Galileo is credited with discovering the first four moons of Jupiter, now known as the Galilean moons, of Io, Europa, Ganymede and Callisto. NASA Solar System Exploration explains that they are the four largest moons of Jupiter. Although not the first moons discovered, they were the first discovered through the use of a telescope.
According to NASA Solar System Exploration, Galileo recorded his first sightings of the moons on Jan. 7, 1610, although he first believed that they were stars. On subsequent evenings, Galileo noted that the objects did not move in the expected direction, prompting him to continue his observations. After noticing that the objects moved in conjunction with Jupiter, Galileo determined that they were moons. Galileo published his observations and conclusions in March of 1610. He originally named the moons I, II, III and IV.
The moons became known by their current names in the mid-19th century, which are credited to fellow astronomers Simon Marius and Johannes Kepler. They were also early observers of the moons of Jupiter. The new names came in response to the discovery of additional moons of Jupiter, numbering up to 50 as of 2014.
Galileo's discovery of planetary bodies orbiting an object other than Earth provided key evidence in support of a heliocentric model of the universe where the planets revolved around the Sun, rather than a geocentric one where the Sun and other planets orbited around Earth.