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Ganymede, what is Ganymede? Ganymede is a moon of Jupiter (Satellite if you want to be smarty). It is the largest moon in the whole solar system. Ganymede is the third Galilean Satellite from Jupiter, and it was the fourth to be discovered. Ganymede was discovered with two other moons on January 11, 1610 by Galileo Galilei. He, at first had thought that they were stars, but the next night he looked they had moved. Then he realized that he had found a new moon, Ganymede.