Io, Europa, Ganymede and Callisto As a result of improvements Galileo Galilei made to the telescope, he was able to see celestial bodies more distinctly than ever before in human history. As a result, sometime between December 1609 and January 1610 Galileo Galilei discovered what came to be known as the Galilean moons although a Chinese historian of astronomy, Xi Zezong, claimed that the Chinese astronomer Gan De observed one of Jupiter’s moons in 362 BC, nearly 2 millennia earlier than Galileo
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