On the night of January 7, 1610, Galileo Galilei, a resident of Padua, walked onto his balcony and tipped his telescope toward space. He spotted three stars near Jupiter and graphed their positions in a notebook. Six days later, he looked through his telescope again and found the same stars—but their positions had shifted. They were, he realized, moons orbiting Jupiter. Galileo had long believed Copernicus’ theory that the Earth was not the center of the universe. Now he had proof.