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NEPTUNE
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NEPTUNE , in See also:astronomy, the outermost known See also:planet of our See also:solar See also:system; its See also:symbol is . Its distance from the See also:sun is a little more than 30 astronomical See also:units, i.e. 30 times the mean distance of the See also:earth from the sun, or about 2,796,000,000 M. It deviates greatly from See also:Bode's See also:law, which would give a distance of nearly 39. Its See also:orbit is more nearly circular than that of any other See also:major planet, See also:Venus excepted. Its See also:time of revolution is 165 years. Being of the 8th stellar magnitude it is invisible to the naked See also:eye. In a small See also:telescope it cannot be distinguished from a fixed See also:star, but in a large one it is seen to have a disk about 2.3" in See also:diameter, of a See also:pale bluish See also:hue. No features and no See also:change of See also:appearance can be detected upon it, so that observation can give no indication of its rotation. Both its See also:optical aspect and the study of its spectrum seem to show that it resembles See also:Uranus. Its spectrum shows marked absorption-bands in the red and yellow, indicating an See also:atmosphere of See also:great See also:depth of which See also:hydrogen would seem to be a constituent. (See PLANET.) Only a single See also:satellite of Neptune is yet known. This was discovered by See also: 119 •35°—o .165° (t—189o) R.A. of See also:node on earth's equator . 185 •15°+ o .148 (t—189o) Distance from node at See also:epoch . 234 .42 Mean daily motion . . . . 61 •2578° Mean distance at See also:log= 1.47814 i6 .271 Epoch, 189o, See also:Jan. o, See also:Greenwich mean See also:noon The eccentricity, if any, is too small to be certainly determined. From the above mean distance is derived as the See also:mass of Neptune ni 65. The motion of Uranus gives a mass To }i•=. 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 See also:celestial motions. So many agencies were concerned in the final discovery that the whole forms one of the most interesting chapters in the See also:history of astronomy. The planet Uranus, before its actual discovery by See also:Sir William See also:Herschel in 1781, had been observed as a fixed star on at least 17 other occasions, beginning with See also:Flamsteed in 169o. In 182o See also:Alexis Bouvard of See also:Paris constructed tables of the motion of See also:Jupiter, See also:Saturn and Uranus, based upon a discussion of observations up to that See also:year. Using the mutual perturbations of these planets as See also:developed by See also:Laplace in the Mecanique See also: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 See also:error, the outstanding See also:differences between theory and observation exceeding 1'. In these circumstances one of two courses had to be adopted, either to obtain the best See also:general See also: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 See also: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 See also:interest. Among the investigators of the question was F. W. See also: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 See also: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 See also:body, was expressed by the Rev. T. J. Hussey, an See also:English See also:amateur astronomer. In a See also:letter to Sir See also:George B. See also:Airy in 1834 he inquired Airy's views of the subject, and offered to See also:search for the planet with his own See also: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 See also:place of a planet which might produce it. In 1837 Bouvard, in See also:conjunction with his See also:nephew See also: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. See also:Nicolai, director of the See also:observatory at See also:Mannheim, in discussing the motion of See also:Halley's See also:comet, considered the possibility that it was acted upon by an ultra- Briefwechsel zwischen See also: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 See also:recent observations? In 1838 Airy showed in a letter to the Astronomische Nachrichten that not only the See also:heliocentric See also:longitude, but the tabulated See also: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 See also:Gottingen offered a See also:prize of 50 ducats for a satisfactory working up of the whole theory of the motions of Uranus, assigning See also:September 1846 as the time within which competing papers should be presented.
It is also recorded that Bessel, during a visit to See also:England in 1842, in a conversation with Sir See also: Meanwhile, at the See also:suggestion of See also:Arago, the investigation had been taken up by U. J. J. See also:Leverrier, who had published some excellent work in theoretical astronomy. Leverrier's first published communication on the subject was made to the See also:French See also:Academy on the loth 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 ?846 (See also:June I). 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 See also:succession various explanations, he found none admissible except that of a planet exterior to Uranus. Considering the distances to be See also: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: Leverrier. Adams. See also:Hypothesis I. Hypothesis II. Semi-major axis . 36.154 38.38 37.27 Eccentricity . 0.1076 o•1610 0.12062 See also:Long. of See also:perihelion 284° 45' 315° 57 299° II' Mean longitude . 318° 47' 325° 8' 323° 2' Epoch 1847, Jan. i 1846, Oct.' 1846, Oct. 1 True longitude 326° 32' 328° 329° 2 Astron. Nach. xiii. § 94. 2 Ibid. xv. § 217. ° See Astron. Nach., F_rganzungsheft, p. 6. The longitude of the actual planet was 327° 57' on the 1st of See also:October 1846. The See also:close agreement of these elements led Airy to suggest to Challis, on the 9th of See also:July 1846, a search for the planet with the See also:Northumberland telescope. He proposed an examination of a See also:part of the heavens 3o° long in the direction of the ecliptic and ro° broad, and estimated the number of See also:hours' work likely to be employed in this sweep. The proposed sweeps were commenced by Challis on the 29th of July. The See also:plan required each region to be swept through twice, and the positions of all the known stars found to be compared, in See also:order that the position of the planet might be detected by its motion. On the 31St of See also:August Leverrier's concluding paper was presented to the French Academy, and on the 18th of September he wrote to John G. See also:Galle (1812-1910), then See also:chief assistant at the See also:Berlin observatory, suggesting that he should search for the computed planet, with the See also:hope of detecting it by its disk, which was probably See also:mere than 3" in diameter. This letter, probably received on the 23rd of September, was communicated to J. F. See also:Encke, the director of the observatory, who approved of the search. H. L. d'See also: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 See also: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 See also: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 See also:sole See also: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 See also: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 See also: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 deter-See also:mining the limits by considering the See also:variations of the elements one at a time, assuming in the case of each that while it varied the others remained See also: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 See also:Benjamin See also:Peirce of Harvard University went so far as to maintain that there was a discontinuity between the See also: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 See also: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 See also:matter of fact the actual motion of the planet during the See also: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 178o to 1845, when the action on Uranus was at its greatest.
The observations of the first opposition enabled Sears See also:Cook See also: 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 See also:Altona, before any word of Walker's work had reached him. Additional information and CommentsThere are no comments yet for this article.
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