[Sidenote: Airy's scepticism.]
NO. 2.--G. B. AIRY _to the_ REV. T. J. HUSSEY.
"'OBSERVATORY, CAMBRIDGE, _1834, Nov. 23_.
"'I have often thought of the irregularity of _Ura.n.u.s_, 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 determining the place of a planet which produced it. I am sure it could not be done till the nature of the irregularity was well determined from several successive revolutions.'"
[Sidenote: Le Verrier's papers.]
[Sidenote: Planet to be detected by disc.]
[Sidenote: Galle's discovery of the planet.]
Although only a sentence or two have been selected from Airy's reply (he was not yet Astronomer 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 solution. 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 November 1845. The great French astronomer Le Verrier, on November 10, 1845, read to the French Academy a paper on the Orbit of Ura.n.u.s, considering specially the disturbances produced by Jupiter and Saturn, and showing clearly that with no possible orbit could the observations be satisfied. On June 1, 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 Ura.n.u.s. And a.s.suming, in accordance with Bode's Law, that the distance of this new planet from the sun would be about double that of Ura.n.u.s (and it is important to note this a.s.sumption), 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 31st, giving a rather completer discussion, and arriving at the conclusion that the planet should be recognisable from its disc. This again is an important point. We remember that in the discovery of Ura.n.u.s 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 a.s.sumed for the new planet) the disc would be practically unrecognisable, and as we shall presently see, this a.s.sumption was made in some searches for the planet which had been commenced even before the publication of this third paper. Le Verrier's courageous announcement, which he deduced from a consideration of the ma.s.s of the planet, that the disc should be recognisable, led immediately to the discovery of the suspected body. He wrote to a German astronomer, Dr.
Galle (still, I am glad to say, 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 same night Dr. Galle, by comparing a star map with the heavens, found the planet.
[Sidenote: Adams' work publicly announced.]
To two points to which I have specially called attention in this brief summary--namely, the preliminary a.s.sumption that the planet would be, according to Bode's Law, twice as far away as Ura.n.u.s; secondly, the confident a.s.sertion 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 announcement of the finding of a planet in this way, the calculation of its place from a belief in the universal 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 circ.u.mstances caused a profound sensation throughout 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 circ.u.mstances, 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 familiar with the story in its abridged form, for it has been scattered broadcast through literature.
In England it generally takes the form of emphasising the wickedness or laziness of the 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.
[Sidenote: Facts undoubted.]
There is fortunately no doubt whatever about any of the _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 satisfactory result that there is practically no doubt about any of the facts. As to the proper interpretations of them there certainly may be wide differences of opinion, nor does this circ.u.mstance 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, 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.
[Sidenote: Airy's "Account."]
[Sidenote: "A movement of the age."]
Airy's account was, as above stated, given to the Royal Astronomical Society at their first meeting (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. His actual words are these: "The princ.i.p.al steps 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 princ.i.p.ally 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 or powers best suited to the different parts of the researches."
[Sidenote: Airy under-estimated Adams' work.]
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 ent.i.tled him to such mention; claims indeed which he proceeded immediately to make clear. It seems almost certain that Airy entirely under-estimated the value of Adams' work throughout. 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 Ura.n.u.s 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 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 Greenwich some eight years earlier.
No. 6.--PROFESSOR CHALLIS _to_ G. B. AIRY.
"'CAMBRIDGE OBSERVATORY, _Feb. 13, 1844_.
[Sidenote: Challis mentions Adams to Airy, and suggests Adams' visit to Greenwich.]
"'A young friend of mine, Mr. Adams of St. John's College, is working at the theory of _Ura.n.u.s_, and is desirous of obtaining errors of the 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 Ura.n.u.s_ besides that of _Jupiter's_ ma.s.s.'
"My answer to him was as follows:--
No. 7.--G. B. AIRY _to_ PROFESSOR CHALLIS.
"'ROYAL OBSERVATORY, GREENWICH, _1844, Feb. 15_.
"'I send all the results of the observations of _Ura.n.u.s_ made with both instruments (that is, the heliocentric errors of _Ura.n.u.s_ in longitude and lat.i.tude 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 Ura.n.u.s_ except in increasing the two equations which depend on _Jupiter_ by 1/50 part. As constants have been added (in the printed tables) to make the equations positive, and as 1/50 part of the numbers in the tables has been added, 1/50 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.
"'CAMBRIDGE OBSERVATORY, _Feb. 16, 1844_.
"'I am exceedingly obliged by your sending so complete a series of tabular errors of _Ura.n.u.s_.... 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 calculations respecting the perturbation of the orbit of _Ura.n.u.s_ by a supposed ulterior planet, and has arrived at results which he would be glad to communicate to you personally, if you could spare 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 Inst.i.tute. 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 Royal Observatory, Greenwich, in my absence and left the following important paper:--
No. 11.--J. C. ADAMS, Esq., _to_ G. B. AIRY.
[Sidenote: Adams' announcement of the new planet.]
"'According to my calculations, the observed irregularities in the motion of _Ura.n.u.s_ may be accounted for by supposing the existence of an exterior planet, the ma.s.s and orbit of which are as follows:--
Mean distance (a.s.sumed nearly in accordance with Bode's Law) 38.4 Mean sidereal motion in 365.25 days 130'.9 Mean longitude, 1st October 1845 323 34 Longitude of perihelion 315 55 Eccentricity 0.1610.
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