The only natural satellite of the Earth, orbiting every 27.322 days at an average distance of 384 400 km. The magnitude of the full Moon is −12.7, but its surface is actually dark, with a mean geometric albedo of only 0.12, lower than for all the planets except Mercury. It is the fifth-largest satellite in the Solar System (diameter 3475 km), over a quarter the diameter of the Earth and about 1/81 the Earth’s mass. Being so similar in size, the Earth and Moon are often considered a double planet. The Moon’s sidereal period of axial rotation, 27.322 days, is the same as its orbital period, so that it keeps the same face towards the Earth. Its equator is inclined by 1°.53 to the plane of the ecliptic. Surface temperatures vary from extremes of 123°C during the day down to −233°C at night; typical values are 107°C (day) and −175°C (night). Polar regions of the Moon contain craters with permanently shadowed floors, where signs of ice have been detected by spacecraft.

The Moon shows two distinctly different types of terrain with very different densities of impact craters: the brighter highlands and the darker lowland mare areas. The lunar highlands have an albedo of 0.11–0.18, and are saturated with large craters of 50 km diameter and greater; the maria have an albedo of 0.07–0.10, and consist of younger plains of basaltic lava with few large craters. The mare basalts are enriched in iron and titanium, and have abundant pyroxene. The highland rocks are chemically different from the maria, being enriched in calcium and aluminium and consisting mainly of feldspar. The highlands date from before 4 billion years ago, whereas the maria were mostly erupted between 2 and 3.9 billion years ago. The farside of the Moon has few dark mare areas but does contain the largest and oldest impact structure on the Moon, the South Pole–Aitken Basin. The prevalence of maria on the nearside may be due to the impact that formed the Imbrium Basin, which created deep fractures within the Moon through which erupting lava later poured.

The Moon has an exceedingly tenuous atmosphere consisting of outgassed elements such as radon arising from radioactive decay in the lunar interior, plus temporarily trapped solar wind particles. Because of the lack of any effective atmosphere, the main erosive process is impact cratering. Lunar craters vary in size from tiny pits less than 1 mm across to major impact basins over 1000 km in diameter. Young impact craters, such as Tycho, are very bright, with prominent central peaks, terraced walls, and bright rays radiating far across the surface. Older craters are gradually worn down and smoothed over by tiny impacts, or obscured by bigger ones or lava flooding. The constant churning of the surface by small impacts has created a soil layer, or regolith, 5–15 m deep over the entire Moon.

Lunar volcanic craters are rare and comparatively small, only a few kilometres in diameter at most. Lunar domes with shallow slopes and summit pits appear to be the equivalent of shield volcanoes on Earth. There are a few tiny cinder cones, plus some bigger collapse pits and calderas. Many of the calderas are the sources of the sinuous rilles, the channels that supplied the extensive fluid lavas of the mare plains. Wrinkle ridges and rilles bear witness to forces of compression and tension on the Moon. They are frequently found in concentric patterns within or around the impact basins.

The interior of the Moon consists of a thick lithosphere down to about 800 km. Below it is an asthenosphere, with perhaps a small core less than 700 km in diameter. Moonquakes are minor events compared with earthquakes, and tremors occur regularly in the same places each month as a result of tidal forces. There is no significant magnetic field.

It is now thought that the Moon formed when the Earth was struck a glancing blow by a passing body similar in size to Mars, sending ejecta from the Earth and the impactor into orbit, where it accreted to form the Moon.

http://photojournal.jpl.nasa.gov/catalog/PIA00302