Galileo discovered Jupiter’s four largest moons in 1610. Using an improved refracting telescope, he began observing Jupiter on 7 January and noticed small points of light arranged near the planet. Their positions changed from night to night, showing that they were orbiting Jupiter rather than being fixed stars.
Galileo saw all four at once on 13 January 1610 and concluded shortly afterward that they were bodies circling Jupiter. He published his observations in March 1610 in Sidereus Nuncius, or The Starry Messenger. The moons were initially called the Medicean Stars in honor of the Medici family, but the names Io, Europa, Ganymede, and Callisto—proposed by Simon Marius—eventually became standard.
Some accounts mention late 1609 because calendar systems and retrospective observations can shift the date. Galileo’s observations began in January 1610 on the Gregorian calendar, and 1610 is the accepted year used in general astronomy and trivia. The discovery provided powerful evidence that Earth was not the only center of celestial motion.