The Moon plays a very largely role in the lore of Touhou, and it's a fascinating mix of Japanese, Chinese and European mythology and real-life conspiracy lore about a hidden civilization lurking on the Moon. In Touhou, the History of Moon goes something like this: on the far side of the Moon which humans can't see is a hidden civilization. They are separated from the outside world through a barrier that is described as being similar to the Hakurei Barrier that splits Gensokyo off from outside world. Within this barrier there are true seas on the moon, forests and a technologicaly and magicaly advanced civilization of Lunarians. They are ancient humans who ascended to the Moon by realizing they could alter probabilities – an incredibly fascinating idea that is sometimes found in real life paranormal and esoteric thought. The Lunarians subjugated the native Moon Rabbits and used them for labor and as soldiers.
The reason why these ancient humans left is that impurity, kegare, was ravaging the land. This makes the Lunarians obsessed with purity. The Moon is a pure land (in Touhou, not Buddhist sense), nothing ages or gets sick there. The Lunarians weren't however content to just enjoy the paradisical conditions. They meddled in Earth's affrais, sowing conflicts to keep humanity from stagnating. Kaguya claims that Lunarians also created the youkai to ”regulate it's filth”. Yukari, who once upon a time attempted an invasion of the Moon, says that the moon is more indirectly responsible for the youkai. She says that the dim light of the moon gave rise to youkai, implicitly because humans projected their fears into the barely visible darkness.
Let's take a closer look at what kind of things have influenced ZUN's fascinating – and convoluted – depiction of the Moon in Touhou.
The Eastern half of ZUN's Moon
The Eastern influences on ZUN's depiction of the Moon don't come from a single source, but are rather a collection of separate influences. Many different tales are woven together into the narrative.
The natives of the Moon, the Moon Rabbits come from East Asian folkloric beliefs about the dark markings on the Moon being a giant rabbit. This rabbit is thought to be constantly working, but the type of work various from culture to culture. In Japan, it was thought that the rabbit is constantly pounding mocchi. In China, the rabbit is thought to be grinding the ingredients for the elixir of immortality in a mortar.
Touhou lore offers fatures both of these elements. The Moon Rabbits pound mocchi and sing to atone for the sins of their former mistress Chang'e. Touhou's lore about Chang'e is quite scant, little is known besides that she was a Lunarian who was the leader of the Moon Rabbits and committed the crime of drinking the elixir of immortality. While it might seem a rather unusual crime, apparently drinking it means that the drinker has become impure because the temptation to avoid death causes them to associate with death, breaking the unaging, timeless purity and bringing in kegare. The Lunarians never meant this elixir for themselves, but rather as a tool of political manipulation of Earthlings.
In real-life mythology, Chang'e is a lunar deity from Chinese mythology. There are many variations of the tale, but all have her drink an elixir of immortality that causes her to ascend to the moon. On the Moon she keeps company to the Moon Rabbit. Her husband is said to be the legendary archer Houyi. He saved the world by shooting down nine suns when one day ten suns rose instead of only one. The traditional Chinese week was ten days, and it was believed that there were ten different suns that reach rose in turn.
There is also another ancient Chinese story featuring a character called Houyi, albeit a much less heroic one. In Zuo Zhuan, Hou Kui, the leader of the Xia Dynasty, is forced into exile by a man called Houyi. After taking the throne, this Houyi kills Hou Kui's son Bo Feng. The nickname of the slain son's mother is Chun Hu – pure fox – written with the same character's as Junko from Touhou. Indeed, the blending of these two myths seems to be the inspiration behind Junko's hell-bent vendetta against Chang'e, whom she wishes to slay as revenge for killing her son.
Chang'e is not the only Lunarian punished for drinking the elixir of immortality. Another such character is Kaguya Houraisan. The character of she comes from The Tale of the Bamboo Cutter, a Japanese tale from the 9th or 10th century. In the story, an old bamboo cutter finds a tiny baby inside a glowing stalk of bamboo. Lacking children of their own, the bamboo cutter and his wife adopt the strange child and name her Nayotake-no-Kaguya-hime. The baby rapidly grows into an exceptionally beatiful young woman. Soon enough five high-ranking men aproach her for marriage. Uninterested, she gives each of them an impossible task. All of the men fail in the tasks. After this, the Emperor of Japan himself proposes to Kaguya-hime. She rejects him too, and tells that she isn't of this land and can not marry him.
Eventually it is revealed that Kaguya-hime is from the Moon, and she has been exiled to Earth for an unspecified crime. Soon after, inhabitants of the Moon come for Kaguya-hime, and take her with them. The inhabitants of the Moon specificaly take her to the Tsuki no Miyako, or Capital of the Moon – probably more well known as the Lunar Capital for fans of Touhou.
The Lunar Capital in Touhou
She leaves behind some of the elixir of immortality for the Emperor, but he refuses to drink it as he does not want to live forever without the company of Kaguya-hime. Instead, the Emperor orders that the elixir and the letter Kaguya-hime left for him to be burned at the ”mountain closest to heaven”. In the story, this mounta is Mt. Fuji. In Touhou, this particular mountain is said to be on some level the Youkai Mountain of Gensokyo. Youkai Mountain is not Mt. Fuji, but rather, it's said that once upon a time Mt. Yatsugatake was slightly taller than Mt. Fuji, therefore closer to heaven. This upset Sakuyahime, a kami living on Mt. Fuji, and she broke Mt. Yatsugatake.
In Touhou, Kaguya's surname is Houraisan. This a reference to a myth within the Tale of the Bamboo Cutter. One of the impossible challenges that Kaguya-hime puts on her suitors is getting a jeweled branch from the mythical land of Hourai. This legendary land comes from Chinese mythology, where it's known as Penglai, a mythical land of plenty, inhabited by immortals. The Chinese myths indeed had jeweled trees growing in Hourai. In Touhou, the bonsai Hourai tree from Moon that Kaguya has blossoms with jewels only when exposed to kegare. Later Japanese myths depict Hourai as a rather diferent place, not a land of plenty, but a strange place where the air is made out of countless souls and it's inhabitants are tiny faery-like beings. It was also called Shinkiro, implying a mirage. Perhaps the latter myths are of some relevance to Touhou too...
While in the Tale of the Bamboo Cutter, Kaguya-hime leaves for the Moon with the emissaries from the Moon, in Touhou the end is quite different and related to another prominent Lunarian character that is tied to mythology beyond this particular tale. In Touhou, Eirin Yagokoro was one of the Lunar emissaries that came to get Kaguya back to Moon. However, Eirin, having made the elixir but not having been punished for it, felt guilty. Drive by a desire for redemption, Eirin killed the other emissaries and decided to stay on Earth with Kaguya as her own punishment. To ensure that she could stay by Kaguya's side, she too drank the Hourai elixir.
Before her exile to Earth, Eirin was the Sage of the Moon, perhaps a role analogous to the Sages of Gensokyo who maintain the Hakurei Barrier. Eirin is described as being extremely inteligent even beyond her main field of medicine. It's very likely that Eirin is based on Ame no Yagokoro Omoikane no Mikoto, the kami who came up with the plan to lure Amaterasu Oomikami out of her hiding from the cave.
Eirin is not the only deity residing on the Moon in Touhou. The Lord Tsukuyomi that is refered to in the narrative is in fact Tsukuyomi-no-Mikoto, the Moon kami of Shinto. In Touhou, Lord Tsukuyomi leaves Earth with his most trusted relatives because he comes to disdain the blood-stained, impure nature of Earth. Tsukuyomi and Yagokoro are far from the only deities on the Moon. In Legacy of Lunatic Kingdom, it's stated that the Lunarians are indeed the heavenly amatsukami. This would make the Moon of Touhou also the Takamanohara of Shinto mythology.
Touhou also blends mythologies related to the underwater kingdom of the Dragon Palace with legends related to the moon. Cage in Lunatic Runagate very straightforwardly refers the legend of Mizue no Uranoshimako. In this popular Japanese folk tale, a young man named Urashima saves a turtle that has been turned over at the beach. Later he encounters the turtle again while fishing, and the turtle speaks to him. The turtle turns into a beatiful young lady, and is revealed to actually be a daughter of the legendary Dragon King. She takes Urashima to the underwater kingom where the two fall in love.
Eventually Urashima however becomes homesick. The daughter of the Dragon King is not happy about this, but she allows Urashima to return with one condition. He is given a box, and she tells him to never open it if he wishes to see her again. Urashima returns to the surface, but is shocked to see how much everything has changed. He has learned that he has in fact been gone for centuries. Distraught, he opens the box and ages hundreds of years in an instant, dying on the beach.
In Touhou's retelling of the tale, Urashima ends on the Moon while chasing after an unusual looking turtle. This is an accident of quantum entanglement and Urashima ends in one of the seas around the Lunar Capital. There he is found by Toyohime no Watatsuki. The two don't quite fall in love, but Toyohime shows him kindness and hides him from the other Lunarians. Much like in the original tale of Urashima, he eventually grows homesick.
However, in Touhou his ultimate fate is difference. He is sent into what was from his perspective the future using Lunarian's magic. The box given to him contains magic devised by Eirin Yagokoro that causes him to age. Rather than killing him outright, he simply turns into an old man. The tales of an old man who has seemingly been gone for centuries raise interest but obfuscate the truth of the Moon. He ends up being worshipped as the Tsutsugawa deity posthumously.
In real-life Japanese mythologies, the Dragon King is not just a king, but a deity, and a very powerful one at that. Known as Owatatsumi-no-kami or Ryujin, he rules the seas from his palace beneath the waves. The Watatsuki sisters, Yorihime and Toyohime, are based on his daughters. This watery element connects the Eastern and Western parts of ZUN's moons togeter.
The Western half of ZUN's Moon
Early Western astronomers mistook the craters of the Moon for seas. In Touhou there are real, albeit lifeless and thus pure, seas on the dark side where the Lunar Capital hides. These seas on the Moon are considered to be actually the closest point to Earth, and can be used to access Moon via magic. This is most likely a reference to how the Moon affects tides on Earth. Another way to access the Moon in Touhou is by using the reflection of full Moon on a body of water.
The Moon can also of course be accessed by spacecraft. Such spacecraft also include Remilia's ”rocket”, which is powered by Sumiyoshi, the Shinto kami of good luck and safe voyages, via Reimu's invocations. Less fantastical spacecraft that have gone to the Moon in Touhou are the Apollo landers on their Saturn rockets. There is an amusing take in Touhou as to why conspiracy theories suggesting that the Moon landing was hoaxed have emerged: the technologies needed for the landing have ”passed into fantasy” and people have started disbelieving it ever happened.
Red ones go faster!
In Touhou, the Moon is also said to be the source of ”many powers”, including magic. The connection of Moon with magic is a very prominent idea in Western culture. This probably largely comes from how the ancient Greek goddess of boundaries and magic, Hekate, was also associated with the Moon. She is present in Touhou as the character of Hecatia Lapislazuli.
Another very Western idea associated with the Moon is the idea that it can drive people insane. In Touhou seeing the ”true full moon” or the far side of the Moon will drive an ordinary person insane. In Touhou, the arrival of Americans to the Moon caused a ”war” with very little actual fighting. It's stated that a member of the Apollo crew was however injured, and the arrival of humans left Lord Toyohime alarmed ever since. This is an extremely fascinating part of the Moon lore, as there are a great deal of real life conspiracy theories about why humans have not returned to the Moon.
Some say that there is an alien civilization present on the Moon, hiding in it's dark side. Some even claim that they are not aliens, but in fact a remnant of an ancient, more advanced human civilization. Whatever the case is, members of the Apollo missions allegedly saw strange and experienced strange things on the moon. An example of such is Buzz Aldrin stating that the Apollo 11 craft was followed by a ”light”. Another case is the one of Edgar Mitchell, where had some kind of a spiritual experience on the Moon and devoted the rest of his life to consciousness research.
There is also a very well documented real-life phenomena called Transient Lunar Phenomena, anomalous events recorded on the Moon by both astronomers and casual observers. Simply put, these are unusual visual phenomena observed on the moon. While some of them might indeed be outgassing or electrostatic events as proposed by current scientific paradigm, some of them seem harder to explain as such. Some of the descriptions such as multicolored pyramid-shaped lights or red streaks across the Moon are certainly quite fantastic.
One last thing of mention is that ZUN has an understanding of the symbolism behind the name of the Apollo missions, stating that it's no surprise that a series of missions named after a Sun deity would ultimately end. Some have found even greater significance behind the rather interesting names behing the American space program. A fascinating rabbit hole for those interested in such things.
