A moon is a natural satellite orbiting a planet. The Earth's only moon is known rather unimaginatively as "the Moon" (or sometimes Luna if you're feeling fancy.) Being the most noticeable object in the night sky, it is a subject of much woo, particularly in astrology where it is usually given a female quality and a high degree of importance in the sky.
 True scientific facts about the moon
 Tides and spins and sides
The Moon's period of rotation around its own axis is equal to its period of revolution around the Earth. As a result, one of the Moon's hemispheres is permanently facing the Earth, though one can actually see more than a half of the Moon's surface due to libration. This synchronous rotation is caused by the gravitational interaction of the Earth and the Moon, in a process known as "tidal locking". This is not unique to the Earth-Moon system, though, and many other moons in the Solar System are tidally locked or near-locked to their parent bodies. In fact, tidal locking of moons is the rule. Moons not tidally locked to their parent-body are indeed the exception. Most of those moons that are not tidally locked to their planets are in some kind of resonance (i.e. the orbit and revolution are in some small integer relation to each other, like 2:3)
The hemisphere facing the Earth is known as the "near side" and the other one is called the "far side". There is no "dark side of the Moon" in the sense of light and dark. Both "sides" experience day and night, which is what causes lunar phases (with a full moon being the result of sunlight illuminating all of the near side; the sun only lighting the far side results in a new moon or no visible moon). The only difference is that on the near side it is a little bit lighter during the night (new moon) due to the illumination provided by the sunlight reflecting off the Earth.
The fact that the same side of the Moon always faces Earth has led to the popular misconception that the Moon does not rotate on its axis. Its tidal locking to the Earth means that it rotates once every time it orbits the Earth; its "day" and its "month" are the same length. In fact if it did not rotate on its axis, the entire surface of the Moon would be visible to us over the course of its orbit around us.
The Moon's gravity causes the tides on Earth. This was independently discovered by many ancient civilizations, including the Maoris and the Babylonians. Later, Isaac Newton showed that tides are a consequence of gravity, but obviously this is only a theory. Noted scientist Bill O'Reilly instead sees the tides as definitive proof of God's direct intervention: tide goes in, tide goes out. Never a miscommunication. You can't explain that.
It is thought that the moon may have been very important in the development of life, as the tidal activity caused by its gravity essentially churned the oceans, mixing up the chemical ingredients that could have caused abiogenesis to occur. It's also thought to have stabilized the Earth's wobble, so that the Earth now stays at a constant 23.5-degree axial tilt, thus regularizing the seasons.
The angular diameter of the Moon varies from 29.3 minutes to 34.1'. The angular diameter of the Sun varies from 31.6' to 32.7'. Thus, they always appear closely similar in size, and for some of the time their apparent sizes are indeed exactly equal. This means that some eclipses create the fabled "diamond ring" effect. This is of course proof that goddidit
The Moon, as the second most hallowed object in the sky, is honored by having first (or second, depending on your calendric issues) day of the week named after it: Monday.
Creationists argue that the moon proves young Earth. In a word, no. It's not, although it's been touted as one.
The Moon has been a source of astronomical study for at least three millennia. To say that science has failed to explain key elements of lunar motion, geography, or composition, is to be willfully blind. Nonetheless, many creationists fabricate "errors" in scientific studies of the Moon, and suggest that, since science has failed, the only alternative is creationism. This is gap-filling with God to an absurd degree. Assuming, for once, that science could not explain the Moon, this does not imply that the only explanation is God. Science evolves. Not everything is known now, nor should science be forced to prove the universe to disprove creationism. The standard for evaluating creationist claims about scientific disciplines is the same that applies to all scientific theories: does the claim match the evidence? Regarding creationism, the answer is a resounding, "no!"
Creationist claims surrounding the moon:
- Lunar recession (the moon is going away from the Earth too fast)
- Lunar dust (there's not enough moon dust)
- Lunar formation (the moon couldn't have formed)
- Lunar radioactivity (the moon is too hot)
- Atmosphere of the Moon (there's too much moon gas)
- Lunar transient phenomena (the moon is too active)
- Lunar bukkake hypothesis (the moon got shot with water during the Flood)
 Moon landings and the idiots that claim they are a hoax
Twelve American astronauts landed on the Moon between 1969 and 1972, took pictures, left behind some junk, and came back home with some rocks. Deal with it.
 The moon gives off light
It seems natural enough for pre-scientific cultures to assume that the sun and the moon both give off light. However, this is not true; the moon is a reflector of light, not a generator, save as a black body, and in that sense, everything gives off light. A good example of this is Genesis 1:16, in which god creates "a greater light to rule the day" (the sun) and "a lesser light to rule the night" (the moon). So there goes any claim that Genesis is scientific.
Bill Nye the Science Guy took the bible to task on this one. It wasn't popular.
 Muhammad: Moon-Smasher
Nevertheless, some truly dedicated followers of the "TEH QUR'AN IS TOTES SCIENCE" argument have still tried to prove it. A few of them stoop to using an article proposing that NASA found a crack that ran all the way around the Moon -- this argument is a total fraud, as anyone with a non-Qur'an-addled brain can spot.
Many Islamic scholars believe that this verse is best understood as a statement of something to occur at the final judgment. This is supported by the fact that the Surah concerns itself with that judgement, and the references to previous judgements are in the middle part of the Surah, and also that the immediate context of the verse makes a future rather than past interpretation more likely. While there is weak support in the Sunnah for a literal past tense interpretation of the event, even in that case, a plausible interpretation would include the possibility of e.g. a large asteroid impact providing the appearance of the Moon splitting (contemporary writings from India record one such event). It should also be noted that the writing here is poetic, so what exactly "the Moon is cleft asunder" indicates is uncertain and based on the Arabic is just as likely to have a metaphorical meaning. In other words, there is no single interpretation of what exactly this verse refers to. If taken literally it refers to some obvious sign in the moon, figuratively it would likely refer to Islam "tearing assunder" the religious fabric of the Middle East (if past tense) or the world (if end times was meant).
Other Muslim apologetics claim that, when Muhammad put the Moon back together, there was no crack because it was joined together perfectly, molecule-to-molecule. This is special pleading on a grand scale and should be ignored.
 Apparent size and the idiots that claim Aliensdidit
It is a fact that the geometry of the Earth-Moon-Sun system is such that the apparent sizes of the Sun and the Moon, as seen from here, are very close to the same. During a total solar eclipse it is often the case that the Moon sits right on top of the Sun like two stacked pennies (not always, however — sometimes the Moon is at perigee and an annular eclipse is the result. Like a US quarter (24.26mm) stacked on a UK 2p (25.9mm).)
Astronomers tend to just accept this as a fact of nature. Pseudoastronomers, when lecturing to, say, MUFON interest groups, are likely to cue up the Powerpoint™ slide and say "Can this be coincidence? I THINK NOT!" The extremist pseudoastronomical position is that the Moon is obviously artificial, towed carefully into place by Aliens.
 Moon cheese!
The moon is quite definitely made of cheese. Wallace and Gromit proved it.
 Pub trivia
Amaze your friends and win free beers for yourself, by taking bets that the Earth has more than one moon. Once everybody is expecting a pint from you, look really smug and say, "What about Cruithne then?" Enjoy your beers. Unless your friends (rightly) point out that 3753 Cruithne[wp] is actually an asteroid in orbital resonance with Earth, and then beat you up for not buying the 14 pints you owe.
 See also
- ↑ Pictured here is the near side, which faces Earth constantly and is completely visible during a full moon. The far side doesn't have nearly as much of the darker spots that the "man in the moon" is made of so it's more pale and boring.
- ↑ See the Wikipedia article on Libration. - it has a nice animation showing the libration of the Moon over a month.
- ↑ Conner, Clifford. A People's History of Science: Miners, Midwives, and Low Mechanicks. p. 206. http://books.google.com/books?id=PIzziChYQvoC&lpg=PA208&ots=G9C-8S-u8Q&dq=tide%20moon%20~sailor%20history&pg=PA206#v=onepage&q=tide%20moon%20~sailor%20history&f=false.
- ↑ "Bill O'Reilly thinks the tides are proof of God's existence: 'You can't explain that'". CrooksAndLiars.com. 2011-01-07. http://crooksandliars.com/david-neiwert/bill-oreilly-thinks-tides-are-proof-.
- ↑ If you want to get all accurate about it, here are the numbers:
- Moon -- 29.3 minutes of arc at apogee, 34.1 at perigee
- Sun -- 31.6 at Earth's aphelion, 32.7 at perihelion
- So it can also happen that the Moon appears too big to stack on the Sun
- ↑ Earth's "second" moon