The Moon

The Moon revolves once on its axis each time it orbits the Earth, thus always presenting the same face to earthbound observers. However, even to the unaided eye this unchanging face shows to contrasting types of landscape dark, plain-like areas of low relief, and brighter, decidedly more rugged regions which cover about two-thirds of the surface. Early astronomers mistakenly referred to the smooth dark areas as maria (or seas) giving the name terrae (or lands) to the bright upland regions. The terms have persisted since, even though the Moon's surface has long been known to be completely waterless.