THIS DAY IN HISTORY:

Galileo Galilei Discovers Three of Jupiter's Four Largest Moons (1610)
Jupiter has more than 60 moons. The four largest—Io, Europa, Ganymede, and Callisto—were the first satellites of a planet other than Earth to be detected. They were discovered by Galileo in 1610, shortly after he invented the telescope, and are therefore known as the Galilean satellites. On January 7, 1610, Galileo observed near Jupiter what he described at the time as "three fixed stars, totally invisible by their smallness."