What did Galileo Galilei discover?
The astronomer Galileo Galilei with the help of a telescope was able to prove Copernicus’ theory that the Earth and other planets in the solar system revolved about the Sun. At this time, most people believed in a geocentric(centered about the Earth) solar system. He made other observations and discoveries in regard to the Moon, Venus, and the moons of Jupiter. For more information see here: http://csep10.phys.utk.edu/astr161/lect/history/galileo.
