1881 to 1890-- " 83 "
1891 to 1900-- " 180 announcements In 1901 " 36 "
" 1902 " 50 "
" 1903 " 41 "
--- Total 609
[_N.B._--Many of the more recent announcements turned out to refer to old discoveries.]
[Sidenote: Scarcity of names.]
The known number of these bodies has accordingly increased so rapidly as to become almost an embarra.s.sment; and in one respect the embarra.s.sment is definite, for it has become quite difficult to find _names_ for the new discoveries. We remember with amus.e.m.e.nt at the present time that for the early discoveries there was sometimes a controversy (of the same kind as in the case of Ura.n.u.s) 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 G.o.ddesses, 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. .h.i.therto considered unsuitable; so, ultimately, the opposition to the name Victoria was withdrawn. Later still the restriction to feminine names has been broken through; one planet has been named Endymion, and another, of which we shall presently speak more particularly, has been called Eros. But before pa.s.sing to him you may care to look at some of the names selected for others:--
248 Lameia 250 Bettina 261 Prymno 264 Libussa 296 Phaetusa 340 Eduarda 341 California 350 Ornamenta 357 Ninina 385 Ilmatar 389 Industria 391 Ingeborg 433 Eros 443 Photographica 457 Alleghenia 462 Eriphyla 475 Ocllo 484 Pittsburghia 503 Evelyn
[Sidenote: The provisional letters.]
In connection with No. 250 there is an interesting little history. In the _Observatory_ for 1885, page 63, appeared the following advertis.e.m.e.nt:--"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 a.s.signing 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 discovered 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 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 pa.s.sing on, the provisional system which was adopted to fill up the interval required 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 discoveries 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, however, 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 AA, AB, AC, &c.; after completing the alphabet again, the letter B was prefixed, and so on; and astronomers began to fear that they had before them a monotonous prospect of continually adding new planets, varied by no incident more exciting than starting the alphabet over again after every score.
Fortunately, however, on running through it for the fifth time, an object of particular interest 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.
[Sidenote: Transit of Venus.]
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 measurement of this important quant.i.ty, which lies at the root of all our knowledge of the exact ma.s.ses and dimensions of not only the sun, but of the planets as well.
[Ill.u.s.tration: FIG. 1.]
[Sidenote: The "Black Drop."]
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 path from that seen by another observer, as will be clear from the diagram. If the size of the earth, the distance of the sun, and the _relative_ distance of Venus be known, it can be calculated 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 quant.i.ty, 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, it was hoped to determine this very important quant.i.ty, the sun's distance. It was known from previous experience that there were certain difficulties in observing very exactly the beginning and end of the transit. There was an appearance called the "Black Drop," which had caused 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 antic.i.p.ated difficulties by careful practice beforehand; models were 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 appearances were found to be totally different in character from those shown by the model; chiefly, perhaps, because it had been impossible to imitate with a model the effect of the atmosphere which surrounds the planet Venus. Observers trained beforehand, using similar instruments, 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.
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 =93-1/4= million (Airy, from British observations of 1874), =92-1/2= million (Stone, from British observations of 1882), and =91-1/2= million (Puiseux, from French observations), a range of 1-3/4 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.
[Sidenote: Modern method for sun's distance.]
[Sidenote: Dr. Gill's expedition to Ascension.]
We have said that the _relative_ distance of Venus from the sun is accurately known from observations of the exact time of revolution. It is easy to see that these times of revolution can be measured accurately by mere acc.u.mulation. 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 distances 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 surrounding it on the same plate, and then by careful 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 photographs taken at stations a known distance apart we can find the distance of the planet from the earth; and hence, as above remarked, the distance 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 by a known interval. For in that interval the station will have been carried by the earth's rotation 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 since in 1878, when Dr. Gill set out on his enterprise of determining the sun's distance, photography 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 then known, was selected for observation with the heliometer.
And now it will be seen why the discovery of 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.
[Sidenote: Victoria, Iris, and Sappho.]
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, somewhat difficult to allow for with certainty, is introduced 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 of Mars, in spite of the fact that they were sensibly 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 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 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, preparations 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 telescopes 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 completed, 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 research, however apparently unimportant or monotonous, which we can afford to neglect. Just when 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.
THE DISCOVERY OF NEPTUNE
[Sidenote: Search for definite objects.]
In the last chapter we saw that the circ.u.mstances under which planets were discovered varied considerably. 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 discovery 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 discovery 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 Ura.n.u.s. I will first briefly recall the main facts relating to the actual discovery.
[Sidenote: Disturbance of Ura.n.u.s.]
After Ura.n.u.s had been discovered and observed sufficiently long for its...o...b..t to be calculated, it was found that the subsequent position of the planet did not always agree with this...o...b..t; 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 Ura.n.u.s was found to have been observed, under the mistaken 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.
[Sidenote: Suspicion of perturbing planet.]
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 correction for the disturbing action of other planets had been applied, it was still impossible to reconcile the old observations with the orbit calculated from the new ones. The idea accordingly grew up that there might be some other body or bodies attracting the planet and causing these discrepancies. Here again it is not easy to say exactly when this notion arose, but it was certainly existent in 1834, as the following letter to the Astronomer Royal will show. I take it from his well-known "Account of some Circ.u.mstances historically connected with the Discovery of the Planet exterior to Ura.n.u.s," which he gave to the Royal Astronomical Society at its first meeting after that famous discovery (Monthly Notices of the R.A.S., vol.
iii., and Memoirs, vol. xvi.).
NO. 1.--_The_ REV. T. J. HUSSEY _to_ G. B. AIRY.
"'HAYES, KENT, _17th November 1834_.
"'With M. Alexis Bouvard I had some conversation 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 _Ura.n.u.s_, I was led to examine closely Bouvard's tables of that planet. The apparently inexplicable discrepancies between the ancient and modern observations suggested to me the possibility of some disturbing body beyond _Ura.n.u.s_, not taken into account because unknown. My first idea was to ascertain some approximate place of this supposed 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 conversation 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 conjectured there were two planets beyond _Ura.n.u.s_. Upon my speaking of obtaining the places empirically, and then sweeping closely for the bodies, he fully acquiesced in the propriety of it, intimating 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 we might expect an eccentricity [inclination?] approaching rather to that of the old planets than 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-gla.s.s, 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 s.p.a.ce 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:--
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