From the earliest traceable mythologies, humankind has dreamed of reaching the moon. It is only comparatively recently that scientific analysis and the Apollo missions discovered that the moon is probably not itself capable of sustaining its own life. However, it has also been found to contain all the elements essential to potentially supporting a self-sustaining human colony.
The moon is one of three natural satellites linked with the earth (the others being Cruithne and J002E3), although it is the only one in long-term orbit around the earth and not in an interlinked co-solar orbit. It orbits at an average centre-to-centre distance of 384,403 kilometres, roughly 30 times the diameter of the earth. With a diameter of 3,450 kilometres, the moon is one of the largest planetary satellites in the solar system. It does have the single largest planetary ratio within the solar system: which is why the Earth-moon pair is sometimes referred to as a double planet. Lunar gravity is approximately one-sixth of Earth's.
The lunar orbit is 27 days, 7 hours, and 43 minutes long, also known as one sidereal month. During this period of time it goes through a full cycle of lunar phases, from new moon to full and back again to new. Since it is tidally locked to the Earth, this is also the same length of time as a single lunar day. However, because the moon's axis is tilted by 1.5 degrees, we are not quite limited only to a single face, but also occasionally see small sections at the edge of the "dark" side: about 59% of the total moon surface.
Although it has a high reflectivity factor, the moon produces no light of its own. Its brilliance is due solely to reflected solar light.
The moon has no magnetic field, and only a trace atmosphere at the bottom of craters: resulting in sharp day-night transitions and severe temperature variance. Daylight temperatures rise over 100 degrees Celsius and plunge to -140 degrees Celsius at night. Such extremes would normally create extreme weather, but since the moon has no appreciable atmosphere, it also has no weather. Thus the two major geological processes involved in the shaping of its surface are vulcanism and meteoric cratering. General geological divisions are impact craters, among the most famous of which is the rayed Tycho; mountainous regions which sometimes ring "splash" craters and sometimes are believed to have a volcanic origin; rills; and large, relatively flat but heavily cratered areas known as maria, or seas.
The moon's origin is unknown. Different theories suggest it was co-created with the Earth or spun off from a pre-Earth mass: but these do not account for its significantly different geology. Another theory suggests that the moon may have been created elsewhere in the solar system or perhaps even in the galaxy, and been captured by the Earth's gravitational field. This is believed to have happened with the asteroid J002E3, the third of Earth's moons.