In 1610, Galileo used his newly improved telescope to observe Jupiter. He noticed four small, star-like objects near the planet that changed positions over a few nights. He correctly deduced that these were not stars, but rather moons orbiting Jupiter. This discovery was a major blow to the geocentric model of the universe, which held that Earth was the center of all celestial motion.

These moons are now known as the Galilean moons in his honor: Io, Europa, Ganymede, and Callisto.