THE DIFFERENCE BETWEEN THE DISCOVERIES OF NEPTUNE AND PLUTO

Abstract

The one point of resemblance between the discoveries of these two planets was the historical one, that there were two claimants in each case. In all other respects the difference is marked. In the case of Neptune the two computers, dealing with the very marked and obvious perturbations of Uranus, arrived by analytical methods at similar results as to the location in the sky of the unknown planet. The planet being very bright, of 7 . 6 magnitude, was found by the German astronomer Galle, by the aid of a new map of this portion of the sky, as soon as its computed position was communicated to him by Leverrier. Adams had no such luck, for the German map used by Galle had not yet reached England, and the English searcher, Chain's, had to construct a map of the region, and then look for Neptune upon it, or else compare his observations individually with one another. As it happened Neptune was already recorded in his notes when Galle's observation was announced, but Challis had not distinguished it from a star, perhaps on account of the low power of his telescope. Adams' preliminary and sufficiently approximate location was completed and communicated to Airy, the English Astronomer Royal, nearly a year before Leverrier's was sent to Galle. Challis' work also began some weeks before that of Galle. The English said it was not Adams' fault that his computations had not led to the earlier discovery of the planet, nor to him as the real discoverer. The French based their claim to Leverrier as the true discoverer on the fact that his location was nearer the truth, and that Galle using his position had actually found the planet in the sky. Turning now to the case of Pluto, Lowell computed its position analytically by the method employed by Adams and Leverrier. By this method it is necessary to secure observations of the perturbed planet throughout nearly the whole of its orbit. Neptune since its discovery in 1846 had only completed a halfrevolution, and Lowell for this reason was obliged to base his