Astronomers first observed it on July 22, 1846. Its full orbit from that point is at 22:27 UT.
The existence of Neptune was actually already predicted years earlier because of some gravitational effect on Uranus.
Only on September 23, 1846 was the planet's existence confirmed. Johann Galle, a German astronomer, had predicted the location of appearance in the sky, looked through his telescope at that exact point and saw a blue-green speck (which was why it was named after the Roman god of the sea).
Galileo actually spotted the planet more than 200 years earlier, but assumed it was only a star because he couldn't see very clearly, as it was cloudy.
According to Erich Karkoschka, an astronomer at the University of Arizona, Neptune 'only gets 0.1 percent as much sunlight as Earth, but Neptune has storms and its appearance changes over time.' People wonder why it has such an active atmosphere. In the 1970s and 1980s, it had appeared darker than it is now.
Neptune seen from Triton, its largest moon.
[Source and image: National Geographic]
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