Jupiter has four moons that are comparable to the Earth's Moon in size and can be relatively easily observed. All other Jupiter's satellites are orders of magnitude smaller.
The four biggest satellites that are relatively easy to see are Ganymede, Callisto, Europa, and Io. Were it not for the brightness of Jupiter, these bodies would be visible even with the naked eye.
These moons are called Galilean satellites because they were the only four moons that Galileo Galilei was able to see. Galileo is the first astronomer to discover these moons. On 7 January 1610 he observed the planet and saw what he thought were three fixed stars near it, aligned out on a line through the planet. Galileo viewed them (together with Jupiter itself) as a tiny model or our Solar system, as serious hint that that planets likely rotate around the Sun. It was revolutionary at that time, when the official view was that Sun and planets rotate around the Earth.
The orbits of Ganymede, Europa and Io are locked in a 1:2:4 orbital resonance. Galilean satellites are among the most massive objects in the Solar System outside the Sun and the eight planets, with a radius exceeding than any of the dwarf planets. Ganymede is the largest satellite in the Solar system.
Applet has been written by Akkana Peck