitudes may be 
 accounted for by supposing a revolution of the 
 axis of rotation of the earth around that of figure, 
 in a period of 427 days, is in such disaccord with 
 the received theory of the earth's rotation that at 
 rst I was disposed to doubt its possibility. But I 
 am now able to point out a vera causa which 
 affords a complete explanation of this period. Up New- 
 to the present time the treatment of this subject 
 has been this : The ratio of the moment of inertia 
 of the earth around its principal axis to the mean 
 of the other two principal moments, admits of very 
 accurate determination from the amount of pre- 
 cession and nutation. This ratio involves what 
 we might call, in a general way, the solid 
 ellipticity of the earth, or the ellipticity of a 
 
196 ASTRONOMICAL DISCOVERY 
 
 homogeneous spheroid having the same moments 
 of inertia as the earth. 
 
 " When the differential equations of the earth's 
 rotation are integrated, there appear two arbitrary 
 constants, representing the position of any as- 
 signed epoch of the axis of rotation relative to 
 that of figure. Theory then shows that the axis 
 of rotation will revolve round that of figure, in a 
 period of 306 days, and in a direction from west 
 toward east. The attempts to determine the 
 value of these constants have seemed to show 
 that both are zero, or that the axes of rotation 
 and figure are coincident. Several years since, 
 Sir William Thomson published the result of 
 a brief computation from the Washington Prime- 
 Vertical observations of a Lyrae which I made at 
 his request and which showed a coefficient of o".O5. 
 This coefficient did not exceed the possible error of 
 the result ; I therefore regarded it as unreal. 
 The " The question now arises whether Mr. Chand- 
 
 forgotten l e r's result can be reconciled with dynamic theory. 
 
 assump- 
 
 tion. I answer that it can, because the theory which 
 assigns 306 days as the time of revolution is based 
 on the hypothesis that the earth is an absolutely 
 rigid body. But, as a matter of fact, the fluidity 
 of the ocean plays an important part in the 
 phenomenon, as does also the elasticity of the 
 earth. The combined effect of this fluidity and 
 elasticity is that if the axis of rotation is displaced 
 by a certain amount, the axis of figure will, by the 
 
THE VARIATION OF LATITUDE 197 
 
 changed action of the centrifugal force, be moved 
 toward coincidence with the new axis of rotation. 
 The result is, that the motion of the latter will be 
 diminished in a corresponding ratio, and thus 
 the time of revolution will be lengthened. An 
 exact computation of the effect is not possible 
 without a knowledge of the earth's modulus of 
 elasticity. But I think the result of investiga- 
 tion will be that the rigidity derived from Mr. 
 Chandler's period is as great as that claimed by 
 Sir William Thomson from the phenomena of the 
 tides." 
 
 This was very satisfactory. Professor New- 
 comb put his finger on the assumption which 
 had been made so long ago that it had been for- 
 gotten : and the lesson is well worth taking to 
 heart, for it is not the first time that mistaken 
 confidence in a supposed fact has been traced to 
 some forgotten preliminary assumption: and we 
 must be ever ready to cast our eyes backward 
 over all our assumptions, when some new fact 
 seems to challenge our conclusions. It might 
 further be expected that this discovery of the way 
 in which theory had been defective would as a 
 secondary consequence inspire confidence in the 
 other conclusions which Mr. Chandler had But 
 arrived at in apparent contradiction to theory ; i^s work 
 or at least suggest the suspension of judgment, 
 But Professor Newcomb did not feel that this 
 was possible in respect of the change of period, 
 
198 ASTRONOMICAL DISCOVERY 
 
 from about twelve months in Bradley's time to 
 fourteen months in ours. We have seen that 
 Mr. Chandler himself regarded this as a " curious 
 result " requiring confirmation : but since the 
 confirmation was forthcoming, he stated it with 
 full confidence, and drew the following remarks 
 from Professor Newcomb in July 22, 1892 : 
 
 " The fact of a periodic variation of terrestrial 
 latitudes, and the general law of that variation, 
 have been established beyond reasonable doubt 
 by the observations collected by Mr. Chandler. 
 But two of his minor conclusions, as enumerated 
 in No. 3 of this volume, do not seem to me well 
 founded. They are 
 
 " i. That the period of the inequality is a vari- 
 able quantity. 
 
 " 2. That the amplitude of the inequality has 
 remained constant for the last half century." 
 
 Professor Newcomb proceeds to give his reasons 
 for scepticism, which are too technical in character 
 to reproduce here. But I will quote the following 
 further sentence from his paper : 
 
 "The question now arises how far we are 
 entitled to assume that the period must be in- 
 variable. I reply that, perturbations aside, any 
 variation of the period is in such direct conflict 
 with the laws of dynamics that we are entitled to 
 pronounce it impossible. But we know that there 
 are perturbations, and I do not see how one can 
 
THE VARIATION OF LATITUDE 199 
 
 doubt that they have so acted as to increase the 
 amplitude of the variation since 1840." 
 
 In other words, while recognising that there may 
 be a way of reconciling one of the "minor" con- 
 clusions with theory, Professor Newcomb con- 
 siders that in this case the other must go. Mr. 
 Chandler's answer will speak for itself. It was chand- 
 delayed a little in order that he might present ler ' sreply ' 
 an immense mass of evidence in support of his 
 conclusions, and was ultimately printed on August 
 23, 1892. 
 
 " The material utilised in the foregoing forty- 
 five series aggregates more than thirty-three thou- 
 sand observations. Of these more than one-third 
 were made in the southern hemisphere, a fact 
 which we owe principally to Cordoba. It com- 
 prises the work of seventeen observatories (four 
 of them in the southern hemisphere) with twenty- 
 one different instruments, and by nine distinct 
 methods of observation. Only three of the series 
 (XXL, XXV., and XXXV.), and these among 
 the least precise intrinsically, give results con- 
 tradictory of the general law developed in 
 No. 267. This degree of general harmony is 
 indeed surprising when the evanescent char- 
 acter of the phenomenon under investigation 
 is considered. 
 
 "The reader has now before him the means for 
 independent scrutiny of the material on which the 
 conclusions already drawn, and those which are 
 
200 ASTRONOMICAL DISCOVERY 
 
 to follow, are based. The space taken in the 
 printing may seem unconscionable, but I hope 
 this will be charged to the extent of the evidence 
 collected, and not to diffuseness or the presenta- 
 tion of needless detail ; for I have studiously 
 sought to compress the form of statement with- 
 out omitting anything essential for searching 
 criticism. That it was important to do this is 
 manifest, since the conclusions, if established, 
 overthrow the existing theory of the earth's 
 rotation, as I have pointed out on p. 21. I am 
 neither surprised nor disconcerted, therefore, that 
 Professor Newcomb should hesitate to accept 
 some of these conclusions on the ground (A. J., 
 No. 271) that they are in such conflict with the 
 laws of dynamics that we are entitled to pro- 
 nounce them impossible. He has been so 
 considerate and courteous in his treatment 
 of my work thus far, that I am sure he will 
 not deem presumptuous the following argument 
 in rebuttal. 
 
 He "put " It should be said, first, that in beginning these 
 teachings investigations last year, I deliberately put aside 
 j^ eo all teachings of theory, because it seemed to me 
 high time that the facts should be examined by 
 a purely inductive process ; that the nugatory 
 results of' all attempts to detect the existence of 
 the Eulerian period probably arose from a defect 
 of the theory itself ; and that the entangled con- 
 dition of the whole subject required that it should 
 be examined afresh by processes unfettered by 
 
THE VARIATION OF LATITUDE 201 
 
 any preconceived notions whatever. The problem 
 which I therefore proposed to myself was to see 
 whether it would not be possible to lay the 
 numerous ghosts in the shape of numerous dis- 
 cordant residual phenomena pertaining to deter- 
 minations of aberration, parallaxes, latitudes, and 
 the like which had heretofore flitted elusively 
 about the astronomy of precision during the 
 century ; or to reduce them to tangible form 
 by some simple consistent hypothesis. It was 
 thought that if this could be done, a study of 
 the nature of the forces, as thus indicated, by 
 which the earth's rotation is influenced, might 
 lead to a physical explanation of them. 
 
 "Naturally, then, I am not much dismayed by and "is 
 the argument of conflict with dynamic laws, since ^ayed." 
 all that such a phrase means must refer merely 
 to the existent state of the theory at any given 
 time. When the 427-day period was propounded, 
 it was as inconsistent with known dynamic law as 
 the variation of it now appears to be. Professor 
 Newcomb's own happy explanation has already 
 set aside the first difficulty, as it would appear, 
 and advanced the theory by an important step. 
 Are we so sure yet of a complete knowledge of 
 all the forces at work as to exclude the chance 
 of a vera causa for the second ? " 
 
 There is a splendid ring of resolution about Faraday's 
 these words. Let us compare them with a notable 
 utterance of Faraday : 
 
202 ASTRONOMICAL DISCOVERY 
 
 "The philosopher should be a man willing to 
 listen to every suggestion, but determined to judge 
 for himself. He should not be biassed by appear- 
 ances ; have no favourite hypothesis ; be of no 
 school ; and in doctrine have no master. He 
 should not be a respecter of persons, but of 
 things. Truth should be his primary object. If 
 to these qualities be added industry, he may in- 
 deed hope to walk within the veil of the temple 
 of Nature." 
 
 Tested by this severe standard, Mr. Chandler 
 fails in no particular, least of all in that of 
 industry. The amount of work he got through 
 about this time was enormous, for besides the 
 main line of investigation, of which we have 
 chandler's onlv had after all a mere glimpse, he had been 
 
 other work * ... n . i 
 
 at this able to turn aside to discuss a subsidiary question 
 with Professor Comstock ; he had examined with 
 great care some puzzling characteristics in the 
 variability of stars ; he computed some comet 
 ephemerides ; and he was preparing a new cata- 
 logue of variable stars a piece of work involving 
 the collection and arrangement of great masses 
 of miscellaneous material. Yet within a few 
 months after replying as above to Professor New- 
 comb's criticism, he was able to announce that 
 he had found the key to the new puzzle, and that 
 
 Hisuiti- "theory and observation were again brought into 
 
 Sctory 1S ~ complete accord." We will as before listen to 
 elation. ^ g accoun j- o f fafe new g ^ e p j n kj s own wor( J Sj 
 
THE VARIATION OF LATITUDE 203 
 
 but a slight preliminary explanation may help 
 those unaccustomed to the terminology. The 
 polar motion was found to be compounded of two 
 independent motions, both periodic, but having 
 different periods. Now, the general results of 
 such a composition are well known in several 
 different branches of physics, especially in the 
 theory of sound. If two notes of nearly .the same 
 pitch be struck at the same time, we hear the 
 resultant sound alternately swell and die away, 
 because the vibrations caused by the two notes interfer- 
 are sometimes going in the same direction, and twowaves. 
 after an interval are going exactly in opposite 
 directions. Diagrammatically we should repre- 
 sent the vibrations by two waves, as below; the 
 
 FIG. 8. 
 
 upper wave goes through its period seven and 
 a half times between A and D, the lower only 
 six times ; and it is easily seen that at A and C 
 the waves are sympathetic, at B and D anti- 
 pathetic. At A and C the compound vibration 
 would be doubled ; at B and D reduced to insensi- 
 bility. The point is. so important that perhaps 
 a numerical illustration of it will not be super- 
 fluous. The waves are now represented by rows 
 
204 ASTRONOMICAL DISCOVERY 
 
 of figures as below. The first series recurs after 
 every 6, the second after every 7. 
 
 First Wave .. . 1234321234321234321234321234321 
 Second Wave . 1234432123443212344321234432123 
 
 Combined Effect . 2468753357764446665555555666444 
 Great disturbance. Calm. 
 
 First Wave . . 2343212343212343212343212343212 
 Second Wave . 4432123443212344321234432123443 
 
 Combined Effect . 6775335786424687533577644466655 
 
 Great disturbance. 
 
 Adding the two rows together, the oscillations 
 at first reinforce one another and we get numbers 
 ranging from 2 to 8 instead of from i to 4 ; but 
 one wave gains on the other, until it is rising 
 when the other is falling, and the numbers add 
 up to a steady series of 5's. It will be seen that 
 there are no less than seven consecutive 5's, and 
 all the variation seems to have disappeared. But 
 presently the waves separate again, and the period 
 of great disturbance recurs ; it will be seen that in 
 the " combined effect" the numbers repeat exactly 
 after the 42nd term. Now those unfamiliar with 
 the subject may not be prepared for the addition of 
 one physical wave to another, as though they were 
 iiiustra- numbers, but the analogy is perfect. Travellers 
 tion from ky some O f the fast twin-screw steamers have had 
 
 OCGclll * 
 
 travel. unpleasant occasion to notice this phenomenon, 
 when the engineer does not run the two screws 
 precisely at the same speed; there come times 
 when the ship vibrates violently, separated by 
 
THE VARIATION OF LATITUDE 205 
 
 periods of comparative stillness. Instances from 
 other walks of life may recur to the memory 
 when once attention is called to the general facts ; 
 but enough has been said to explain the point 
 numbered (2) in the subjoined statement. To 
 understand the rest, we must remember that if the 
 two waves are not equal in " amplitude," i.e. if the 
 backward and forward motion is not the same in 
 both, they cannot annul one another, but the 
 greater will always predominate. Those interested 
 in following the matter further should have no 
 difficulty in constructing simple examples to illus- 
 trate such points. We will proceed to give Mr. 
 Chandler's statements : 
 
 " We now come upon a new line of investiga- chand- 
 tion. Heretofore, as has been seen, the method formulae. 
 has been to condense the results of each series 
 of observations into the interval comprised by a 
 single period, then to determine the mean epoch 
 of minimum and the mean range for each series, 
 and, finally, by a discussion of these quantities, 
 to establish the general character of the law of 
 the rotation of the pole. It is now requisite to 
 analyse the observations in a different way, and 
 discover whether the deviations from the general 
 provisional law, in the last column of Table II. , 
 are real, and also in what manner the variation 
 of the period is brought about. The outcome 
 of this discussion, which is to be presented in 
 the [present paper, is extremely satisfactory. The 
 
206 ASTRONOMICAL DISCOVERY 
 
 real nature of the phenomenon is most distinctly 
 revealed, and may be described as follows :- 
 
 " i . The observed variation of the latitude is the 
 resultant curve arising from two periodic fluctua- 
 tions superposed upon each other. The first of 
 these, and in general the more considerable, has 
 a period of about 427 days, and a semi-amplitude 
 of about o". 12. The second has an annual period 
 with a range variable between 0^.04 and o".2o 
 during the last half-century. During the middle 
 portion of this interval, roughly characterised as 
 between 1860 and 1880, the value represented by 
 the lower limit has prevailed, but before and after 
 those dates, the higher one. The minimum and 
 maximum of this annual component of the varia- 
 tion occur at the meridian of Greenwich, about 
 ten days before the vernal and autumnal equinoxes 
 respectively, and it becomes zero just before the 
 solstices. 
 
 "2. As the resultant of these two motions, the 
 effective variation of the latitude is subject to a 
 systematic alternation in a cycle of seven years' 
 duration, resulting from the commensurability of 
 the two terms. According as they conspire or 
 interfere, the total range varies between two* 
 thirds of a second as a maximum, to but a few 
 hundredths of a second, generally speaking, as 
 a minimum. 
 
 "3. In consequence of the variability of the co- 
 efficient of the annual term above mentioned, the 
 apparent average period between 1840 and 1855 
 
THE VARIATION OF LATITUDE 207 
 
 approximated to 380 or 390 days ; widely fluctu- 
 ated from 1855 to 1865 ; from 1865 to about 1885 
 was very nearly 427 days, with minor, fluctuations ; 
 afterwards increased to near 440 days, and very 
 recently fell to somewhat below 400 days. The 
 general course of these fluctuations is quite faith- 
 fully represented by the law of eq. (3), (No. 267), 
 and accurately, even down to the minor oscilla- 
 tions of individual periods, by the law of eq. (15), 
 hereafter given, and verbally interpreted above. 
 This law also gives a similarly accurate account 
 of the corresponding oscillations in the amplitude. 
 The closeness of the accordance between observa- 
 tion and the numerical theory, in both particulars, 
 places the reality of the law beyond reasonable 
 doubt." 
 
 Those who cannot follow the details of the 
 above statement will nevertheless catch the general 
 purport that the difficulties felt by Professor 
 Newcomb have been surmounted; and this, is 
 made clearer by a later extract : 
 
 "A very important conclusion necessarily fol- 
 lows from the agreement of the values of the 427^ 
 day term, deduced from the intervals between the 
 consecutive values of T in Table XII. , namely, 
 that there has been no discontinuity in the revo : 
 lution, such as Professor Newcomb regarded as so 
 probable that he doubted the possibility of draw- 
 ing any conclusions from the comparison of obser- 
 vations before and after 1860 (A. J. 271, p. 50). 
 
2o8 ASTRONOMICAL DISCOVERY 
 
 ''The present investigation demonstrates that 
 the way out of the apparently irreconcilable con- 
 Theory tradiction of theory and observation in this matter 
 it win not does not lie in the direction of discrediting the 
 vatkm? 1 " observations, as he is inclined to do. On the 
 contrary, the result is a beautiful vindication of 
 the trustworthiness of the latter, and, at the same 
 time, of the theory that demands an invariable 
 rate of motion ; providing a perfectly fitting key 
 to the riddle by showing that another cause has 
 intervened to produce the variability of the period. 
 I feel confident that Professor Newcomb will agree 
 with the reality of the explanation here set forth, 
 and will reconsider his view that the perturbations 
 in the position of the Pole must be of the nature 
 of chance accumulations of motion, a view which 
 he then considered necessary to the maintenance 
 of the constancy in the period of latitude-varia- 
 tion." 
 
 The paper from which these words are taken 
 appeared on November 4, 1892. The next paper 
 on the main theme did not appear till a year later, 
 though much work was being done in the mean- 
 time on the constant of aberration and other 
 matters arising immediately after the discovery. 
 On November 14, 1893, Mr. Chandler winds up 
 The final the series of eight papers " On the Variation of 
 Latitude," which he had commenced just two 
 years before. His work was by no means done ; 
 rather was it only beginning, for the torch he had 
 
THE VARIATION OF LATITUDE 209 
 
 lit illuminated many dark corners. But he rightly 
 regarded his discovery as now so firmly estab- 
 lished that the series of papers dealing with it as 
 still under consideration might be terminated. 
 In this final paper he first devotes the most care- 
 ful attention to one point of detail. He had shown 
 earlier in the series that the North Pole must be 
 revolving from West to East, and not from East to 
 West ; but this was when the motion was supposed 
 to be simple and not complex, and it was neces- 
 sary to re-examine the question of direction for 
 each of the components. After establishing con- 
 clusively that the original direction holds for each 
 of the components, he almost apologises for the 
 trouble he has taken, thus : 
 
 " It is therefore proved beyond reasonable doubt 
 that the directions of the rotations r i from West 
 to East in both elements ; whence the general 
 form of the equation for the variation of latitude 
 adopted in A. </., 284, p. 154, eq. (19). It may be 
 thought that too much pains have been here be- 
 stowed upon a point which might be trusted to 
 theory to decide. I cannot think so. One of 
 the most salient results of these articles has been 
 the proof of the fact that theory has been a blind 
 guide with regard to the velocity of the Polar 
 rotation, obscuring truth and misleading investi- 
 gators for a half a century. And even if we were 
 certain, which we are not, that the fourteen 
 months' term is the Eulerian period in a modi- 
 
 o 
 
210 ASTRONOMICAL DISCOVERY 
 i 
 
 fied forin, It would still be necessary to settle 
 
 by observation the direction of the annual motion, 
 with regard to which theory is powerless to in- 
 form us. To save repetition of argument, I must 
 refer to the statement in A. J., 273, pp. 68, 70, of 
 the principles adopted in beginning these inquiries 
 in 1891." 
 
 Finally, he answers one of the few objectors of 
 eminence who still lingered, the great French 
 physicist Cornu : 
 
 " The ground is now cleared for examination of 
 ed * the only topic remaining to be covered, to estab- 
 lish, upon the foundation of fact, every point in 
 the present theory of these remarkable movements 
 of the earth's axis. This is the question of the 
 possibility that these movements are not real, but 
 merely misinterpretations of the observed pheno- 
 mena ; being in whole or in part an illusory effect 
 of instrumental error due to the influence of tem- 
 perature. Such a possibility has been a night- 
 mare in practical astronomy from the first, 
 frightening us in every series of unexplained 
 residuals, brought to light continually in nearly 
 all attempts at delicate instrumental research. A 
 source of danger so subtile could not fail to be 
 ever present in the mind of every astronomer and 
 physicist who has given even a superficial atten- 
 tion to the question of the latitude variations, 
 and there is no doubt that some are even now 
 thus deterred from accepting these variations as 
 
THE VARIATION OF LATITUDE 211 
 
 proved facts. Perhaps the most explicit and for- 
 cible statement of the doubts that may arise on 
 this subject has been given very recently by Mr. 
 Cornu. The views of so distinguished a physicist, 
 and of others who are inclined to agree with him, 
 call for careful attention, and cannot be neglected 
 in the present closing argument upon the theory 
 presented in these articles. It is unnecessary, 
 for the purpose of disposing of objections of the 
 sort raised by Cornu, to insist that it is not suffi- 
 cient to show that the observed variations, attri- 
 buted to the unsteadiness of the Earth's Pole, are 
 near the limit of precision attainable in linear 
 differential measures, and in the indication of the 
 direction of gravity by means of the air bubble of 
 the level ; or to show that there are known varia- 
 tions in divided circles and in levels, dependent 
 on temperature and seasons. Nor need we re- 
 quire of objectors the difficult, although essential, 
 task which they have not distinctly attempted 
 of showing that these errors are not eliminated, 
 as they appear to be, by the modes in which 
 astronomers use their instruments. Neither need 
 we even urge the fact that a large portion of the 
 data which have been utilised in the present re- 
 searches on the latitude were derived by methods 
 which dispense with levels, or with circles, a part 
 of them indeed with both, and yet that the results 
 of all are harmonious. On the contrary, let us 
 admit, although merely for argument's sake, that 
 all the known means of determining the direction 
 
212 ASTRONOMICAL DISCOVERY 
 
 of gravity including the plumb-line, the level, 
 and a fluid at rest, whether used for a reflecting 
 surface or as a support for a floating instrument 
 are subject to a common law of periodical error 
 which vitiates the result of astronomical observa- 
 tion, obtained by whatever methods, and in pre- 
 cisely the same manner. Now, the observed law 
 of latitude variation includes two terms, with 
 periods of fourteen and twelve months respec- 
 tively. Since the phases of the first term are 
 repeated at intervals of two months in successive 
 years, and hence in a series of years come into all 
 possible relations to conditions of temperature 
 dependent on season, the argument against the 
 reality of this term, on this ground, absolutely 
 fails, and needs no further notice. As to the 
 second, or annual term, while the phases, as 
 observed in any given longitude, are indeed 
 synchronical with the seasons, they are not so 
 as regards different longitudes. If, therefore, the 
 times of any given phase, as observed in the same 
 latitude, but in successively increasing longitudes, 
 occurred at the same date in all of them, there 
 would be a fatal presumption against the existence 
 of an annual period in the polar motion. If, on 
 the contrary, they occur at times successively 
 corresponding to the differences of longitude, the 
 presumption is equally fatal to the hypothesis 
 that they can possibly be due to temperature 
 variation as affecting instrumental measurement. 
 But the facts given in the foregoing section cor- 
 
THE VARIATION OF LATITUDE 213 
 
 respond most distinctly to the latter condition. 
 Therefore, unless additional facts can be brought 
 to disprove successively these observed results, we 
 may dismiss for ever the bugbear which has un- 
 doubtedly led many to distrust the reality of the 
 annual component of the latitude-variation, while 
 they admit the existence of the 427-day term." 
 
 At this point we must leave the fascinating 
 account of the manner in which this great dis- 
 covery was established, in the teeth of opposition 
 such as might have dismayed and dissuaded a less 
 clear-sighted or courageous man. It is my purpose 
 to lay more stress upon the method of making the 
 discovery than upon its results ; but we may afford 
 a brief glance at some of the consequences which Conse- 
 have already begun to flow from this step in 
 advance. Some of them have indeed already come C( 
 before us, especially that large class represented 
 by the explanation of anomalies in series of obser- 
 vations which had been put aside as inexplicable. 
 We have seen how the observations made in 
 Russia, or in Washington, or at Greenwich, in 
 all of which there was some puzzling error, were 
 immediately straightened out when Chandler 
 applied his new rule to them. We in England suspected 
 have special cause to be grateful to Chandler ; not acquitted. 
 only has he demonstrated more clearly than ever 
 the greatness of Bradley, but he has rehabilitated 
 Pond, the Astronomer Royal of the beginning of 
 the nineteenth century ; showing that his obser- 
 
214 ASTRONOMICAL DISCOVERY 
 
 vations, which had been condemned as in some 
 way erroneous, were really far more accurate than 
 might have been expected ; and further he has 
 shown that the beautiful instrument designed by 
 Airy, and called the Eeflex Zenith Tube, which 
 seemed to have unaccountably failed in the purpose 
 for which it was designed, was really all the time 
 accumulating observations of this new phenomenon, 
 the Variation of Latitude. Instead of Airy having 
 failed in his design, he had in Chandler's words 
 " builded better than he knew." 
 Constant Secondly, there is the modifying influence of 
 
 of Aberra- . -, . i ,1 ' i i 
 
 tionim- this new phenomenon on other phenomena al- 
 proved. rea( jy known, such, for instance, as that of 
 " aberration." We saw in the third chapter 
 how Bradley discovered this effect of the velo- 
 city of light, and how the measure of it is 
 obtained by comparing the velocity of light with 
 that of the earth. This comparison can be effected 
 in a variety of ways, and we should expect all the 
 results to agree within certain limits ; but this 
 agreement was not obtained, and Chandler has 
 been able to show one reason why, and to remove 
 some of the more troublesome differences. It is 
 impossible to give here an idea of the far-reaching 
 consequences which such work as this may have ; 
 so long as there are differences of this kind we 
 cannot trust any part of the chain of evidence, 
 and there is in prospect the enormous labour of 
 examining each separate link until the error is 
 found. The velocity of light, for instance, may be 
 
THE VARIATION OF LATITUDE 215 
 
 measured by a terrestrial experiment ; was there 
 anything wrong in the apparatus ? The velocity of 
 the earth in its journey round the sun depends 
 directly upon the distance of the sun : have we 
 measured this distance wrongly, and if so what 
 was the error in the observations made? These 
 are some of the questions which may arise so long 
 as the values for the Constant of Aberration are 
 still conflicting ; but it requires considerable know- 
 ledge of astronomy to appreciate them fully. 
 
 Another example will, perhaps, be of more Latitude 
 general interest. If the axis of the earth is Tide, 
 executing small oscillations of this kind, there 
 should be an effect upon the tides ; the liquid 
 ocean should feel the wobble of the earth's axis 
 in some way ; and an examination of tidal registers 
 showed that there was in fact a distinct effect. It 
 may cause some amusement when I say that the 
 rise and fall are only a few inches in any case ; 
 but they are unmistakable evidences that the 
 earth is not spinning smoothly, but has this kind 
 of unbalanced vibration, which I have compared 
 to the vibrations felt by passengers on an im- 
 perfectly engineered twin-screw steamer. A more 
 sensational effect is that apparently earthquakes 
 are more numerous at the time when the vibration 
 is greatest. We remarked that the vibration 
 waxes and wanes, much as that of the steamer Earth- 
 waxes and wanes if the twin-screws are not qua 
 running quite together. Now the passengers on 
 the steamer would be prepared to find that break- 
 
216 ASTRONOMICAL DISCOVERY 
 
 ages would be more numerous during the times 
 of vigorous oscillation ; and it seems probable that 
 in a similar way the little cracks of the earth's 
 skin which we call great earthquakes are more 
 numerous when these unbalanced vibrations are at 
 their maximum ; that is to say, about once every 
 seven years. This result is scarcely yet worthy of 
 complete confidence, for our observations of earth- 
 quakes have only very recently been reduced to 
 proper order ; but if it should turn out to be true, 
 it is scarcely necessary to add any words of mine 
 to demonstrate the importance of this rather un- 
 expected result of the Latitude Variation. 
 
 Finally I will mention another phenomenon 
 which seems to be at present more of a curiosity 
 than anything else, but which may lead to some 
 future great discovery. It is the outcome of obser- 
 vations which have been recently made to watch 
 these motions of the Pole ; for although there 
 seems good reason to accept Mr. Chandler's laws 
 of variation as accurate, it is necessary to establish 
 their accuracy and complete the details by making 
 observations for some time yet to come ; and there 
 could be no better proof of this necessity than the 
 The discovery recently made by Mr. Kimura, one of 
 ^heno- a those engaged in this watch of the Pole in Japan, 
 menon. Perhaps f can give the best idea of it by men- 
 tioning one possible explanation, which, however, 
 I must caution you may not be by any means the 
 right one. We are accustomed to think of this 
 great earth as being sufficiently constant in shape; 
 
THE VARIATION OF LATITUDE 217 
 
 if asked, for instance, whether its centre of gravity 
 remains constantly in the same place inside it, we 
 should almost certainly answer in the affirmative, 
 just as only twenty years ago we thought that the 
 North Pole remained in the same place. But it 
 seems possible that the centre of gravity moves 
 a few feet backwards and forwards each year 
 this would at any rate explain certain curious 
 features in the observations to which Mr. Kimura 
 has drawn attention. Whatever the explanation 
 of them may be, or to settle whether this expla- 
 nation is correct, we want more observations, 
 especially observations in the Southern Hemis- 
 phere ; and it is a project under consideration 
 by astronomers at the present moment whether 
 three stations can be established in the Southern 
 Hemisphere for the further observation of this 
 curious phenomenon. The question resolves itself 
 chiefly into a question of money ; indeed, most 
 astronomical projects do ultimately resolve them- 
 selves into questions of money ; and I fear the 
 world looks upon scientific men as insatiable in 
 this respect. One can only hope that on the 
 whole the money is expended so as to give a 
 satisfactory return. In this instance I have no 
 hesitation in saying that an immediate return of 
 value for a comparatively modest expenditure is 
 practically certain, if only in some way we can get 
 the means of making the observations. 
 
 ^ 4*-** 
 
218 ASTRONOMICAL DISCOVERY 
 
 It would be natural, at the conclusion of this 
 brief review of some types of astronomical dis- 
 covery, to summarise the lessons indicated : but 
 there is the important difficulty that there appear 
 to be none. It has been pointed out as we pro- 
 ceeded that what seemed to be a safe deduction 
 from one piece of history has been flatly contra- 
 dicted by another ; no sooner have we learnt that 
 important results may be obtained by pursuing 
 steadily a line of work in spite of the fact that it 
 seems to have become tedious and unprofitable 
 (as in the search for minor planets) than we are 
 confronted with the possibility that by such 
 simple devotion to the day's work we may be 
 losing a great opportunity, as Challis did. We 
 can scarcely go wrong in following up the study 
 of residual phenomena in the wake of Bradley ; 
 but there is the important difficulty that we may 
 be wholly unable to find a clue for the arrange- 
 ment of our residuals, as is at present largely the 
 case in meteorology. And, in general, human 
 expectations are likely to be quite misleading, as 
 has been shown in the last two chapters ; the 
 discoveries we desire may lie in the direction 
 precisely opposite to that indicated by the best 
 opinion at present available. There is no royal 
 road to discovery, and though this statement may 
 meet with such ready acceptance that it seems 
 scarcely worth making, it is hoped that there may 
 be sufficient of interest in the illustrations of its 
 truth. 
 
THE VARIATION OF LATITUDE 219 
 
 The one positive conclusion which we may 
 derive from the examples studied is that dis- 
 coveries are seldom made without both hard work 
 and conspicuous ability. A new planet, even 
 as large as Uranus, does not reveal itself to a 
 passive observer : thirteen times it may appear 
 to such a one without fear of detection, until at 
 last it encounters an alert Herschel, who suspects, 
 tests, and verifies, and even then announces a 
 comet so little did he realise the whole truth. 
 Fifteen years of unrequited labour before Astrsea 
 was found, nineteen years of observation before 
 the discovery of nutation could be announced : 
 how seldom do these years of toil present them- 
 selves to our imaginations when we glibly say 
 that ** Bradley discovered nutation," or " Hencke 
 discovered Astrsea " ! That the necessary labour 
 is so often forgotten must be my excuse for re- 
 calling attention to it somewhat persistently in 
 these examples. 
 
 But beyond the fact that he must work hard, 
 it would seem as though there were little of value 
 to tell the would-be discoverer. The situation 
 has been well summarised by Jevons in his 
 chapter on Induction in the " Principles of 
 Science ; " and his words will form a fitting con- 
 clusion to these chapters : 
 
 "It would seem as if the mind of the great 
 discoverer must combine contradictory attributes. 
 He must be fertile in theories and hypotheses, 
 
220 ASTRONOMICAL DISCOVERY 
 
 and yet full of facts and precise results of ex- 
 perience. He must entertain the feeblest analo- 
 gies, and the merest guesses at truth, and yet he 
 must hold them as worthless till they are verified 
 in experiment. When there are any grounds 
 of probability he must hold tenaciously to an 
 old opinion, and yet he must be prepared at any 
 moment to relinquish it when a clearly contra- 
 dictory fact is encountered." 
 
INDEX 
 
 ABERRATION, 105-109, in, 112, 
 
 117, 118, 185, 188, 192, 214, 
 
 215 
 Accidental discovery, 15, 73, 
 
 121-154 
 
 Adams, 12, 45-85; resolution, 55 
 Airy, 32, 40-85, 214 
 Algiers, 130 
 Alleghenia, 26 
 Almucantar, 180, 181 
 Alphabet used for planets, 27 
 Anderson, Dr. T. C., 8, 142, 143, 
 
 144, 146 
 Anthelm, 142 
 Apollo, 9 
 Argon, 109 
 Ascension, 34 
 
 Assumption, forgotten, 196 
 Astraea, 22, 23,219 
 Astrographic chart, 122, 125, 130 
 AstronomicalJournal, 177-217 
 Astronomische Nachrichten, 52, 
 
 158 
 
 Astrophil, 143 
 Auwers, 142 
 
 BALL, Sir R, 24 
 Balliol College, 87 
 Banks, Sir J., 9 
 Barnard, E. E., 146, 220 
 Berlin, 181, 183, 184, 188, 193 
 Berlin star-map, 45, 66, 83, 124 
 Bessel, 192 
 Bettina, 26, 27 
 Birmingham, 142 
 
 321 
 
 "Black Drop" (in transit of 
 
 Venus), 30 
 Bliss, 114 
 Board of Visitors of Greenwich 
 
 Observatory, 63 
 Bode, u, 14, 15, 22 
 Bode's Law, 12, 13, 38, 43, 45, 
 
 52, 72, 76, 77, 84 
 Bourdeaux, 130 
 Bouvard, 39, 40, 42, 48, 49, 50, 
 
 61 
 Bradley, 39, 86-120, 188-192, 
 
 213,214, 218, 219 
 Bradley, John, 1 1 5 
 Bremen, 20 
 Bridstow, 87, 88, 94 
 Briggs, 119 
 Brinkley, 192 
 British Association, 63 
 Briinnow, 193 
 
 CALIFORNIA, 26 
 
 Cambridge (Mass.), 180, 184, 
 
 1 88 
 Cambridge Observatory, 23, 42, 
 
 49,52,63,65,66, 135, 193 
 Cambridge University, 68-71, 
 
 114 
 
 Cape Observatory, 123, 124, 130 
 Cards, n 
 Cassini II., 1 56 
 Catania, 130 
 Ceres, 14-22 
 Chacornac, 124 
 Challis, 49-54, 63-68, 71, 85, 218 
 
222 
 
 INDEX 
 
 Chandler, S. C., 118, 177-217 
 
 Chapman's " Homer," 2 
 
 Chicago, 157 
 
 Chromosphere, 170 
 
 Clarke, C. C., 2 
 
 Coelostat, 94 
 
 Columbus, 63 
 
 Comet, 4-8, 88, 108, 117, 123, 
 
 125 
 
 Commission, planetary, 27 
 Common, A. A., 124, 127 
 Compte Rendu, 62 
 Comstock, 202 
 Conference, Astrographic, 125- 
 
 136 
 
 Copernicus, 79, 95 
 Cordoba, 130, 199 
 Cornu, 210-213 
 Corona, 170-175 
 Cosmos (Humboldfs), 160 
 
 DELAMBRE, 157 
 
 Deviation of Pole, 187 
 
 Disc of Neptune, 44, 64, 79 
 
 Disc of Uranus, 4-7 
 
 Dorpat, 192 
 
 Doublet (photographic), 127-129 
 
 Draconis, -y, 96-104 
 
 Draconis, |8, 193 
 
 Driessen, 23 
 
 Dry plate, 122 
 
 Dublin, 192 
 
 EARTHQUAKES, 215 
 Earth's Pole, 177-217 
 Eccentricity, 41, 83 
 Eclipses, 1707176 
 Edinburgh, 143 
 Eduarda, 26 
 Egeria, 22 
 Endymion, 25 
 Eriphyla, 26 
 
 Eros, 25, 26, 28, 35, 37, 68 
 Eulerian, 200, 209 
 
 Evelyn, 26 
 
 Exposure, times of, 122, 131 
 
 FACUL^E, 170 
 
 Faraday, 201 
 
 Flamsteed, 39, 53, 115 
 
 Fleming, Mrs., 142 
 
 Flora, 22 
 
 Foulkes, Martin, 94 
 
 French Academy, 43, 51, 62 
 
 GALILEO, 95, 163 
 
 Galle, 44, 45, 47, 66, 67, 83 
 
 Gasparis, 22 
 
 Gauge (railways), 56 
 
 Gauss, 17-20 
 
 Geminorum, H., 4 
 
 George III., 8, 10 
 
 "Georgian," 11 
 
 Georgium Sidus, 8, 10, II 
 
 Gill, Sir D., 32, 34, 35, 123 
 
 Gilliss, 32 
 
 Gotha, 20 
 
 Gould, 32 
 
 Graham, 22, 23 
 
 Gravitation, law of, 38, 45, 59, 
 84, 105 
 
 Greaves, 119 
 
 Greenwich Observatory, 48-64, 
 88, 89, 114-117, 130, 160-169, 
 182, 192, 193, 206, 213 
 
 Gregory, 93, 119 
 
 HALE, G. E., 170, 171 
 
 Hall, A., 184, 185 
 
 Halley, 88-92, 1 08, 112-116, 119 
 
 Hansen, 41, 59 
 
 Harkness, 184 
 
 Hartwig, 142 
 
 Harvard College Observatory, 
 
 128, 142, 144, 145 
 Hebe, 22 
 Hegel, 15 
 Heidelberg, 145 
 Heliometer, 32, 34 
 
INDEX 
 
 223 
 
 Helium, 109 
 
 Helsingfors, 130 
 
 Hencke, 22. 23, 64, 153, 219 
 
 Henry brothers, 124-129 
 
 Herschel, Sir John, 63, 75, 83 
 
 Herschel, Sir William, 2-11, 39, 
 
 44,82,219 
 
 Herschel (Uranus), 11, 12 
 Hind, 22, 23, 25, 142 
 Hooke, 96, 97 
 Hubbard, 184 
 Humboldt, 160 
 Hussey, Rev. T. J., 40, 42 
 Hygeia, 22 
 
 ILMATA, 26 
 
 Industria, 26 
 
 Ingeborg, 26 
 
 Instruments at Greenwich, 114- 
 
 116 
 Iris. 22, 23, 32, 35 
 
 JANSON, 142 
 Jevons, 219 
 Johnson, M., 156, 1 60 
 Juno, 9, 21, 22 
 
 Jupiter, 9, 28, 43, 49> 5> 6l '> 
 satellites, 92, 117 
 
 KEATS, 1-3, 7, 8 
 Keill, 94, 112, 119, I5 6 
 Kelvin, Lord, 196, 197 
 Kepler, 95, 142 
 Kew, 95, 96, 1 88, 190 
 Kiel, 141 
 Kimura, 2 1 6 
 Konigsberg, 192 
 Kiistner, 118, 181, 183 
 
 LALANDE, 7, 11, 107, 157 
 Lameia 26 
 Laplace, 61 
 La Plata, 130 
 
 Latitude variation, 99, 100, 117 
 118, 177-217 
 
 Lemonnier, 39, 53, 157 
 Le Verrier, 12, 43-85 
 Libussa, 26 
 Lick Observatory, 152 
 Liouville's Journal, 73 
 Lisbon, longitude of, 92 
 London, 23, 25, 96 
 Long, 157 
 Longitude, 92, 117 
 Lowth, Bishop, 119 
 Lyrae, a, 184, 196 
 
 MACCLESFIELD, Earl of, 94, 113 
 
 Madler, 192 
 
 Magnetic observations, 161, 164, 
 
 174 
 
 Magnitude equation, 135 
 Markree, 23 
 
 Mars, 9, 28, 32, 34, 35, 91 
 Mayer. 39 
 
 Measurement of plates, 132-135 
 M&anique Celeste, 61 
 Melbourne, 130, 193 
 Memorandum (Adams), 55 
 Mercury, 9 
 Messier, 7 
 Meteorites, 59 
 Meteors (November), 60 
 Metis, 22, 23 
 Micrometer, 5, 133 
 Milky Way, 125 
 Minerva, 9 
 Minor planets, 1 3-28 
 Minor planets tables, 22, 24, 26 
 Mistakes, 71-83 
 Molyneux, Samuel, 94-96, 101, 
 
 104 
 
 Monte Video, 130 
 Moon, tables of, 117 
 
 NAMES of minor planets, 22-28 
 Nasmyth, 162 
 " Nautieal Almanac," 1 1 
 Nebula, 124, 146-152 
 
224 
 
 INDEX 
 
 Neptune, n, 12, 38-85, 124 
 New College Lane, 112 
 Newcomb, Simon, 81, 183, 184, 
 
 195-202, 207, 208 
 New stars, 121, 140-154 
 Newton, 38, 84, 90-95, 105, 113 
 New York, longitude, 92 
 Ninina, 26 
 Northleach, 87 
 Northumberland, 65 
 Nova Geminorum, 141, 145, 146 
 Nova Persei, 143, 146-152 
 Nutation, 99, 100, no, 115, 117, 
 
 118, 188, 219 
 
 Observatory (magazine), 26 
 
 Ocllo, 26 
 
 Olbers, 20-22 
 
 Olympic games, 119 
 
 Oriani, 15 
 
 Ornamenta, 26 
 
 Oxford University, 87-89, 94, 
 
 105-119 
 Oxford University Observatory, 
 
 121,130,132,136,142,145,154 
 
 PALERMO, Observatory of, 18 
 
 Palisa, 26 
 
 Pallas, 9, 21, 22 
 
 Parallax, 34, 91, 95-98, 109, 185 
 
 Paris, 130 
 
 Parkhurst, J. A., 145 
 
 Parthenope, 22 
 
 Peirce, 73, 80-83 
 
 Pendulum, 117 
 
 Perseus, 8, 143 
 
 Personal equation, 31, 134, 135, 
 
 185 
 
 Perth, 130 
 Perturbations of Uranus, 1 2, 42, 
 
 51,54, 55,6i,75 
 Peters, 188, 192 
 Phaetusa, 26 
 Philosopher, 201, 219 
 
 Philosophical Transactions, 3, 4, 9 
 Photographica, 26 
 Photographic methods, 24, 33, 
 36, 121-139; lenses, 125, 126 
 Photographs of sun, 163, 170- 
 
 173 
 
 Piazzi, 13-18, 22 
 
 Pickering, E. 0., 128, 144 
 
 Pittsburghia, 26 
 
 Plana, 61 
 
 Planetary distances, 13; com- 
 mission, 27 ; numbering, 27 
 
 Planets by photography, 24 
 
 Pole Star (Polaris), 177, 178, 
 192, 193 
 
 Pond, 192, 213 
 
 Potsdam, 130, 181 
 
 Pound, Mrs., 104, 110-112 
 
 Pound, Rev. James, 89-94, 104, 
 
 "5 
 
 Prague, 181 
 Precession, 96, 178 
 Prymno, 26 
 Puiseux, 32 
 Pulfrich, 154 
 Pulkowa, 181-188, 213 
 
 QUADRANTS at Greenwich, 116 
 
 RADIUM, 175 
 
 Radius vector, 52-58, 60-62, 79, 
 
 83 
 
 Rayleigh, Lord, 109 
 Records before discovery, 144 
 Reflector, 93, 127, 128 
 Reflex zenith tube, 192, 214 
 Refraction, 96, 101-103, IJ 7 
 Refractor, 93, 128 
 Roseau, 133 
 Residual phenomena, 108-110, 
 
 118, 120, 218 
 
 Rigaud, S. P., 87, 115, 119 
 Rome, 130 
 Rothschild, 27 
 
INDEX 
 
 Royal Astronomical Society, 40, ' Toulouse Observatory, 1 30 
 
 225 
 
 47, 68, 74, 124, 155, 157 
 Royal Society, 4, 9, 10, 92, 94 
 
 SAMPSON, R. A., 74-76, 84 
 
 San Fernando, 130 
 
 Santiago, 130 
 
 Sappho, 32, 35 
 
 Saturn, 9, 43,61, 149, 150 
 
 Savile, Sir H., 119 
 
 Savilian professorship, 87-94, 
 
 108-119 
 
 Schmidt, Julius, 142, 160 
 Schuster, A., 169 
 Schwabe, 155-163, 176, 177 
 Sheldonian Theatre, 119 
 Sherbourn, 87 
 Solar eclipse, 26, 170-176 
 Spectro-heliograph, 170, 171 
 Star-maps, 45, 65, 83, 124 
 " Star-trap," 24 
 Stereo-comparator, 154 
 Stone, E. J., 32 
 Struve, 184, 1 88, 192 
 Sun's distance, 28-37 
 Sun-spots, 155-176 
 Sydney Observatory, 130 
 
 TACUBAYA OBSERVATORY, 130 
 Telescopes, 92, 124-129 
 Thames River, 105 
 Themistocles, 119 
 Theoria Motus, 17 
 Theory and observation, 208 
 Thomson, Sir W., 196, 197 
 Tides, 215 
 Titius, 13 
 
 Tycho Brahe, 95, 140, 142 
 
 URANUS, 2-14, 25, 38-85, 144, 
 219 
 
 VARIABLE stars, 140 
 Variation of latitude, 99, 100, 
 
 117, 118, 177-217 
 Venus, 9, 79 ; diameter of, 92 ; 
 
 transit of, 28-32, 34 
 Vesta, 21, 22 
 Victoria, 22, 25, 32, 35 
 Von Zach, 20 
 
 WALLACE, 119 
 
 Wansted, 88-94, 104, no, 115. 
 
 1 88, 190 
 Ward, 119 
 Washington Observatory, 184- 
 
 188, 193, 196,213 
 Weather and sun-spots, 161, 
 
 167-169 
 Weyer, 193 
 Whiteside, 112 
 Williams, Mrs. E., no, in 
 Wind -vane, revolutions, 167- 
 
 169 
 
 Winnecke, 32 
 Wolf, Dr. Max, 145 
 Wolf, Rudolf, 163 
 Wren, Sir 0., 119 
 
 YERKES Observatory, 145, 146, 
 152, 157, 170, 176 
 
 ZEISS, 154 
 
 Zodiac, 64, 124, 137 
 
 THE END 
 
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