Arago said that the discovery made by Leverrier was one of the most brilliant manifestations of the precision of modern astronomic science. It would encourage the best geometers to seek with renewed ardor the eternal truths which, in Pliny's phrase, are latent in the majesty of theory.
Professor Challis received Leverrier's third paper on September 29, and in the evening turned his magnificent refractor to the part of the heavens that Leverrier had so definitely and so confidently indicated. Among the three hundred stars observed Challis was struck by the appearance of one which presented a disk and shone with the brightness of a star of the eighth magnitude. This proved to be the planet. On October 1 Challis heard that the German observer had anticipated him.
Arago, while recognizing the excellent work done by Adams in his calculations, thought that the fact that the young mathematician had failed to publish his results should deprive him of any share whatever in the glory of the discovery of the new planet, and that history would confirm this definite judgment. Arago named the new planet after the French discoverer, but soon acquiesced in the name Neptune, which has since prevailed.
Airy, in whose possession Adams's results had remained for months unpublished and unheeded, wrote Leverrier: "You are to be recognized beyond doubt as the predictor of the planet's place." A vigorous official himself, Airy was deeply impressed by the calm decisiveness and definite directions of the French mathematician. "It is here, if I mistake not, that we see a character far superior to that of the able, or enterprising, or industrious mathematician; it is here that we see the philosopher." This explains, if anything could, his view that a distant mathematical result is the subject of ethical rather than of mathematical evidence.
Adams's friends felt that he had not received from either of the astronomers, to whom he confided his results, the kind of help or advice he should have received. Challis was kindly, but wanting in initiative. Although he had command of the great Northumberland telescope, he had no thought of commencing the search in 1845, for, without mistrusting the evidence which the theory gave of the existence of the planet, it might be reasonable to suppose that its position was determined but roughly, and that a search for it must necessarily be long and laborious. In the view of Simon Newcomb,[3] Adams's results, which were delivered at the Greenwich Observatory October 21, 1845, were so near to the mark that a few hours' close search could not have failed to make the planet known.
Both Adams and Leverrier had assumed as a rough approximation at starting that the orbit of the new planet was circular and that, in accordance with Bode's Law, its distance was twice that of Uranus. S. C. Walker, of the Smithsonian Institution, Washington, was able to determine the elements of the orbit of Neptune accurately in 1847. In February of that year he had found (as had Petersen of Altona about the same time) that Lalande had in May, 1795, observed Neptune and mistaken it for a fixed star. When Lalande's records in Paris were studied, it was found that he had made two observations of Neptune on May 8 and 10. Their failure to agree caused the observer to reject one and mark the other as doubtful. Had he repeated the observation, he might have noted that the star moved, and was in reality a planet.
Neptune's orbit is more nearly circular than that of any of the major planets except Venus. Its distance is thirty times that of the earth from the sun instead of thirty-nine times, as Bode's Law would require. That generalization was a presupposition of the calculations leading to the discovery. It was then rejected like a discredited ladder. Man's conception of the universe is widened at the thought that the outmost known planet of our solar system is about 2,796,000,000 miles from the sun and requires about 165 years for one revolution.
Professor Peirce, of Harvard University, pointing to the difference between the calculations of Leverrier and the facts, put forward the view that the discovery made by Galle must be regarded as a happy accident. This view, however, has not been sustained.
REFERENCES
Sir Robert Ball, Neptune's Jubilee Year, Scientific American, Supplement, Oct. 10, 1896.