The planet Jupiter's four largest moons, or satellites, are called the Galilean moons, after Italian astronomer Galileo Galilei, who observed them in 1610. The German astronomer Simon Marius apparently discovered them around the same time. The names Marius proposed for the moons in 1614 (suggested to him by a fellow astronomer, Johannes Kepler) are the ones we use today - Io, Europa, Ganymede, and Callisto.
Note: This lithograph was revised in September 2013.
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