The four large inner moons of Jupiter were discovered by Galileo
Galilei, in January of 1610. As a result, they are often
referred to as the Galilean satellites. Aside from their common
discovery, the moons are closely linked by gravitational
interactions with each other and Jupiter. Their mutual
interactions cause them to have almost exactly commensurate
orbits -- that is, the orbital period of each moon is closely
linked to that of its neighboring companions, according to the
ratio of two small whole numbers. Thus, the orbital period of
Europa is almost exactly twice (2/1) that of Io, the orbital
period of Ganymede is almost exactly twice that of Europa, and
the orbital period of Callisto is almost exactly 7/3 that of
Ganymede. This sort of regularity is very common in the Solar
System, being found in satellite systems (as noted here), in the
spacing of the planets, in the spacing of gaps in the rings of
Saturn, and in the "Kirkwood gaps" in the asteroid belt (see the
second half of Orbital Regularities). I