On January 13, 1610, Galileo Galilei, peering through his homemade telescope from Padua, recorded a fourth point of light near Jupiter, later named Callisto. Combined with his sightings of Io, Europa, and Ganymede in the preceding days, the discovery shattered the notion that all celestial bodies orbited Earth.
Galileo had begun his nightly vigil over Jupiter just days earlier, noting three small 'stars' that shifted position from night to night. By January 13, a fourth had appeared, and he quickly realized these were not stars at all but moons circling Jupiter itself.
The implication was seismic: if other worlds had satellites that didn't orbit Earth, then Earth was not the unique center of all celestial motion, lending powerful observational support to the Copernican model. Galileo published his findings within weeks in Sidereus Nuncius (Starry Messenger), a slim pamphlet that made him instantly famous — and eventually got him into serious trouble with the Roman Inquisition.
The four moons, forever known as the Galilean satellites, remain visible today through even modest backyard telescopes, a direct link to that cold January night in Padua.