Neptune was the first planet discovered based on observations of another planet. In the mid-1840’s, a young astronomer and mathematician named John C. Adams began the hunt for the farthest planet from the sun by observing Uranus. Subtle changes in the planet’s orbit hinted that another body was tugging on Uranus with its gravitation pull. Adams concluded that the unknown planet should be about 1 billion miles farther from the sun than Uranus. He completed his work in September 1845 and sent his computed orbit to Sir George B. Airy, the astronomer royal of England. However, Adams did not provide Airy with sufficient information to find Neptune.
Meanwhile, the French mathematician Urbain J. J. Le Verrier also became interested in the unseen planet. By mid-1846, Le Verrier had also calculated Neptune’s orbit. However, Le Verrier also used his calculations to predict Neptune’s position in the sky. He sent his predictions to the Urania Observatory in Berlin, Germany. Johann G. Galle, the director of the observatory, had just charted the stars in the area where the planet was believed to be. On Sept. 23, 1846, Galle and his assistant, Heinrich L. d’Arrest, spotted Neptune. The planet was in almost exactly the position predicted by Le Verrier.
Traditionally, Adams and Le Verrier have been jointly credited with the discovery. However, after Adams’s papers were rediscovered in 1998, some historians began to grant sole credit to Le Verrier. Adams’s papers revealed that he had not fully explained where in the sky Neptune could be found and that his calculations lacked the accuracy of Le Verrier’s. By contrast, Le Verrier’s calculations of position led Galle and d’Arrest to find Neptune after only a 30-minute search.