The Moon



Earth's only -- or at least, only significant -- natural satellite.

It orbits our planet some 400,000 kilometers away, taking 27.3 days to go all the way around once. (Since the Earth will have moved some distance around the sun by the time the moon has orbited once, it takes a little longer -- 29.5 days total, to be precise -- for the lunar light-cycle to get back around to the same phase it started in.) Tidal forces long ago caused the moon to lock in synchronous rotation with the Earth, so that the same side is always facing us.

Compared to other moons in The Solar System, Earth's moon is really huge compared with the planet it orbits, weighing in at a whopping 1/81 of Earth's mass and 1/6 of Earth's surface gravity. By comparison, even the largest moon of Saturn is only 1/4000 of Saturn's mass. Currently, our best guess at how such a humongous companion came into existence is that a Mars-sized planetessimal struck the Earth early in its formation period, which knocked loose a huge chunk of material that eventually cooled, congealed, and settled into the moon's current nearly-circular orbit. However, a recent comparison of the Earth-moon titanium isotope ratio has thrown this model into question.

The moon has been with us since before the dawn of the human race, progressing through its utterly predictable phases night after night. For most of human prehistory, it was the only light source available to us at night, which lent it a good deal of mystique. Lunar deities are almost as prevalent as Solar deities in human culture; in fact, the very name Luna was originally the name of a goddess from Classical Mythology who personified the moon. The fact that the moon's 29-and-a-half-day light cycle is very, very similar in duration to the average woman's menstrual cycle has also not escaped the notice of poets and philosophers, and indeed the moon is often associated with femininity in literature and myth. Loss of personal control and going berserk are also associated with the full moon, and not just when dealing with werewolves; the very word "lunatic" refers to the moon.

The moon is also the major cause of tides on the Earth. When the moon is directly above you or directly below you (i.e. on the opposite side of the Earth), tides are highest; when it's 90 degrees off to one side of you, tides are lowest. The Sun also causes tides, but these tides are much weaker than the moon's.

Sadly, the moon will not be with us forever. Those same tidal forces that pull on the Earth's oceans and locked the same face of the moon toward the Earth are also, very slowly, widening the moon's orbit. In a short time (on a geological scale, at least), the moon will be too far away to cause total solar eclipses. Eventually, it may leave Earth orbit entirely, and wander through space just like in Space: 1999.

The moon's surface is covered in craters, caused by comet and asteroid impacts in the ancient (and, occasionally, recent) past. Each of those craters has a name, and most are named after scientists and philosophers. For instance, the great big crater with the huge white rays coming out of it in all directions is Tycho Crater, named after Tycho Brahe. The moon's surface is also partially covered by "Seas" (maria in Latin), dark areas where ancient volcanoes spilled lava all over the place. Like the craters, each Sea has a name, but unlike the craters the Sea names are derived from things that sailors might be concerned about -- the Sea of Tranquility, the Sea of Rains, the Sea of Fecundity, etc..

Despite how bright the moon may appear in the night sky, its surface is very very dark. Its albedo is a dismal 7%, which means that 93% of all incident light is absorbed without being reflected back into space. For comparison, Earth's albedo is around 38%. The difference in color between the light-colored regolith and the dark-colored maria is like the difference between coal dust and extra-dark coal dust.

Since the moon keeps the same face pointed toward Earth at all times, the far side of the moon can't be seen from the Earth's surface, and it wasn't until the advent of the first space probes that we had any idea what the far side looked like. (It's got a lot less maria and a lot more craters than the near side; the slightly greater density of the dark maria material may be why the maria-rich side ended up facing Earth.) Both the near side and the far side wax and wane through light-and-dark phases, so it's incorrect to call the far side the "Dark Side of the Moon" except during the brief period every month while the moon appears Full in Earth's skies.

Pre-Apollo

Film

 * A Trip to the Moon, the first movie to rely on special effects to tell the story of a trip, featured people getting shot to the moon inside a giant cannon shell -- which gave the Man in the Moon a black eye.

Comic Books

 * The Tintin graphic novel Explorers on the Moon features a surprisingly realistic take on what travelling to the moon would be like, despite being written pre-Sputnik.

Literature

 * H. G. Wells's The First Men in the Moon has its protagonist travel to the moon through the use of "cavorite", a magical Anti Gravity metal.
 * The Mouse on the Moon, the 3rd installment in the Mouse that Roared series, features the mini-country of Grand Fenwick embroiled in The Space Race with the Americans and Soviets. They get to the moon with a rocket powered by wine fermentation.
 * Jules Verne's From the Earth to the Moon has a huge bullet-like spacecraft shot off a cannon with three people in it. The book ends there, but the sequel, Around the Moon, deals with the space travel..

Post-Apollo

Film

 * The Walter Koenig vehicle Moontrap involves alien intelligences lying in wait on the moon.
 * Apollo 13, being based on the Real Life Apollo moon mission, had the moon as the crew's ultimate (original) destination.
 * The film Moon takes place on the titular worldlet.

Live Action TV

 * Space: 1999 takes place on Moonbase Alpha in the far distant future year of 1999. A nuclear explosion on the moon's surface knocks it out of Earth orbit, sending it drifting through the galaxy rapidly enough to pass through a new star system every week.
 * The pilot episode of Salvage 1 features Andy Griffith managing a mission to the moon in a homemade rocket. (They can get away with this because their NASA reject friend has concocted a rocket fuel hundreds of times more efficient than anything the space program has yet put into production.) His intent is to salvage all the "junk" the Apollo astronauts left lying around on the moon and sell it.