Galileo Galilei was the first to point a telescope toward the sky and observed the heavens. Galileo discovered four moons around Jupiter. This discovery proved that objects could orbit more than just the Earth. He also measured the other planets, and determined they also orbited the Sun, not the Earth. Galileo was punished for his work by the church. He published his findings late in his life, only to be placed on house arrest and forced to abandon his work. He was not allowed to even teach his findings.
Galileo discovered Jupiter's four largest moons: Io, Ganymede, Europa, and Callisto.