The French mathematical astronomer Urbain Jean Joseph Leverrier (1811-1877) made theoretical investigations which led to the discovery of the planet Neptune.
Born at Saint-Lô in Normandy on March 11, 1811, U. J. J. Leverrier entered the highly competitive école Polytechnique to prepare for a career as a professional scientist. His early interest was in chemistry; but when the teaching post in astronomy fell vacant at the Polytechnique in 1837, Leverrier took it and thereby entered the discipline in which he was to spend the rest of his life.
The aspect of astronomy with which Leverrier was primarily concerned was celestial mechanics, the mathematical analysis of the planetary motions. According to the principles of celestial mechanics, each planet was supposed to move around the sun in an essentially elliptical orbit with minor deviations due to attractions by the rest of the planets. The computations involved were very complicated, but the results were generally sufficient to provide predictions of considerable accuracy. There was, however, one prominent exception—the planet Uranus. Although it had been the subject of a great deal of study since its discovery in 1781, attempts to reduce its motion to rule had yet to meet with complete success. The remaining error was small by ordinary standards (1 minute of arc, or the angle subtended by a nickel at a distance of 100 yards), but it was a scandal in a profession accustomed to accounting for angles less than one-tenth that size.
In 1845 Leverrier decided to look into the question. After concluding that the difficulty was probably due to the action of an unknown planet whose effects were not being taken into account, he undertook a series of detailed calculations which culminated in an estimation of the location of the unknown planet. On Sept. 23, 1846, the planet, later named Neptune at Leverrier's suggestion, was discovered by J. G. Galle, the director of the Berlin Observatory, less than a degree from the spot indicated by Leverrier.
Leverrier's work was universally acclaimed as one of the outstanding scientific achievements of all time, and he received honors from virtually every country and scientific society in Europe. He embarked on similar but less successful investigations of a slight anomaly in the motion of Mercury which was resolved only in the 20th century through the work of Albert Einstein. Leverrier continued with exhaustive examinations and revisions of all the existing planetary theories. In addition, he served with distinction as director of the Paris Observatory, organized the French meteorological service, and worked for the inclusion of scientific instruction in the French educational system. He died in Paris on Sept. 23, 1877.
Further Reading on Urbain Jean Joseph Leverrier
There is no biography of Leverrier, nor is there any thorough discussion of his technical contributions. Most of what has been written about him is in French; but Morton Grosser, The Discovery of Neptune (1962), presents a good account of Leverrier and one aspect of his work.