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Jupiter and the Galilean moons © Branko Šimunek/Alamy
On January 7, 1610, Galileo was the first person to train a telescope on Jupiter—and what he saw surprised him. Strung in a line beside the planet were three tiny stars, one to the left of the planet and two to the right. But when he observed the formation the next night, he saw that now all three were on the same side of Jupiter. Over the following week, he watched as the tiny stars (now joined by a fourth) changed their position relative to the planet while remaining beside it. By January 15, he had it figured out: he was observing four moons orbiting Jupiter.