By modern convention, compass directions when applied to the Moon correspond with the directions used on terrestrial maps. Thus for an observer in the Northern hemisphere North is at the top, South at the bottom, West to the left and East to the right.

Maps of the night sky reverse East and West so that each will face the appropriate horizon. Older maps of the Moon, drawn before it was envisaged that men would visit there, also used this system. This is why the Mare Orientale, the Eastern Sea, is actually on the Moon's western limb.

Inconstant Moon