moon

A moon is a natural satellite of a planet. (A satellite is an object orbiting a planet, and can either be natural, like the Moon, or artificial, like HST.)

More specifically, the Earth's natural satellite is known as the Moon:

• perigee: 362600 km
• apogee: 405400 km
• semi-major axis: 384399 km
• eccentricity: 0.0549
• orbital period: 27.321661 days (sidereal)
• solar day: 29.5 days
• average orbital speed: 1.022 km/s
• orbital inclination: 5.145° to the ecliptic.
• axial tilt: 1.5424° from perpendicular to the ecliptic or 6.687° from perpendicular to its orbital plane.
• 1737 km radius (0.27 Earth radii).
• 0.012 Earth masses.

Current theory on the formation of the Moon favors the giant-impact hypothesis, that the Moon coalesced from orbiting debris from an impact between Earth and a Mars-sized planet, very early in solar system history. Characteristics of lunar material show a striking similarity to that of Earth in abundances and radioactivity, as compared to other planets such as Mars. This supports the giant impact hypothesis, yet it is too much like Earth, providing no evidence of different material from the presumed impacting planet.

The moon is equivalent to a rocky planet within the Sun's habitable zone but that hasn't led to its habitability. It essentially lacks water though some exists as ice in shadows and Moon surface material may contain a very small amount.

The Moon is thought to have a relatively small iron core and does not have the type of magnetic field that suggests a current dynamo and it is thought the Moon is internally solid (unlike Earth). The Moon's extant magnetic rocks might reveal its early thermal history, from before it cooled.

Tidal locking keeps the Moon's same face toward the Earth, and it rotates roughly perpendicular to the ecliptic, so valleys at its poles have shadows that never receive light from the Sun. Places cold enough to keep water ice frozen are called Lunar cold traps.

Other well-known solar system moons:

No moons are known for Mercury or Venus, but moons have been found for more than 400 minor planets (minor planet moons), e.g., minor planet Orcus. Currently, moons are being discovered every year around the outer planets and minor planets.