The longitude of the actual planet was 327° 57′ on the 1st 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 May 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 Ergänzungsheft 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 planète 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.)
NÉRAC, a town of south-western France, capital of an arrondissement in the department of Lot-et-Garonne, 16 m. W.S.W. of Agen by road. Pop. (1906) town, 4018; commune, 6318. The town, once the capital of the dukes of Albret, is divided by the Baise into two parts, Grand-Nérac on the left bank and Petit-Nérac on the right bank. The river is spanned by a bridge of the 16th century, called the Pont Vieux, and by the Pont Neuf, of modern construction. Narrow winding streets often bordered by old houses ascend from the narrow quays on both banks. From the left bank a staircase leads to the Rue Henri Quatre, where stands a wing of the castle in which Henry IV. lived. A statue of the king stands in one of the squares. The former palace of the Chambre des Comptes is now occupied by the tribunal of commerce, the library and the museum. The church of Grand-Nérac of the 18th century and the church of Petit-Nérac of the 19th century offer no remarkable features. On the left bank of the Baise, above Grand-Nérac, market gardens have taken the place of the old gardens of the Sires d’Albret, but remains of the Palais des Mariannes and of the Pavillon des Bains du Roi de Navarre, both of Renaissance architecture, are left. The famous promenade of La Garenne laid out by Antoine de Bourbon, king of Navarre, stretches for more than a mile along the opposite bank of the river. The remains of a Roman villa, including a fragment of mosaic, have been found there. A road leads from the south end of La Garenne to the ruins of the feudal castle of Nazareth. The Château du Tasta of the 15th century is within a short distance of Nérac. The town has a sub-prefecture, and the industries include brewing and cork-working.
Nérac appears at the beginning of the 11th century as a possession of the monks of St Pierre de Condom. The lords of Albret gradually deprived them of their authority over the town, and at the beginning of the 14th century founded a castle on the left bank of the Baise. In the 16th century the castle was the residence of Henry IV. during much of his youth and of
