The Rugged Lunar Limb

The Moon is not a perfectly smooth sphere. The mean equatorial diameter is 3,476 km and the mean polar diameter is 3,470 km. But the actual surface, although the lunar maria are relatively smooth, contains some very rugged terrain. Indeed, at the lunar limb, as viewed from the Earth, there is precious little in the way of smooth plains. Most of the maria regions are directly facing the Earth. Also, due to the so-called “librations”, the limb of the Moon never presents the same jagged outline to us.