Full Moon. Computer artwork of the full Moon on a starfield background rising above a limb of the Earth. The Moon is the only natural satellite of the Earth. The Moon's rotation period around the Earth is equal to its period of revolution of 27.3 days. For this reason the Moon keeps the same face turned towards the Earth at all times. The visible side of the Moon is marked with large dark areas called the lunar maria (or seas) and craters caused by the impacts of falling meteorites. The lunar maria are relatively empty of craters suggesting that these basins were filled recently by volcanic material.