An observatory is a building that holds one or many very big telescopes. Observatories are used by astronomers to look at the sky. Observatories are usually very high up on mountains or hills, because they need clear and clean air to be able to see properly. The higher up you go, the clearer and cleaner the air gets. Observatories also have to be away from places that have lots of light, like cities. Too much light can make it so that the astronomers cannot see out of the telescope very well.