The diameter of the Moon is 3,474 km. (Diameter of the Moon in miles: 2,159 miles)
Need to put this in context? The diameter of the Earth is 12,742 km, so the Moon’s diameter is about 1/4 that of the Earth (27.3% to be exact). Need another comparison? The diameter of Jupiter’s moon Ganymede – the largest moon in the Solar System – is 5,268 km across. This makes it about 1.5 times larger than the Moon. The Moon is the only natural satellite of the Earth, and the fifth largest moon in the Solar System.
Like most objects in the Solar System, the Moon spins on its axis, completing a day in 27.3 Earth days. Because it’s rotating, the Moon slightly flattens out. I say slightly, because we’re going to need decimals to really tell the difference. The equatorial diameter of the Moon is 3,476.28 km. And the polar diameter of the Moon is 3,471.94 km. In other words, the Moon’s diameter from side to side is 4.34 km more than its distance from pole to pole.
You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?