Neptune (Astronomy) - Encyclopedia

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NEPTUNE, in astronomy, the outermost known planet of our solar system; its symbol is W. Its distance from the sun is a little more than 30 astronomical units, i.e. 30 times the mean distance of the earth from the sun, or about 2,796,000,000 m. It deviates greatly from Bode's law, which would give a distance of nearly 39. Its orbit is more nearly circular than that of any other major planet, Venus excepted. Its time of revolution is 165 years. Being of the 8th stellar magnitude it is invisible to the naked eye. In a small telescope it cannot be distinguished from a fixed star, but in a large one it is seen to have a disk about 2.3" in diameter, of a pale bluish hue. No features and no change of appearance can be detected upon it, so that observation can give no indication of its rotation. Both its optical aspect and the study of its spectrum seem to show that it resembles Uranus. Its spectrum shows marked absorption-bands in the red and yellow, indicating an atmosphere of great depth of which hydrogen would seem to be a constituent. (See Planet.) Only a single satellite of Neptune is yet known. This was dis covered by William Lassell soon after the discovery of the planet. Its period of revolution is 5d. 21 h. Its motion is retrograde, in a plane making an angle of about 35° with the orbit of the planet. This was the first case of retrograde motion found in any of the xix. 13 planets or satellites of the solar system. The most noteworthy feature connected with the satellite is a secular change which is going on in the position of its orbital plane. Were the planet spherical in form, no such change could occur, except an extremely slow one produced by the action of the sun. The change is therefore attributed to a considerable ellipticity of the planet, which is thus inferred to be in rapid rotation. It will ultimately be possible to determine from this motion the position of the axis of rotation of Neptune with much greater precision than it could possibly be directly observed.

Inclination to earth's equator .


. 35° - o. 165° (t-1890)

R.A. of node on earth's equator .


15°+ o. 148 (t-1890)

Distance from node at epoch

2 34


Mean daily motion. .. .



Mean distance at A =1.47814



The following elements of the satellite were determined by H. Struve from all the observations available up to 1892: Varying Elements of Neptune's Satellite. Epoch, 1890, Jan. 0, Greenwich mean noon The eccentricity, if any, is too small to be certainly determined. From the above mean distance is derived as the mass of Neptune The motion of Uranus gives a mass Discovery of Neptune. - The detection of Neptune through its action upon Uranus before its existence had been made known by observation is a striking example of the precision reached by the theory of the celestial motions. So many agencies were concerned in the final discovery that the whole forms one of the most interesting chapters in the history of astronomy. The planet Uranus, before its actual discovery by Sir William Herschel in 1781, had been observed as a fixed star on at least 17 other occasions, beginning with Flamsteed in 1690. In 1820 Alexis Bouvard of Paris constructed tables of the motion of Jupiter, Saturn and Uranus, based upon a discussion of observations up to that year. Using the mutual perturbations of these planets as developed by Laplace in the Mecanique Celeste, he was enabled satisfactorily to represent the observed positions of Jupiter and Saturn; but the case was entirely different with Uranus. It was found impossible to represent all the observations within admissible limits of error, the outstanding differences between theory and observation exceeding 1'. In these circumstances one of two courses had to be adopted, either to obtain the best general representation of all the observations, which would result in the tables being certainly erroneous, or to reject the older observations which might be affected with errors, and base the tables only on those made since the discovery by Herschel. A few years of observation showed that Uranus was deviating from the new tables to an extent greater than could be attributed to legitimate errors of theory of observation, and the question of the cause thus became of growing interest. Among the investigators of the question was F. W. Bessel," who tried to reconcile the difficulty by an increase of the mass of Saturn, but found that he could do so only by assigning a mass not otherwise admissible. Although the idea that the deviations were probably due to the action of an ultra-Uranian planet was entertained by Bouvard, Bessel and doubtless others, it would seem that the first clear statement of a conviction that such was the case, and that it was advisable to reach some conclusion as to the position of the disturbing body, was expressed by the Rev. T. J. Hussey, an English amateur astronomer. In a letter to Sir George B. Airy in 1834 he inquired Airy's views of the subject, and offered to search for the planet with his own equatorial if the required estimate of its position could be supplied. Airy expressed himself as not fully satisfied that the deviation might not arise from errors in the perturbations. He therefore was not certain of any extraneous action; but even if there was, he doubted the possibility of determining the place of a planet which might produce it. In 1837 Bouvard, in conjunction with his nephew Eugene, was again working on the problem; but it does not seem that they went farther than to collect observations and to compare the results with Bouvard's tables.

In 1835 F. B. G. Nicolai, director of the observatory at Mannheim, in discussing the motion of Halley's comet, considered the possibility that it was acted upon by an ultra ' Briefwechsel zwischen Olbers u. Bessel, ii. 250 (Oct. 9, 1823).

Uranian planet, the existence of which was made probable by the disagreement between the older and more recent observations.2 In 1838 Airy showed in a letter to the Astronomische Nachrichten that not only the heliocentric longitude, but the tabulated radius vector of Uranus was largely in error, but made no suggestions as to the cause.3 In 1843 the Royal Society of Sciences of Gottingen offered a prize of 50 ducats for a satisfactory working up of the whole theory of the motions of Uranus, assigning September 1846 as the time within which competing papers should be presented. It is also recorded that Bessel, during a visit to England in 1842, in a conversation with Sir John Herschel, expressed the conviction that Uranus was disturbed by an unknown planet, and announced his intention of taking up the subject.4 He went so far as to set his assistant Fleming at the work of reducing the observations, but died before more was done.

The question had now reached a stage when it needed only a vigorous effort by an able mathematician to solve the problem. Such a man was found in John Couch Adams, then a student of St John's College, Cambridge, who seriously attacked the problem in 1843, the year in which he took his bachelor's degree. He soon found that the observations of Uranus could be fairly well represented by the action of a planet moving in a radius of twice the mean distance of Uranus, which would closely correspond to Bode's law. During the two following years he investigated the possible eccentricity of the orbit, and in September 1845 communicated his results to Professor James Challis. In 1845, about the 1st of November, Adams also sent his completed elements to Airy, stating that according to his calculations the observed irregularities in the motion of Uranus could be accounted for by the action of an exterior planet, of which the motions and orbital elements, were given. It is worthy of note that the heliocentric longitude of the unknown body as derived from these elements is only between one and two degrees in error, while the planet was within half a degree of the ecliptic. Two or three evenings assiduously devoted to the search could not therefore have failed to make the planet known. Adams's paper was accompanied by a comparison of his theory with the observations of Uranus from 1780, showing an excellent agreement. Airy in replying to this letter inquired whether the assumed perturbation would also explain the error of the radius-vector of Uranus, which he seemed to consider the crucial test of correctness. It does not seem that any categorical reply to this question was made by Adams.



Hypothesis I.

Hypothesis II.

Semi-major axis .




Eccentricity .




Long. of perihelion

28 4° 45'

315° 57'

299° I I'

Mean longitude .

318° 47'

325° 8 '

32e3° 2'

Epoch.. .

1847, Jan. 1

1846, Oct.'

1846, Oct.'

True longitude .

326° 32'



Meanwhile, at the suggestion of Arago, the investigation had been taken up by U. J. J. Leverrier, who had published some excellent work in theoretical astronomy. Leverrier's first published communication on the subject was made to the French Academy on the 10th of November 1845, a few days after Adams's results were in the hands of Airy and Challis. A second memoir was presented by Leverrier in 1846 (June 1). His investigation was more thorough than that of Adams. He first showed that the observations of Uranus could not be accounted for by the attraction of known bodies. Considering in succession various explanations, he found none admissible except that of a planet exterior to Uranus. Considering the distances to be double that of Uranus he then investigated the other elements of the orbit. He also attempted, but by a faulty method, to determine the limits within which the elements must be contained. The following are the elements found by Adams and Leverrier: 2 Astron. Nach. xiii. § 94. s Ibid. xv. § 217.4 See Astron. Nach., Erganzungsheft, p. 6.

The longitude of the actual planet was 3 2 7° 57' on the ist of October 1846.

The close agreement of these elements led Airy to suggest to Challis, on the 9th of July 1846, a search for the planet with the Northumberland telescope. He proposed an examination of a part of the heavens 30° long in the direction of the ecliptic and 10° broad, and estimated the number of hours' work likely to be employed in this sweep. The proposed sweeps were commenced by Challis on the 29th of July. The plan required each region to be swept through twice, and the positions of all the known stars found to be compared, in order that the position of the planet might be detected by its motion. On the 31st of August Leverrier's concluding paper was presented to the French Academy, and on the 18th of September he wrote to John G. Galle (1812-1910), then chief assistant at the Berlin observatory, suggesting that he should search for the computed planet, with the hope of detecting it by its disk, which was probably more than 3" in diameter. This letter, probably received on the 23rd of September, was communicated to J. F. Encke, the director of the observatory, who approved of the search. H. L. d'Arrest, a student living at the observatory, expressed a wish to assist. In the evening the search was commenced, but it was not found possible to detect any planet by its disk. Star charts were at the time being prepared at the observatory under the auspices of the Berlin Academy of Sciences. It was suggested by d'Arrest that this region might be covered by one of the charts. Referring to the chart, which was lying in a drawer, it was found that such was the case. Comparing the stars on the chart one by one with the heavens it was found that an eighth magnitude star now visible was not on the chart. This object was observed until after midnight, but no certain motion was detected. On the following evening the object was again looked for, and found to have actually moved. The existence of the planet was thus established. It was afterwards found that Challis in his sweeps had observed the planet on the 4th of August, but, not having compared his observations with those made subsequently, had failed to detect it.

The question whether Leverrier should receive the sole credit of the discovery was warmly discussed. Arago took the extreme ground that actual publication alone should be considered, rejecting Adams's communications to Airy and Challis as quite unworthy of consideration. He also suggested that the name of Leverrier should be given to the planet, but this proposal was received with so little favour outside of France that he speedily withdrew it, proposing that of Neptune instead.

The observations at the first opposition showed that the planet was moving in a nearly circular orbit, and was at a mean distance from the sun much less than that set by Leverrier as the smallest possible. The latter had in fact committed the error of determining the limits by considering the variations of the elements one at a time, assuming in the case of each that while it varied the others remained constant. But a simultaneous variation of all the elements would have shown that the representation of the observations of Uranus would be improved by a simultaneous diminution of both the eccentricity and the mean distance, the orbit becoming more nearly circular and the planet being brought nearer to the sun. But this was not at first clearly seen, and Benjamin Peirce of Harvard University went so far as to maintain that there was a discontinuity between the solution of Adams and Leverrier and the solution offered by the planet itself, and that the coincidence in direction of the actual and computed planet was an accident. But this view was not well founded, and the only explanation needed was to be found in Leverrier's faulty method of determining the limits within which the planet must be situated. As a matter of fact the actual motion of the planet during the century preceding, as derived from Leverrier's elements, was much nearer the truth than the elements themselves were. This arose from the fact that his very elliptic orbit, by its large eccentricity, brought the planet near to the sun, and therefore near to its true position, during the period from 1780 to 1845, when the action on Uranus was at its greatest.

The observations of the first opposition enabled Sears Cook Walker of the National Observatory, Washington, in February 1847 to compute the past positions of the planet, and identify it with a star observed by Lalande at Paris in Ma y 1795. This being communicated to the Paris observatory, an examination of Lalande's manuscript showed that he had made two observations of the planet, on the 8th and 10th of May, and finding them discordant had rejected one as probably in error, and marked the other as questionable. A mere re-examination of the region to see which observation was in error would have led him to the discovery of the planet more than half a century before it was actually recognized. The identity of Lalande's star with Neptune was also independently shown by Petersen of Altona, before any word of Walker's work had reached him. BIBLIOGRAPHY. - The principal sources for the history of the discovery of Neptune are the Astronomische Nachrichten, vols. xxv., xxvi., xxviii., and Lindenau's paper in the Erganzungsheft to this publication, pp. 1-31 (Altona, 1849). In the Memoirs of the Royal Astronomical Society, vol. xvi., Airy gave a detailed history of the circumstances connected with the discovery, so far as he was cognizant of them. Documents pertaining to the subject are found in the Monthly Notices of the Royal Astron. Society. B. A. Gould, Report to the Smithsonian Institution on the History of the Discovery of Neptune, published by the Smithsonian Institution (Washington, 1850), is the most complete and detailed history of all the circumstances connected with the discovery, and with the early investigations on the orbit of the planet, that has been published. Leverrier's investigation was published in extenso as an addition to the Connaissance des temps, and Adams's as an appendix to the Nautical Almanac for 1851. Peirce's discussions, so far as published at all, are found in the Proceedings of the American Academy of Arts and Sciences. The first computations of the orbit after the discovery were made by Sears Cook Walker, and published by the Smithsonian Institution (1848-1850). General tables of the motion of Neptune are in Kowalski's Tables du mouvement de la planete Neptune; Newcomb's Investigation of the Orbit of Neptune, Washington, Smithsonian Institution (1866); Leverrier's Annales de l'Observatoire de Paris; Memoirs, vol. xiv. (1877), and lastly Newcomb's " Tables " in Astron. Papers of the American Ephemeris, vol. vii., part iv. Tables of the satellite are found in Newcomb, The Uranian and Neptunian Systems; appendix to the Washington observations for 1873. (S. N.)

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