The moon does things to people. Makes women crazy, drives the lunatics, or maybe enhances your supernatural powers. The full moon might bring out the monster in someone, or a new moon may bring the human out of a monster. In general, the moon is often a catalyst in magical things.
More mundane things are stealth on the night of a new moon, or illumination on the night of a full moon. A full moon is associated with lycanthropy as the most common way to trigger the transformation (a trope that is somewhat Newer Than They Think). The duality of the sun and moon is also seen as mirroring the duality of men and women. From that, the moon is usually a feminine symbol (this may be because both have monthly cycles).
The words "lunacy", "lunatic", and "loony" are derived from "Luna" because of the folk belief in the moon as a cause of periodic insanity. When planning stealthy invasions some characters will wait for the night of a new moon when the sky is the darkest.
See Bad Moon Rising for situations where the moon itself is changed. See also Melancholy Moon for when used as a sad backdrop.
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Anime and Manga
Inverted In Inuyasha. Inuyasha loses his demonic powers during the new moon. As do other half demons, like Jiniji.
Saiyans with tails on turn into giant monkeys by seeing the moon on Dragon Ball. In one of the few times that the author tries to rationalize anything in this manga, Vegeta explains that Saiyans absorb the special frequency of light caused by the full moon and a certain gland in their tails prompts their transformation.
In Tsukihime, power of vampires is dependent on the phase of the moon. On full moon, even regenerating from one's ankles up is possible. The reason for this is all vampires are descended from what is essentially the Anthropomorphic Personification of the moon: Crimson Moon Brunestud.
Similar to the Tsukihime example, vampires (or at least Evangeline, the only vampire we've actually met) in Mahou Sensei Negima! grow stronger as the moon gets closer to full.
Promptly subverted in that she gets more mileage out of people knowing this and thus being less prepared for what she can do when the moon's not full. We haven't actually seen her and a full moon together since learning about the connection. From her.
Koromo Amae from Saki. While normally sweet and innocent, her aggression and strength go up while underneath a full moon. Way up.
In A Certain Magical Index, the Archangel Gabriel is strengthened by the Moon due to its elemental alignment. Both times it appears, the Moon is not up, so it solves this by casually and instantly causing nightfall and a full moon. Yeah, angels are powerful... Since Acqua of the Back is empowered by Gabriel, he is also powered up by the moon.
In Naruto the demon Tanuki sealed inside of Gaara is at its most powerful, and it's easier for Gaara to transform into Shukaku. (Thus, at least pre-time-skip, Gaara arguably counts as a were-tanuki.)
In Daily Life with Monster Girl, females of the monster races essentially go into heat during the full moon. This scares the hell out of our hero, not because he doesn't want to sleep with the girls (he does), but because he knows that having sex with a lamia/centaur/harpy that isn't holding back their strength would kill him.
In Cardcaptor Sakura, Syaoran develops a crush on Yukito, because Yukito is really Yue, and thus has a lot of moon-energy, and Syaoran draws energy from the moon himself. (Strangely, Kaho is another moon-energy-manipulator, and yet Syaoran doesn't develop any sort of attraction to her. In fact, he's afraid and suspicious of her.)
In Hataraku Maou-sama!, the angel Sariel is powered up by the moon. He can power up even more by drawing the moon closer to Earth.
The character Moon Knight in Marvel comics is a former special forces operative that met and became the Avatar of the Egyptian god of the Moon and Revenge. Or he is just bat shit crazy as he suffers from extreme multiple personality disorder at times. His ability to be a hero just makes up for how full blown bonkers he can become, as he actually cut the face off his arch enemy at one point.
Discworld subverts the "full moon = magic" cliché: the effect of the full moon on werewolves is mentioned but rarely has much impact (though there is the humorous consequence of werewolves suffering from their 'time of the month' - PLT, Pre-Lunar Tension). Instead, on Discworld the most magical phase is a half moon, because it's on the edge between light and darkness.
The Night Watch averts it completely. Werevolves do suffer from periods of uncontrollable transformation and feral rage, but the timing depends on their biology and has nothing to do with the state of the moon.
Ursula K. Le Guin has a short story that involves a transformation on moonless nights. It's made obvious early on that something even weirder than usual is going on. The narrator's a wolf, and the "monster" transforms into a human.
In Kelley Armstrong's The Otherworld, werewolves have to change approximately weekly, unless they want to have weird things happen like their hands turning into paws in the middle of a grocery store (or just changing spontaneous, with an associated loss of control). At one point the Alpha explains that while they do often change on the full moon, it has more to do with the added light being good for hunting than the phase the moon is in.
In The Dresden Files, there are four types of werewolves, however only one is bound by the full moon: loup-garou. These are cursed people and it is generally inherited through the bloodline. The victim of the curse becomes an insanely powerful feral wolf who will kill anything that gets in its path from moon rise to either moon set or sun rise. A loup-garou can only be killed by silver that was inherited. No other type of attack would take one down as it heals from any wound instantly and resists mind-altering magic.
In the C.J. Henderson novel All Things Under the Moon, the villain explains that the moon doesn't trigger his transformations: "The moon does not make me into a monster. I did that to myself - a long time ago. The moon only makes the monster kill."
This is a major theme in most of Simon R Green's works. Most notably his "Blue Moon Rising" series in which the world is plunged into chaos and darkness when the titular moon rises and a demon horde is unleashed, and his standalone novel Drinking Midnight Wine which features an Anthropomorphic Personification of the moon itself who is stark staring bonkers!
"There is witchcraft in the moon," she shuddered. "He pointed at the moon; while the moon shines on them, they live. So I believe."
In The Name of the Wind, when Elodin and Kvothe visit the Rookery, it's commented that the moon makes the patients worse. This may actually be true, since the moon seems to be linked to both their world and the Fae realm, and the insane may be seeing through a weak spot between worlds.
Live Action TV
An episode of Castle focuses on two murders committed on the evening of the full moon; the beginning features a scene of utter chaos in the station house, with a whole load of lunatics and maniacs causing havoc in the squad room, and all the phones ringing off the hook... with Detective Beckett calmly sitting at her desk doing paperwork and Richard Castle happily sitting beside her with a bowl of popcorn watching the chaos.
In Power Rangers Wild Force, Zen-Aku is like Inuyasha - on the night of the new moon, he loses his powers and becomes his human self again. However, he gets full moon power Inuyasha doesn't get. Basically, his power waxes and wanes with the moon's phases as a rule. (This also accounts for the traditional Strong as They Need to Be factor the show has. Invincible in his first and last battles; whenever one Ranger is a match for him, it must not be near full moon time!)
In the Charmed episode; "Once in a Blue Moon", when two Blue Moons occur in a year, weird things happen. Including The Charmed Ones turning into werewolves. And yes, it did coincide with "that time of the month".
A rather famous song by the Spanish group Mecano, "Hijo de la Luna" (Son of the Moon in Spanish) tells the story about a gypsy girl who in exchange of her first born son, the moon will grant her with husband. Afterwards the son is born, he is an albino kid, the husband notice this, and confronts his wife about it, killing her in the process and abandoning the child in the mountains at the end is told that that the moon is full when the child is in good mood, and becomes half to make him a cradle if he cries.
For the beast is coming to life./Taking form in the glimmer of this tainted moonlight./Death approaches on this night.
In Emilie Autumn's "Girls! Girls! Girls!", the host of a Bedlam Housefreakshow explains that women are predisposed to insanity because of the link between "lunacy" and ovulating by the cycle of the moon.
Many lunar deities, of course; unlike solar deities, they are consistently powerful, perhaps because of the associations with insanity. Selene fought against the Typhon, an evil monster, to the point of scarring his freaking throat - not to mention Artemis, her late Antiquity replacement, who was infamous for her violence. Máni battles nightly against wolves. Khonsu is so violent that many invocations are about him killing the enemies of the pharaoh. Sin is basically one of the greatest authorities among the gods, above Shamash the Sun.
Actively used in Forgotten Realms with the goddess Selune-the oldest deity in existence (barring her evil twin), and extremely difficult to get a handle on in regards to her actual portfolio, which changes like the moon. Specifically the goddess of lycanthropes and women, but also encompasses divination, travelling, questing, tolerance, female spellcasters...
In Magic: The Gathering, magic associated with the Moon is usually either red (chaos) or blue (intellect). Depending on the setting, the local Moon might be of other colours, like white (order) and black (ambition), such as in Kamigawa (where it is blue and black) and the new Innistrad setting (where the Moon empowers both the local white flavoured church and the green and red werewolves, as well as a blue Planeswalker, though she is from Kamigawa).
In the Dragonlance setting, arcane magic is governed by the three moons Solinari, Lunitari, and Nuitari (and the deities of the same names).
Solinari is silver, Lunitari is red and Nuitari is black. The last governs evil magic, and is, for obvious reasons, really hard to see.
Luna, the mother of all werewolves, gets similar treatment in Werewolf: The Forsaken. "Ever-Shifting Luna" is a more polite way of saying "Bitch crazy." For instance, she has spirits, known as the Lunes, who oversee each of the auspices and dispense wisdom to the Forsaken. The reason no werewolves ever try to make one a pack totem is because continued exposure to them can drive a werewolf insane.
And then there's what the moon phase does for werewolves. Like in the predecessor game, a werewolf's phase of the moon (the one they were born under in the old game, the one they first change under in the new game) determines their basic role in werewolf society. Full moons are warriors, gibbous moons are bards and prophets, half moons are judges, crescent moons are shamans, and new moons are tricksters and rogues.
Changeling: The Lost gets in on it too. There's a Contract that Changelings can forge with the moon that revolves around madness; from sensing it with a glance to inducing it in whole crowds of people at once. And if you're a Darkling of the Moonborn kith, you have a spiritual connection to the moon that lets you drive people insane with a touch. Of course, you get zapped with a lesser version of that madness youself when you use it.
Exalted gives similar treatment to Luna, specifically how her chosen, the Lunar Exalted, regard her. Like all Exalted, Lunars accrue Limit if they go against their guiding Virtues; however, they also gain Limit whenever the full moon's in the sky. Instead of realizing there may be some external reason for their occasional crazy rampages, they just ascribe it to Luna being chaotic like that.
And as in Werewolf, a Lunar's role is linked to a phase of the moon; here, however, it's chosen by the Lunar, as they lost the inherent assignation of Caste when the Lunars as a whole fled into the Wyld after the Usurpation. Those that aren't marked are the Casteless, whose Caste abilities shift with the moon and bear the risk of becoming Chimerae. The other three positions are Full Moon (warriors), Changing Moon (tricksters and diplomats), and No Moon (sorcerers and scholars); there used to be five Castes, but again, the Wyld screwed with that.
RuneQuest has Lunes, which are basically "Moon Elementals". Their touch causes madness.
Then there's the Lunar Empire, with their Red Moon, who worship the Moon Goddess. Unlike most examples, though, the Goddess and the Empire are both completely evil.
Dungeons & Dragons 3.5 had a monster called Moon Rats. They are perfectly ordinary rats most of the time, but when exposed to the full moon they become intelligent, organized, and evil. They're also very, very patient - don't think you're safe if they don't do anything of note on a full moon, because they're willing to make plans that take dozens of them to come full flower.
Yu-Gi-Oh! has Bujin Arasuda and Bujintei Tsukuyomi, as they are based off the Shinto moon god, Tsukiyomi.
Ōkami has Yumigami, a rabbit god who gives you the power of Crescent, allowing you to change day to night. It's rarely useful. Certain events can only happen at night, and one boss allows it to somehow be used as an attack.
Then there are Mr. and Mrs. Cutter, two crow tengus who disguise themselves as human. Mrs. Cutter specifically says that during the full moon their kind gets too excited for them to move outside without exposing themselves.
Yami, the final boss is meant to represent the cold and lifeless moon in contrast to Amaterasu, the sun goddess.
In Touhou Imperishable Night, if you reach the "true" moon, the Big Bad reveals that "pure" moonbeams can drive humans insane. In one of Remilia and Sakuya's endings, it drives all the non-human characters insane.
The waxing and waning of the moon is a key gameplay feature in every Shin Megami Tensei game. The fuller the moon is, the more damage your attacks do, the more likely an accident is to occur during fusion, and the crazier the monsters act. During a full moon they're practically drunk off those moonbeams, which makes for entertaining conversation. There are also some abilities that are more or less effective depending on the phase of the moon. Certain games have their own quirks:
Persona 3 has important storyline events occur during a full moon, including a general rise in Shadow activity and, as a result, increases in the number of cases of Apathy Syndrome, which is caused by Shadow attacks.
There's a period of time in Shin Megami Tensei I where the main character would take damage during a full moon. This is because of his psychic link to the Heroine, whose reincarnated self is currently undergoing torture from a demon that has invaded her mind. The full moon makes him feel her pain. This is solved by rescuing her.
In Digital Devil Saga, there's a 50% chance during every new moon- excuse us, MIN Solar Noise that your characters will be cured of any ailments that they are suffering from. Also, the selling price of Cells is at its highest during MAX Solar Noise. In the sequel, Digital Devil Saga 2, there is a chance during 7/8 or MAX Solar Noise- sorry, Solar Data that you will enter battle in Berserk Form.
You can guarantee that you get the best items from Shin Megami Tensei III: Nocturne's Mystical Chests by opening them during a full moon. The drop rate of Gems is also highest at this point.
One Sub App in Shin Megami Tensei: Strange Journey lets you speak to demons during the Full Moon (something otherwise impossible). Because they're drunk on the moonlight, they don't really know what they're saying, and will ask bizarre questions. You have a 50/50 chance of impressing them or pissing them off; impressing them can earn you rewards up to and including instantly recruiting them - this is one of the only two ways to recruit Dark demons without having to fuse them (The other being them joining on their own accord with yet another Sub App).
Hecate is a moon goddess, so in her appearance as a boss in Shin Megami Tensei II she is invulnerable as long as any part of the moon is visible - you have to wait until new moon to fight her.
The Dragon Saïx in Kingdom Hearts II. He uses the ominous Kingdom Hearts floating over The World That Never Was as the focus of his powers, turning him from The Stoic into The Berserker. The aforementioned MacGuffin is, for all intents and purposes, the moon of The World That Never Was; the theme of the world is entitled Sacred Moon, the city is draped in eternal night with the only light source being Kingdom Hearts...plus, Saïx has fairly obvious connections to being a werewolf, what with his title being the Luna Diviner.
Skies of Arcadia. There are six moons, all powering different types of magic, all dropping meteorites onto the land below which everyone then uses to power just about everything. And that's not even counting their plot-critical role...
Luna returns in Seiken Densetsu 3. Very few classes have access to any of Luna's spells, but Hawk's Wanderer class can cast all but one of them. The game also features the Moonlight Forest, home to the werewolf-like Beastmen (including Kevin, who can gain access to the one Moon spell that Hawk can't cast) who transform and get stronger at night. Unsurprisingly, the Moonlight Forest is where you eventually find Luna. The game also includes Dolan, the God-Beast of the Moon's power. He resembles a titanic werewolf-like creature, and can summon the moon to drain the party's health, cut their lives exactly in half, and grant himself the same berserker buffs as the party can cast.
Pokémon has a move called "Moonlight" that restores HP based on the weather, and in the games they debuted in, the time of day. In later games, it works identically to its counterpart, "Morning Sun," which results of a Dub-Induced Plot Hole due to the clear skies field effect being translated as strong sunlight - Moonlight restores the most health when the sun is shining.
According to the Pokédex, Eevee evolves into its Dark-type evolution Umbreon by exposure to the light of the moon, Lunatone becomes active during the full moon, and Cresselia extends its aurora during the quarter moon.
There's also the Moon Stone, which allows a miscellaneous grouping of Pokémon to evolve. Only one of them, Clefairy, has any connections to the moon, being said to have originated on it.
As of generation 6, there is a fairy type move named "Moonblast", which is a beam made by borrowing power from the moon. It has the strongest base power of all Fairy-Type moves. Additionally, the previously-mentioned "Moonlight" has been changed to Fairy type in Gen IV.
In Chrono Cross, Harle, the harlequin minion of villain Lynx, is revealed late in the game to be the Dark Moon Dragon, created by the other six Dragons to be a servant of the Dragon God. She was created secretly so that she could release the seal on the other six Dragons and they would be able to merge once more into the Dragon God and unleash havoc on humans once again. She is eventually abandoned by them, as she did not merge with the other dragons. She's even named Tsukuyomi in the Japanese version, after the Japanese moon god. Her special attacks are all moon-based.
League of Legends has Diana, the Scorn of the Moon. She's a melee assassin whose abilities revolve around using moonlight to attack her foes, making her the opposite of the sun-powered Leona. While she is a dark character, her goal is more tragic: for discovering the long-hidden secret moon cult of the Lunari, the sun-worshiping Solari almost executed her before she was endowed with her lunar powers and killed them.
In Final Fantasy VIII, the Lunar Cry not only drops a torrent of monsters onto the earth from the moon, it also changes terrestrial animals into monsters.
One of the many running gags in Sinfest is "Aah! Full moon! I'm turning into [whatever the character is most afraid of becoming]." As with most of Sinfest's running gags, the precise results can vary wildly—for instance, a demoness turns into a housewife.
Futurama with the were-car. Even though the transformation had nothing to do with the moon, the were-cars howled/honked at it anyway.
Waterbenders of Avatar: The Last Airbender are powered by the moon's cycles (the Moon being the first water-bender (think about tides) and the one who taught humans how to do it), with their abilities at their peak during a full moon. Most notably, it allows them the power of People Puppetry.
By The Legend of Korra, the Sequel Series to ATLA, it's shown that a full moon is not required to pull off Bloodbending if the bender is powerful enough, as shown by Tarrlok and Yakone.
Superjail! has an interesting variation: In the pilot episode, The Warden decides to order bunny suits for all of the inmates. The Twins screw up his order, of course, and half of the suits are now wolf suits. The Warden decides to 'congratulate' The Twins while they're working out in the prison yard with the inmates (who are now clad in either bunny suits or wolf suits.) The Warden then proceeds to 'start the big show' and raises the full moon in the sky. The prisoners in the wolf suits suddenly start foaming at the mouth and transform into actual werewolves.
An episode of Dexter's Laboratory had Dexter experiencing a bad day. Dee Dee convinces him that it's because of the zodiac - the Moon is blocking him from his constellation. He solves it by pushing the Moon... and accidentally causing it to fall into his city.
In the Phineas and Ferb episode "The Doof Side of the Moon", Card-Carrying Villain Dr. Doofenshmirtz claims that although his own evil remains constant, the level of "background evil" waxes and wanes with the phases of the moon.
In SilverHawks, the light of the Moonstar of Limbo transforms Mon*Star (already a pretty imposing guy) into an even more powerful armored monster. The series begins when a burst of light from the Moonstar gives Mon*Star the strength to escape his prison. In one episode, a small piece of the Moonstar broke off and Mon*Star carried it around with him. This vastly amplified his power until the heroes got rid of it.
It was once a common urban legend that people behave more rowdily under the full Moon, that people go crazy, that more crimes are committed, and that more people get hurt and go to the hospital. Many pseudoscienctific explanations were proposed to account for these supposed phenomena. As it turns out, none of it is true; studies have failed to find any correlation between the phase of the Moon and human behavior.