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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). 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>

The '''Moon''' is a natural satellite orbiting the Earth at an average distance of about 238,000 miles (380,000 km).  

 

Although the Moon is often described as revolving around the Earth, this is slightly inaccurate: the Earth and the Moon ''both'' revolve around their ''barycenter'', their common [[centroid|center of mass]]. The combined motion is like an athlete performing an Olympic hammer throw: one object swings in a large circle, the other almost pivots around a fixed point but actually performs a small opposing circle of its own. <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 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'').

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'').