Neptune - One orbit and counting

Neptune and Triton
Neptune and Triton one orbit after discovery Credit: Dr Robert Smith and the LT

Last night Neptune passed a very special milestone - exactly one orbit since it was discovered!

Neptune was discovered by Johann Galle on the 23rd September 1846, but since it takes 164.79 Earth years for it to orbit around the Sun, this is the first time that we have seen it return to that place in its orbit. This is particularly important for Neptune as it was the first planet discovered not by lucky searching of the heavens, but by prediction. By studying the orbits of the other planets, mathematicians Alexis Bouvard, Urbain Le Verrier and John Adams were able to work out that there must be another, unknown planet slightly distorting the orbit of Uranus with its gravity, and they were even able to predict where in the sky to look. Indeed, Galle found Neptune within a degree of the position predicted by Le Verrier - astonishingly accurate!

To celebrate this very special occasion, the observation above was taken by the Liverpool Telescope last night as Neptune returned to its "starting point". You can also see the moon Triton - the only large moon to have a significant atmosphere.