|The four largest and most prominent of Jupiter's moons. The nearest to Jupiter is Io, followed by Europa, Ganymede (the largest moon in the Solar System), and finally - orbiting nearly two million kilometres from its parent planet - Callisto. All four are clearly visible through all but the smallest telescopes, changing position nightly as they travel around Jupiter. They carry the name of Galileo, who discovered them in 1610. Though he was initally baffled by the sight of the shifting objects, he soon came to understand what he had found, and used them as an important part of his evidence against the old geocentric view of the universe.