In 1610, Galileo Galilei peered through a telescope and observed, “I have seen Jupiter accompanied by three fixed stars, totally invisible by their smallness. The planets are seen very rotund, like little full moons.” In fact, what he saw with his eyes, magnified by his early telescope, were the largest moons of our solar system’s largest planet, Jupiter. Galileo eventually identified Europa, Callisto, Io, and Ganymede, and they are now sometimes known as Jupiter’s “Galilean’ satellites.
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Source: Phys.org