He discovered Jupiter's four largest moons: Io, Europa, Ganymede and Callisto. He also found the Milky Way to be a multitude of stars instead of merely a cloud as Earth scientists had previously believed. He confirmed Copernican theories that Earth was not the center of the universe, although religious authorities made him renounce his support for those claims on the premise of a death sentence for disobedience.
He was one of six Human astronomers honored with a monument located in front of the Griffith Observatory, the others being Hipparchus, Nicolaus Copernicus, Johannes Kepler, Isaac Newton and William Herschel. (VOY: "Future's End")