The celestial body, sometimes also called Luna, that is the only natural satellite of the Earth. The Moon is in synchronous rotation (i.e., it is "spin-locked") with the Earth, and so always keeps the same face pointed towards the planet. The far-side of the Moon (also called the "dark side") was therefore completely unknown until it was photographed by space probes. The Moon's rotation and orbital periods are therefore the same, days. Moon phases occur with a period of 29.530587 days (the "synodic period"), since

where is the sidereal year, giving

The orbit of the Moon has a 5° inclination to the ecliptic plane which results from perturbations caused by the Sun. Eclipses (either lunar or solar) occur when the line of nodes points towards Earth. This periodicity is called the Saros cycle and has a period of 18 years and 11.3 days. The Moon's inclination to the Earth's equator therefore varies between 18 and 28 degrees.

As a result of torques produced by the Earth's oblateness, the rotation of the Moon is being gradually slowed, and the lost angular momentum causes the Moon to recede 3 cm a year away from the Earth.

The Moon is deficient in siderophiles and volatiles (Na, K, Rb). It is enriched in refractories (Ca, Al, Ti, Ba, Sr, U, Th, and the rare earth elements). Principle moment of inertia measurements indicate that the crustal thickness varies from 40 km and the poles to 150 km on the far side. A seismic velocity of 7.7 km^-1 continues from km, the deepest depth of penetration of seismic energy from man-made objects. The velocity is appropriate for pyroxene or plagioclase. If it is the latter, the crust must extend further down to an unknown depth.

Eclipse, Full Moon, Lunar Cycles, Lunar Eclipse, Lunar Transient Phenomena, Lunation, Month, Moon Phase, Moonrise, Moonset, New Moon, Old Moon in the New Moon's Arms, Saros Cycle, Satellite, Sidereal Year, Solar Eclipse, Synodic Month, Terminator