Galilean moons
![Montage of Jupiter's four Galilean moons, in a composite image comparing their sizes and the size of Jupiter. From top to bottom: Io, Europa, Ganymede, Callisto.](/Images/godic/202501/21/Jupiter_and_the_Galilean_Satellites3434.jpg")
![Galileo Galilei, the discoverer of the four moons](/Images/godic/202501/21/Galileo.arp.300pix3435.jpg")
![The Medician stars in the Sidereus Nuncius (the 'starry messenger'), 1610. The moons are drawn in changing positions.](/Images/godic/202501/21/Sidereus_Nuncius_Medicean_Stars3435.jpg")
![A Jovilabe:[7] an apparatus from the mid-18th century for demonstrating the orbits of Jupiter's satellites](/Images/godic/202501/21/Apparatus_to_demonstrate_the_motion_of_Jupiter's_satellites_in_Putnam_Gallery,_2009-11-243435.jpg")
The Galilean moons are the four largest moons of Jupiter—Io, Europa, Ganymede, and Callisto. They were discovered by Galileo Galilei around January 1610 and were the first group of objects found to orbit another planet. Their names derive from the lovers of Zeus. They are among the largest objects in the Solar System with the exception of the Sun and the eight planets, with radii larger than any of the dwarf planets. Ganymede is the largest moon in the Solar System, and is even bigger than the planet Mercury. The three inner moons—Io, Europa, and Ganymede—are in a 4:2:1 orbital resonance with each other.