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[[image:Moon2.jpg|right|thumb|350px|The Moon, age 21 days]]  

[[image:Moon2.jpg|right|thumb|350px|The Moon, age 21 days]]  

The '''Moon''' is a natural satellite orbiting the Earth at an average distance of about 238,000 miles (380,000 km). Its mass is great enough that the earth and the moon both move around a common point located at their common [[centroid|center of mass]] or ''barycenter''. <ref>The Earth and the Moon orbit each other around a gravitational midpoint called the ''barycenter.'' This point is, however, actually inside the Earth, about 3/4 from its center. It is not the Earth, but the barycenter of the Earth/Moon system, that orbits the Sun in a near-perfect ellipse. Relative to this ellipse, Earth is 3,000 miles closer to the Sun at full moon and 3,000 miles further from the Sun at new moon.</ref>

The '''Moon''' is a natural satellite orbiting the Earth at an average distance of about 238,000 miles (380,000 km). The Earth and the Moon both move around a ''barycenter'', their common [[centroid|center of mass]].<ref>The Earth and the Moon orbit each other around a gravitational midpoint called the ''barycenter.'' This point is, however, actually inside the Earth, about 3/4 of its radius from its center. It is not the Earth, but the barycenter of the Earth-Moon system, that orbits the Sun in a near-perfect ellipse. Relative to this ellipse, Earth is 3,000 miles closer to the Sun at full moon and 3,000 miles further from the Sun at new moon.</ref>

When speaking of things having to do with the moon, the adjective ''lunar'' is used. The Greek-derived prefix ''seleno-'' is used as well; for example, the science of mapping the moon is called [[selenography]].

When speaking of things having to do with the moon, the adjective ''lunar'' is used. The Greek-derived prefixed root ''selen-'' followed by the morpheme ''-o-'' is used as well; for example, the science of mapping the moon is called ''[[selenography]]'' (morphemes: ''selen-o-graph-y'').

The gravitational force of the Moon causes the ocean [[tides]]; these tidal forces have other effects as well. These tidal forces have the effect of creating resistance which slows down the rotation of both objects. In the case of the Earth, the result is that the Earth's rotation slows on the order of one millisecond per century. In the case of the Moon, over time the Moon has entirely lost its rotation relative to the Earth. The Moon's rotation is now locked in to its period of revolution, and the same side of the Moon always faces the Earth.

The gravitational force of the Moon causes the ocean [[tides]]; these tidal forces have other effects as well. These tidal forces have the effect of creating resistance which slows down the rotation of both objects. In the case of the Earth, the result is that the Earth's rotation slows on the order of one millisecond per century. In the case of the Moon, over time the Moon has entirely lost its rotation relative to the Earth. The Moon's rotation is now locked in to its period of revolution, and the same side of the Moon always faces the Earth.