Galilean moons

The Galilean moons (ˌgælᵻ'liː.ən), or Galilean satellites, are the four largest moons of Jupiter: Io, Europa, Ganymede, and Callisto. They are the most readily visible Solar System objects after Saturn, the dimmest of the classical planets, which are readily visible from Earth by the unaided eye, even under night sky conditions of high light pollution. Visible with common binoculars, the invention of the telescope enabled the discovery of the moons in 1610. Through this they became the first Solar System objects discovered since humans have started tracking the classical planets, and the first objects to be found to orbit a planet other than the Earth. They are among the largest objects in the Solar System with the exception of the Sun and the eight planets, with radii greater than any of the dwarf planets. The largest of the four are Ganymede, which is the largest moon in the Solar System, and Callisto, both of which are either larger or as large as the planet Mercury, though not nearly as massive. The smaller ones, Io and Europa, are about the size of the Moon. The three inner moons—Io, Europa, Ganymede—are in a 4:2:1 orbital resonance with each other. While the Galilean moons are spherical, all of Jupiter's, much smaller, remaining moons have irregular forms because of their weaker self-gravitation. Named after Galileo Galilei, who observed them in either December 1609 or January 1610, and recognized them as satellites of Jupiter in March 1610, remaining the only known moons of Jupiter until the discovery of the fifth largest moon of Jupiter Amalthea in 1892. Galileo initially named his discovery the Cosmica Sidera ("Cosimo's stars"), but the names that eventually prevailed were chosen by Simon Marius. Marius discovered the moons independently at nearly the same time as Galileo, 8 January 1610, and gave them their present individual names, derived from rape victims of Zeus, which were suggested by Johannes Kepler, in his Mundus Jovialis, published in 1614.