Earth: The Moon

Facts in Brief

Mass	7.349 x 10²² kg
Diameter	3,474.8 km
Orbital Period	27.322 Days
Average Lunar Day	29.53 Days
Surface Gravity	1.62 m/s²
Escape Velocity	8,568 kph
Mean Surface Temperature (Day)	107^oC
Mean Surface Temperature (Night)	-153^oC

The Moon, brightest object in the night-time sky, is one of the most potent symbols in mankind's psyche. In mythology it is Artemis-Diana, chaste goddess of the hunt, or Chang O, who stole herbs from her husband the sun-killer. In folklore it inspires lunacy, excites werewolves, and cures warts. Rare events happen once in a blue moon. The cow jumped over the moon. Even now, the moon lays hold of the popular imagination. People turn out for lunar eclipses. Almanacs maintain an elaborate system of lunar nomenclature. Astronomers find the moon even more fascinating. So close to the Earth, our nearest neighbor affects us in greater ways than any other non-solar body. Thanks to the recent discovery of water on the surface, a lunar space station is now more probable. The regolith could be mined for vast quantities of hydrogen fuel. Even today, when spirits and magic have been banished, the Moon is still quite exciting.

Orbit

The Moon orbits around the Earth every 29.5 hours. The basis for the year's twelve months, the first calendars, among them the Hebrew and ancient Chinese calendars, were exclusively lunar. Since twelve lunar orbital periods do not equate to one of the Earth's orbital periods, however, these calendars had to be continually adjusted. As the Moon orbits, its phases change. It appears full, than wanes (appears to narrow), then waxes (regains its lost slivers). This is due to the difference in the angle between the Moon and Sun, whose reflected light gives the Moon its brilliance.

Structure

     Due to its size and structure, the Moon is sometimes grouped with the terrestrial planets. Its innermost layer is a small core that constitutes 2% of its core. Surrounding that is a partially-molten mantle. The outermost layer is a crust an average 68 km thick. The Earth appears to influence the Moon's structure. The lunar center of mass is offset 2 km towards Earth. The crust is on average thinner on the Earthward side.
     The surface of the Moon can be divided into two distinct terrains: the smooth maria (Latin for "seas") and the cratered highlands, which are older. The maria, enormous impact craters, make up scarcely more than 16% of the surface. The maria's names are a tour of illustrious scientists: Tycho, Copernicus, and Ptolemaeus. One of them, the South Pole-Aitken Crater, measures 2,250 km in diameter and 12 km in depth, and is the solar system's largest impact basin. The remainder of the surface, ancient and battered, is reminiscent of the surface found on Mercury. Nearly all of the crust is covered with ground and gritty debris, termed the regolith. There is now conclusive evidence that water exists on the lunar surface, particularly in the North Pole region.
     There is no atmosphere on the Moon. The Moon has no magnetosphere.

Missions to the Moon

The Moon is the first body visited by a spacecraft, USSR's Luna 2 in 1959. It is also the first and (so far) only non-terrestrial body to have felt the footsteps of man. This was the culmination of the US's Apollo Program, which landed Neil Armstrong and Buzz Aldrin on July 20, 1969. Over the years more than 375 kg of rock samples have been ferried from the Moon to Earth. Most recently, the US's Lunar Prospector thoroughly mapped the Moon in 1999.

The Tides

The side of Earth closer to the Moon is more influenced by the latter's gravitational pull. Speaking in gross simplifications (tides would be another webpage in themselves), this attraction creates a bulge in the direction of the Moon and a bulge directly opposite it. This and the rotation of Earth, which is faster than the Moon's, daily creates two high tides.
The gravitational attraction has more effect than merely creating tides, however. It also transfers rotational energy from the Earth to the Moon. The Earth's rotation slows by 1.5 milliseconds a century. This will eventually result in a rotation of Earth that matches the lunar rotational period, as is the case with Pluto and its moon Charon.

An Almanac's Listing of the Moons

January: Wolf Moon
February: Ice Moon
March: Storm Moon
April: Growing or Pink Moon
May: Flower Moon
June: Strawberry Moon
July: Hay Moon
August: Corn or Sturgeon Moon
September: Harvest Moon
October: Blood or Hunter's Moon
November: Beaver Moon
December: Cold Moon