The Moon, Earth's major satellite
The Moon orbits the Earth and is its only significant natural satellite. It is believed that about 4.5 billion years ago, a planet slightly larger than Mars struck the young Earth obliquely and later disintegrated. A small portion of the planet's original mass, trapped in orbit around the Earth, re-amalgamated to form the Moon. The early Moon was probably molten but, when it cooled, low-density plagioclase crystals grew within the liquid and floated to the surface where they collected to form a thick layer of anorthosite rock. The Moon is currently receding from the Earth by a few centimetres every year and will eventually be free of Earth's gravitational pull if the Sun does not melt the Earth-Moon system first. Because of its incredibly weak gravity, in order to walk on the Moon, an astronaut would have to practice a completely different way of walking.
Seen from the Earth, the Moon's surface appears bright with distinct, darker areas. The bright regions form a rugged terrain, heavily marked with circular meteorite impact craters known as the lunar highlands. We now know that the highlands are made of rocks called anorthosite and polymict breccia. The anorthosite is a rock made largely from a white calcium aluminum silicate mineral called plagioclase, while the polymict breccia consists mainly of angular pieces of anorthosite and other materials broken and compacted together by many meteorite impacts.
In contrast to the highlands, the dark areas are quite smooth and low-lying. They are known as the lunar maria (which are seas, though they contain no water). We now know that the maria are made from basalt, formed as molten rock [in a previously hot interior] welled up and flowed out over the lunar surface, covering the older, cratered landscape. Seen in close-up, the lunar surface is strewn with a covering of loose fragments of rock and dust known as the lunar regolith. The regolith is simply the debris created by meteorites that struck the Moon at high velocity and then exploded.
In the late sixties and early seventies, NASA sent seven Apollo missions to the Moon. The first man to step onto the lunar surface, Neil Armstrong, was an Apollo 11 astronaut; the Apollo 17 crew were the last to set foot on the Moon. NASA, ESA, along with the Russian and Chinese space agencies, have recently begun new programs to realise again, a quarter of a century later, mankind's presence on the Moon.
As evidenced by the film 2001: A Space Odyssey, the Moon has been a subject of intense science fiction propaganda. The term lunar is actually derived from the name Luna, which was given to the Moon in various forms by the many ancient civilizations. The Moon has been featured in much folklore, and the Chinese used to believe that during a solar eclipse, a dragon was eating the Sun. Both solar and lunar eclipses occur frequently. During a solar eclipse the Moon blocks the Sun on a particular region of the Earth for a few seconds, creating a "diamond ring" effect moments before the actual eclipse. During a lunar eclipse, the entirety of the moon passes behind Earth’s shadow, giving it a very faint glow.
Author: Astronomy Today Staff