The four largest moons of Jupiter are called the Galilean moons. They are Io, Europa, Ganymede, and Callisto, named for Galileo Galilei, whose telescopic observations revealed them in 1610.
The moons are not simply four similar objects. Io is the most volcanically active world known, driven by tidal heating from Jupiter and its neighboring moons. Europa is covered by an icy shell and is believed to conceal a global subsurface ocean. Ganymede is the largest moon in the Solar System and is even larger in diameter than Mercury, while Callisto is heavily cratered and lies farthest from Jupiter among the four.
The term “largest” matters. Jupiter has many additional moons, most of them small, irregular bodies discovered through modern telescopes. The Galilean group refers specifically to Jupiter’s four largest natural satellites. Their discovery also changed astronomy by showing that not every celestial object revolves around Earth, challenging the traditional geocentric model.