The four largest moons of Jupiter. These moons, Ganymede, Io, Callisto, and Europa, were discovered by Galileo in 1610; Galileo was using his 20-power telescope. These satellites were the first bodies known to orbit another planet.
This refers to the 4 largest moons around Jupiter. Galileo was the first to discover them in 1640 and are thus credited to him in name. The four satellites are known as Io, Europa, Ganymede and Callisto. Interestingly these same satellites were discovered independently by Marius, and it is he who is responsible for their individual names.
The four Jovian moons first observed by Galileo ( Io, Europa, Ganymede and Callisto). They are observable with small amateur telescopes.