January 7, 1610Galileo Galilei makes his first observation of the four Galilean moons
: Ganymede, Callisto, Io and Europa, although he is not able istinguish the last two until the following day. The Galilean moons are the four moons of Jupiter discovered by Galileo Galilei in January 1610. They are the largest of the many moons of Jupiter and derive their names from the lovers of Zeus: Io, Europa, Ganymede and Callisto. Ganymede, Europa and Io participate in a 1:2:4 orbital resonance. They are among the most massive objects in the Solar System outside the Sun and the eight planets, with radii larger than any of the dwarf planets.