921 



NEPTUNE. 



NEPTUNE. 



922 



from observations made subsequently to its discovery, assign 324 48 

 as its longitude for the same day. The difference therefore between the 

 theoretical and the actual places of the planet amounts to only 1 49'. 

 The errors in the longitude of Uranus were shown to be accounted 

 for most satisfactorily by the disturbing action of the hypothetic 

 planet. 



" Thus it appears that as early as the month of October, 1845, seven 

 months before any other person had arrived at a similar conclusion, 

 Mr. Adams had solved the inverse problem of planetary perturbation ; 

 that by means of his solution he had discovered, theoretically, the 

 existence of a planet exterior to Uranus ; and that he had assigned to 

 the unknown body a place in the heavens which was subsequently 

 found to differ little more than one degree from its actual place. All 

 that was now wanting therefore to assure, both to Mr. Adams and, to 

 his country, the undivided honour of one of the noblest discoveries 

 recorded in the annals of science, was some zealous observer to give 

 effect to his results by carefully searching the heavens in the vicinity 

 of the place indicated by his theory as that occupied by the planet. 

 If such a scrutiny had been undertaken, and prosecuted for some time, 

 it would beyond all doubt have resulted in the actual discovery 'of the 

 planet, and the name of Adams would have been alone associated with 

 that remarkable triumph of science. Such a consummation was not 

 destined to be the reward of Mr. Adams; but this circumstance 

 does not detract in the slightest degree from the merits of his 

 researches ; for it is now universally admitted that he was the first 

 theoretical discoverer of the planet, and that as far as the task of 

 the mathematician was concerned he left no part of the problem 

 relative to the determination of its actual position to be completed by 

 others." * 



Shortly after receiving Mr. Adams's communication respecting the 

 trans-Uranian planet, Mr. Airy wrote to Mr. Adams requesting to know 

 whether the errors in the radius vector of Uranus were accounted for 

 by the action of the undiscovered planet as satisfactorily as the errors 

 in longitude. He considered that the explanation of the errors of the 

 radius vector would form an experimentum cruets for testing the 

 validity of Mr. Adams's researches, and he has stated that if a satis- 

 factory reply had been returned to his communication he would at 

 once have taken steps to institute a search for the planet.t Mr. Adams 

 inadvertently omitted to furnish Mr. Airy with the desired information. 

 This is more especially to be regretted as it would not have cost him 

 the slightest effort to have returned a satisfactory reply to the 

 Astronomer Royal. 



It is right to state that the remarkable researches of Mr. Adams 

 were not hitherto published. In fact it does not appear that at this 

 period any one was acquainted with his labours except a few of his 

 friends at Cambridge and the Astronomer Royal. 



Allusion has been made to the failure of Bouvard's attempt to con- 

 struct table* of Uranus capable of representing all the observations ol 

 the planet. A similar attempt of his nephew, Eugene Bouvard, in 1845 

 was not attended with much greater success. It was probably this 

 circumstance which induced the late M. Arago, in the same ye.ir, to 

 propose to M. Le Verrier, as a subject of investigation, the irregularities 

 in the motion of Uranus. M. Le Verrier was engaged at the time in 

 researches on the movements of comets, but he did not hesitate to 

 adopt the suggestion of M. Arago, and laying aside his cometary 

 labours for a future occasion, he commenced a rigorous investigation of 

 the theory of Uranus-. On the 10th of November, 1845, he communi 

 cated to the Academy of Sciences of Paris an account of the first part o; 

 his researches. They consisted in a new determination of the element? 

 of the planet and a careful investigation of the perturbations producec 

 in its motion by all the known bodies of the planetary system which 

 are capable of exercising a sensible influence. On the 1st of June 

 1846, M. Le Verrier communicated to the Academy of Sciences an 

 account of the second part of his researches on the theory of Uranus 

 Finding it impossible to account satisfactorily for all the observations 

 of the planet by the existing theory, he was led to consider whether 

 the outstanding discordances between theory and observation migh 

 nt be due to the disturbing action of some planet revolving beyouc 

 Uranus, and the problem which now presented itself to him was to 

 determine the whereabouts of the unknown body, corresponding to a 

 given instant of time. The final result which he deduced at this 

 stage of his labours was, that on the 1st of January, 1847, the true 

 hdiocentric longitude of the planet was 325, and thiit the probable 

 error did not exceed 10. 



The close accordance exhibited between the results arrived at by Mr 

 Ailams and M. Le Verrier, independently of each other, afforded strong 

 evidence in favour of the theory on which they were founded. Mi- 

 Airy was consequently induced to suggest to Professor Chain's, o 

 Cambridge, the desirableness of instituting a search for the planet wit] 

 the large telescope of the Observatory, known as the Northumberland 

 refractor. Professor Challis adopted the views of the Astrouome 

 Royal, and commenced a systematic search for the planet on the 29tl 

 of July. 



On the 31st of August, 1846, M. Le Verrier communicated to th 

 Academy of Sciences an account of the third part of his researches o 



* Grant' 'History of Physical Astronomy,' p. 173. 

 t Mem. At. Soc., Tol. xvl. 



theory of Uranus. The following are his final results in regard to 

 ie orbit of the unknown planet : 



Semi-axis of the orhit .... 



Sidereal revolution .... 



Excentiicity ...... 



Longitude of the perihelion ... 

 Mean longitude, January 1, 1817 . . 



Mass ....... 



True heliocentric longitude, January 1, 1847 

 Distance from the sun, January 1, 1847 . 



38-154 

 217-387 ycara. 



0-10761 

 281 45' 

 S1847' 

 1 



9300 

 320 32' 

 33-06 



On the 2nd of September, 1846, Mr. Adams communicated to the 

 Astronomer Royal the final results of his researches on the disturbing 

 jlanets. On this occasion he diminished the hypothetic value of the 

 mean distance from 38 to 37'5. The following are the values of the 

 ither elements as deduced by him. 



. 323^ 2' 



Mean longitude, October 1, 18-16 

 Longitude of the perihelion . 

 Escentricity .... 

 Mass ..... 



. 299 11' 

 . 0-12062 

 . 0-00015003 



On the 18th of September, 184B, M. le Verrier addressed a letter t 

 5 rofessor Encke, the Director of the Berlin Observatory, announcing 

 o him the results of his researches on the trans-Uranian planet, and 

 equesting him to institute a search for it in the region indicated by 

 theory. This letter was received at Berlin on the 23rd of the month. 

 On the same evening, Dr. Galle, Professor Encke's assistant, examined 

 the regions of the heavens indicated by M. Le Verrier, as comprising 

 the locus of the disturbing planet, with the assistance of the corres- 

 >ondiug map of the Berlin Academy, which had just been engraved, 

 and he found that a star of the eighth magnitude, situate very 

 lear the place assigned by M. Le Verrier to the disturbing planet, was 

 not laid down on the map. An examination of the same region on the 

 following evening established beyond doubt the interesting fact that 

 the object thus observed was the planet. The following comparison 

 will serve t.o show how nearly M. Le Verrier approximated in his 

 researches to the actual position of the planet : 



True heliocentiic longitude, September 23, 1846 . 326 57' 

 Longitude assigned by the theory of M. Le Ycrrier .326 



Difference between observation and theory . 57' 



Professor Challis, who had commenced a systematic search for the 

 planet, as early as the 29th of July, first received information of the 

 discovery of the planet by Galle, on the 1st of October. He had 

 already noted down the positions of 3150 stars comprised in the 

 region which he proposed to explore. He now proceeded to discuss 

 the results, in order to ascertain whether they included any places of 

 the disturbing planet. It appeared from this examination that he had 

 determined the position of the planet on the evening of August 4, and 

 also on August 12, and it consequently followed that his plan of search, 

 when carried into complete effect, would have infallibly led to the 

 discovery of the planet. He also observed a star on the 29t.h of 

 September, which seemed to have a disk, but unfortunately he did not 

 establish the real nature of the object, which turned out afterwards to 

 be the planet. 



By the unanimous concurrence of astronomers, the new member of 

 the planetary system has been called Neptune. It is plain from the 

 brief account which we have given, that its theoretical discovery is due 

 to the independent researches of Mr. Adams and M. Le Verrier. It is 

 true that the optical discovery of the planet was effected solely by the 

 aid of M. Le Verrier's indications, but it is equally true that the same 

 object might have been achieved by means of Mr. Adams' theoretical 

 results ; and indeed we have seen that Professor Challis obtained two 

 positions of the planet, solely by relying on the latter. 



Soon after the discovery of the planet Neptune, it was found that 

 it had been observed as a star by Lalande, on the 10th of May, 1795. 

 This circumstance enabled astronomers to determine the orbit of the 

 planet with a greater degree of precision than could otherwise have 

 been expected until after the lapse of several years. The following 

 elements of the orbit of the planet are due to the late Mr. Walker, of 

 Washington, U. S. 



Mean distance 30-03G3 



Mean longitude, January 1, 1847, M.T. Greenwich , 328 32' 44 -20 



Eccentricity ........ 0-00871946 



Longitude of perihelion 47 1 'M 



Longitude of ascending node 130'4'20"'8 



Inclination 1 48 88 ? 



Mean daily motion 21 '-55448 



Periodic time 164-6181 trop. years. 



These elements represent the observations of the planet with a 

 remarkable degree of precision. It will be seen that they differ in 

 several respects very considerably from the corresponding results 

 obtained by Mr. Adams and M. Le Verrier. This circumstance pre- 

 sented a difficulty to the minds of several persons, who even went so 

 far as to assert that the planet Neptune did not represent the theo- 

 retical planet indicated by the researches of those geometers, and that 



