Looking up - 17 Dec

The Andromeda Galaxy was first seen through a telescope 500 years ago. It is famous for not only being the nearest full-on galaxy to our own, but because it was used to determine the distance between us. This marked it out not as a cloud, as was thought at the time, but an entirely separate galaxy. Suddenly in 1924 we knew for sure that the Milky Way was not the entire universe.