The first person to observe Jupiter’s moons was Galileo Galilei in 1610. Using a telescope he had improved, Galileo discovered four of Jupiter’s largest moons: Io, Europa, Ganymede, and Callisto. These are now known as the Galilean moons in his honor.
His discovery was groundbreaking—it provided strong evidence against the geocentric (Earth-centered) model of the universe, since it showed that not everything revolved around the Earth. Pretty big moment in science history!