Most of the other planets in our solar system also have moons. The number of moons ranges from one for the earth, to as many as 21 for Uranus . Two of our planets, Mercury and Venus , have no moons. Jupiter has 16 known moons, Saturn has 18, and Neptune has 8. Pluto has one moon, but this moon is so large in comparison that many astronomers consider the two objects to be a double planet rather than a planet and a moon.
Moons also vary greatly in size.
This makes Ganymede larger than the planet Mercury. All moons orbit their host planets as well as rotating about their own axis. Our moon takes a little over 27 days to make one complete orbit of the earth. Some moons, including our own, orbit in what is called a synchronous rotation. In this case, the time it takes the Moon to make one orbit of the planet is the same as the time it takes the Moon to make one complete rotation on its axis.