GALILEAN SATELLITE

any of the four large satellites of Jupiter discovered by Italian astronomer Galileo Galilei in 1610. Originally christened the Medicean stars by Galileo (who was then seeking the patronage of the Medici family), the satellites were individually named by German astronomer Simon Marius after four figures in Greek mythology with whom Jupiter fell in love. In order of distance from Jupiter, the satellites are Io, Europa, Ganymede, and Callisto. The amount of ice relative to rock in these bodies increases systematically with distance from Jupiter and is thought to reflect a primordial high-luminosity phase of the planet at the time of their formation. The orbits of the inner three satellites are in a resonant state, such that conjunctions of Io and Europa and conjunctions of Europa and Ganymede always occur on opposite sides of Jupiter.