On this night in 1610 in Padua, Galileo Galilei turned his small telescope toward Jupiter. He saw three "fixed stars" in a straight line - two on one side of Jupiter's disk, one on the other. The next night, all three were on the planet's right side. Galileo soon spied a fourth "star." The quartet seemed to shift positions, or duck behind the planet, nightly, always staying close. He concluded they were moons orbiting Jupiter. By March 12, he'd published his discovery, changing astronomy forever.