Earth's moon is somewhat of a puzzle. It's size (3476km dia) is way out of proportion to that of the planet, and in fact the only larger planetary satellites in the solar system are those of the gas giants - Jupiter (Io: 3630, Ganymede: 5262, Callisto: 4800) and Saturn (Titan: 5150) which makes our moon 5th in size. The next largest moon orbiting one of the inner planets is Mars' Phobos which is only 22km in dia. In fact the moon is larger than the planet Pluto, and omly about 2/3 of the diameter of Venus.

I seem to remember at sometime that without the moon, Earth's atmosphere would be much smaller - does anyone have any ideas about that ?