Mean Radius of the Moon

The Moon is the roundest large body in the inner solar system: its polar and equatorial radii differ by only a few kilometres, because it is small enough that its own gravity dominates over any rotational bulge, and it long ago spun down to synchronous rotation. Its mean density of 3344 kg/m³ is far below Earth's 5514, telling us immediately that it has only a tiny iron core.

The far side is systematically higher and its crust roughly twice as thick — a hemispheric asymmetry mapped by GRAIL and still not fully explained. The radius also sets the coincidence that makes total solar eclipses possible: the Moon is about 400 times smaller than the Sun and currently about 400 times closer, so the two discs match almost exactly, a temporary alignment in the history of the solar system.