32 



PHYSICAL ASTRONOMY. M. LEVERRIER MR ADAMS. 



PISS. VI. 



deduced a complete list of elements as regards longi- 

 tude ; and diminished the mean distance considerably. 

 The true longitude by this calculation differed only 

 1 from his previous estimation. He finds for the 

 mean distance from the Sun 36 times that of our 

 Earth (that of Uranus being 19), the period 217 

 years, and the mass 5-^-5- of the Sun's. Assuming 

 the density of the planet to be the same as that of 

 Uranus, he conjectures that the apparent diameter 

 will be 3"' 3 ; that it will therefore have a visible disk 

 sufficient to distinguish it from a fixed star, and that 

 its brilliancy should equal that of a star of the 8th 

 or 9th magnitude. As the result of these supposi- 

 tions, he found that the whole errors in the places 

 of Uranus from 1781 to 1844 were reconciled within 

 quantities amounting but in one instance to 5" ; that 

 the ancient observations of the 18th century were 

 reconciled within 7" or 8" ; and the oldest observa- 

 tion of all, that of Flamsteed in 1690, had an out- 

 standing error of 20", a quantity very far from exces- 

 sive, considering the state of astronomy at that period. 

 (137.) No one who read at the time the abstract of this 

 resulting in remarkable paper in the Comptes Rendus failed to 

 ler d of " ke struc k ^l 1 i*> not on l v as regarded the weighty 

 Neptune by matter, thus publicly announced, but also on account 

 M. Galle. of the calm and well-grounded conviction which the 

 author manifested in the truth of his bold conclusions, 

 and the definite manner in which he gives the chal- 

 lenge to practical astronomers to verify or disprove 

 them. " Since Copernicus declared" (according to the 

 prevalent tradition) " that when means should be dis- 

 covered for improving the vision, it would be found that 

 Venus had phases like the moon, nothing," writes Mr 

 Airy, "so bold, and so justifiably bold, has been uttered 

 in astronomical prediction." M.Leverrier had hastened 

 his calculations in anticipation of the approaching 

 opposition of the new planet in the autumn of 1846, 

 but it is very doubtful whether astronomers would 

 have made the discovery at that time, but for his per- 

 sonal application to M. Galle, then assistant-astro- 

 nomer at Berlin, where a powerful refractor suitable 

 for the search existed. So ardent a conviction in a 

 manner compelled the proof which the geometer 

 claimed, and M. Galle, whose intelligence and zeal 

 are well known, pointed his telescope to the sky the 

 very evening that M. Leverrier's letter reached him. 

 Fortunately provided with a newly published star 

 map by Brenicker of that region of the heavens, which 

 was not at that time diffused generally amongst 

 European observatories, he detected that same night 

 (the 23d September 1846) a star-like body of the 8th 

 magnitude, not noted in the star chart, therefore a 

 wandering body, having a manifest disk from 2J" to 

 3" in diameter, and distant only fifty-four minutes of 

 a degree from the predicted place. 

 (138.) It will be remarked that the discovery in question 



was anticipated and completed in France and Ger- 

 many alone; England had no direct participation. We 

 mustnow, however, state briefly what occurred there of 

 a similar character at the same time, and even earlier. 



Mr JOHN COUCH ADA.MS, when a student at St (139.) 

 John's College, Cambridge, in 1841, formed the de- 

 sign of detecting the position of a perturbing planet 

 which should account for the anomalous motions of 

 Uranus. He made a preliminary essay on the pro- 

 blem in 1843, assuming the distance of the suspected 

 body from the Sun to be double that of Uranus. 

 I learn from good authority that he obtained a place 

 for the unseen planet not very different from that 

 which he finally adopted. Early in 1844 he ob- 

 tained from Greenwich the valuable series of places 

 of Uranus which were afterwards in like manner ap- 

 plied for by M. Leverrier. In September 1845, he 

 communicated to Professor Challis the elements of the 

 new planet's orbit (neglecting the inclination) and an 

 ephemeris of its geocentric place ; and in October he 

 transmitted the elements also to the Astronomer Royal. Mr Adams 

 This, it will be observed, was soon after M. Leverrier's preceded 

 attention was first directed to the subject, and nine r ^ r in a 

 months previous to his announcement of the locality similar in- 

 where the new body should be sought. Mr Adams vestiga- 

 afterwards repeated his calculation with a mean dis- ll 

 tance ^ O tli less than before, and considered himself 

 warranted, by the improvement thus produced on the 

 residual errors, in inferring that a farther consider- 

 able diminution of the mean distance would satisfy 

 the observations still better. This was communicated 

 to Mr Airy on the 2d September 1846 ; subsequently 

 therefore to the publication of M. Leverrier's Ele- 

 ments. Mr Adams, in communicating his results (at 

 a later time) to the Astronomical Society, with charac- 

 teristic modesty says, " I mention these dates merely 

 to show that my results were arrived at independently, 

 and previously to the publication of M. Leverrier, 

 and not with the intention of interfering with his just 

 claims to the honors of the discovery, for there is no 

 doubt that his researches were first published to the 

 world and led to the actual discovery of the planet by 

 Dr Galle." 



And such is no doubt the fact. The priority of Mr 

 Adams in the mathematical investigation is as certain 

 as that the researches of M. Leverrier alone produced discovered 

 the discovery of Neptune. Even the search for the in conse- 

 planet which took place in August and September 1 uence - 

 1846 at Cambridge by Professor Challis, was not oc- 

 casioned by Mr Adams' researches only ; it was the 

 near coincidence of the longitude assigned by Mr 

 Adams in the previous October with that published 

 by M. Leverrier in June 1846 which induced Mr 

 Airy to suggest this investigation of the heavens, 

 and to offer (if need were) himself to bear the ex- 

 pense. Had the planet been discovered 1 at Cambridge 



1 The planet was indeed seen at Cambridge by Mr Challis, for it was recorded more than once amongst the numerous fixed 

 stars whose places were taken down in the progress of the search ; but as the comparisons of the " sweeps" were not made at 

 the time, the discovery was anticipated by M. Galle. 



