Neptune and Triton — Image credit: NASA/JPL

1846: German astronomer Johann Gottfried Galle, knowing exactly where to look, confirms the existence of an eighth planet in the solar system, Neptune.

Galle was not the first astronomer to see Neptune -- Galileo, puzzled by the changing position of what he thought was a fixed star, had sketched the movement in 1613 but never published his findings. Others had seen it, too, but Galle, working at the Berlin Observatory, was the first to observe Neptune while understanding exactly what he was looking at.

By the time Galle fixed his gaze upon Neptune, the existence of a planet beyond Uranus was widely predicted and its position had been intensely calculated. In fact, other astronomers were quarreling over who owned the priority of discovery.

Link to Wired article by Tony Long