Galileo first observed Neptune on December 28, 1612, but he mistook Neptune for a fixed star when it appeared very close to Jupiter in the night sky.
The search for Neptune began when astronomers noticed that Uranus’s movements, particularly its orbit, didn’t correspond to their expectations.
One explanation was that another planet existed beyond Uranus, and exerted its gravitational force on Uranus.
In 1846 astronomers calculated exactly where such a planet would have to be, pointed their telescopes to that spot in the sky, and found Neptune.
In 2011, Neptune will complete its first orbit since it was discovered in 1846.