Urbain Leverrier (1811-1877)
French astronomer who predicted the existence of the planet Neptune, from the discrepancy between the theoretical orbit and the observed data of Uranus. He was educated at the Ecole Polytechnique, and began to teach astronomy there in 1837. Arago, then the director of the Paris Observatory, was impressed by Leverrier's ability, and Leverrier was to spend most of his time at the Paris Observatory. It was Arago who suggested Leverrier to work on the problems of Mercury and Uranus.
As regards Uranus, Leverrier's work achieved a most dramatic success, leading to the discovery of the new planet Neptune, as was already mentioned. Although Adams (British astronomer) was independently working on the same problem, Leverrier was more fortunate in that his prediction was confirmed by Galle (German astronomer), in 1846. This incident was regarded by many people as one of the most impressive successes of the Newtonian astronomy (called celestial mechanics), since Leverrier's prediction was made by means of only "a pen and calculations".
Leverrier eventually succeeded Arago's position, the director of the Paris Observatory, in 1854. Then he noticed the problem of the perihelion of Mercury. But this time, his speculation (postulating another planet inside of Mercury's orbit) was not successful; this problem was evetually solved by Einstein's general relativity in 1915 (see Perihelion of Mercury).
See a biography in MacTutor History of Mathematics.