HST Pointing Control System Diagram
Hubble uses some very basic physics to turn itself around and look at different parts of the sky. Located on the telescope are six gyroscopes (which, like a compass, always point in the same direction) and four free-spinning steering devices called reaction wheels.
The gyroscopes sense when the telescope needs to be repointed. When they "tell" Hubble that it needs to turn itself, a computer gives a command to give the reaction wheels a "push" or "spin."