Earth's only or at least, only significant natural satellite. While it's referred to as the Moon, it's called by its Roman name, Luna, when differentiating it with the other satellites in the Solar System. It has been named Selene, Cynthia, and Diane by the Roman and Greek ancients as well. Of course, this is where the word "lunar" comes from, as well as the Greek prefix seleno- (e.g selenophobia, fear of the Moon.)
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. The Moon also has roughly 2/9 the mass of Mercury, and is about 1.8 times more massive than all five recognized dwarf planets (Eris, Pluto, Makemake, Haumea, and Ceres), Pluto's satellite Charon (which is more massive than Ceres), and the Asteroid Belt (Ceres excluded) combined. Among the moons of Jupiter and Saturn, only Ganymede, Titan, Callisto, and Io are more massive than the Moon. Of them only Io is dense enough to have a higher surface gravity than the Moon. The Moon is so large in comparison to the Earth that the center of gravity between the two is about two thirds of the Earth's radius out from its center, causing the Earth to noticeably wobble around it. This has led some to believe the Earth-Moon system should be considered a double planet.
Currently, our best guess at how such a humongous companion came into existence is that a Mars-sized planetesimal 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. New discoveries support this theory, as scientists have found evidence that the Moon was tidally locked only a hundred days after the collision, "baking" one side of the Moon, with the other side thickening from the vaporized crust, explaining the odd crust dichotomy of the satellite. Even more amazingly, the "strange lights" that have been seen in the last few decades are probable indication that the Moon's core is Not Quite Dead, such as the flashes of light from Aristarchus, one of the ancient lunar volcanoes. In reality, instead of being UFO's, it's actually the heated expulsion of dust from the Moon's crust.
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 mythology. The fact that the Moon's 29-and-a-half-day light cycle is very similar in duration to the average woman's menstrual cycle has also not escaped the notice of poets, philosophers, and biologists; indeed the moon is often associated with femininity in literature and myth. However, there are more male lunar deities than female lunar deities. While Greeks and Romans considered the moon a woman, to the Germanic peoples it apparently was a man, which is how we get The Man in the Moon.
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.
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 Tranquilitynote , the Sea of Rainsnote , the Sea of Fecunditynote , etc. Another interesting note is that the Moon does have a thin atmosphere, called an "exosphere". It's visible when there's a new moon and the Moon shows off a sulfurous "tail".
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 dustnote .
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.
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. note In a short time (on a geological scale, at least), the Moon will be too far away to cause total solar eclipses. note Eventually, it will be far enough away to leave Earth orbit entirely, and wander through space just like in Space: 1999, though by the time that will actually happen, the Sun will have expanded into a red giant and engulfed the Earth already.note
Examples with the Moon as a place
- 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.
- In 2001: A Space Odyssey (written and premiered in the years prior to the Apollo landings) mankind has an established moonbase and space travel to the moon is a semi-mundane trip.
- Radar Men from the Moon: The Moon is inhabited by Human Aliens, who are preparing to invade Earth because the Moon's atmosphere is getting too thin and dry.
- 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.
- Cat-Women of the Moon and the remake Missile to the Moon inform us that the Moon is populated by a Lady Land of gorgeous babes. We are not meant to take this seriously.
- Destination Moon is a hard sci-fi depiction of the first landing on the Moon, albeit involving a single-stage nuclear rocket.
- Despite being made in 1929, Frau im Mond (aka Woman in the Moon) also was a serious attempt to depict such a landing, with some Necessary Weasel due to it being a silent movie (e.g. having atmosphere on the Moon because the actors rely entirely on facial expressions).
- Project Moonbase was co-written by Robert A. Heinlein but is not one of his better efforts, thanks to a blatantly sexist portrayal of the female protagonist even for a movie made in The '50s.
- 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.
- 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. However, they never reach the Moon, only orbit around it and back (as the title implies).
- 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.
- Doctor Who: The 1966 story "The Moonbase", set in 2070, has the Second Doctor visit a Moonbase that controls Earth's weather and stop the Cybermen (in their second appearance) taking over the Moonbase.
- Apollo 13, being based on the Real Life Apollo moon mission, has the Moon as the crew's ultimate (original) destination.
- Apollo 18 is a fictionalized account of a top secret final Moon landing, presented as a Found Footage Film.
- The opening of Independence Day features the alien mothership flying past the Moon, so close that its gravity (or engine-emission vibrations, we can't really tell) disturbs the Apollo 11 landing site.
- The film Moon takes place on the titular worldlet.
- The Walter Koenig vehicle Moontrap involves alien intelligences lying in wait on the Moon.
- In Superman II, the escaped Krypton criminals land on the moon before they reach the Earth. Woe to the lunar astronauts who happened to be there at the time.
- Doctor Who:
- In 2007's "Smith and Jones", an entire hospital is teleported to the Moon by Space Police hunting an alien fugitive. The Doctor and Martha Jones meet and become a team in the confusion.
- 2014's "Kill the Moon" pulls That's No Moon! with the Moon, claiming that it's actually a giant egg that's about to hatch. Fortunately, the newborn creature lays a new Moon at the end.
- The 1998 miniseries From the Earth to the Moon is about the race for the moon in the 1960's.
- 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.
- 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.
- In Star Trek, the Moon has been colonized by Earth, with cities like Tycho City and New Berlin. According to Will Riker, the total lunar population is over 50 million.
- This is the main battle ground in Asura's Wrath for the final fight between Asura and Augus, as well as the fights against Evil Ryu and Akuma/Oni.
- Although it hasn't yet played any major role, in the Halo universe the Moon is the site of the UNSC's officer candidate academy; more than one Naval character from the series, including the famous Captain Keyes, received their training there.
- Mass Effect has a mission on the Moon to stop a rogue military Virtual Intelligence (VI) which has taken over some defense and training systems.
- The sixth mission of the Soviet campaign in the Yuri's Revenge expansion of Red Alert 2 has you chase the titular villain to the Moon so that he can't set up shop there. The gameplay becomes somewhat different in the mission compared to normal - there is no ore to mine, for instance, and you can produce high-flying Cosmonauts (basically normally-Allied Rocketeers specific to this mission) from Soviet barracks.
- At the beginning of Star Control II the quest you must do in order to have Commander Hayes and the Earth Starbase in your side is to deal with a base left in the Moon by the Ur-Quan Hierarchy. Once you go there, and especially when you find Fwiffo, you'll find Hayes' reports were very inaccurate.