inner moons -- Io, Europa, and Ganymede -- are in a 4:2:1 orbital resonance with each other. While the Galilean moons are spherical, all of Jupiter's remaining moons have irregular forms because they are too small for their self-gravitation to pull them into spheres.

The Galilean moons are named after Galileo Galilei, who observed them in either December 1609 or January 1610, and