The fiftieth anniversary of Apollo 11 turned our thoughts to the moon and thus, in the context of Shinto, to Tsukuyomi (or Tsukiyomi) no Mikoto, a brother of Amaterasu Ōmikami and generally accepted to be the kami of the moon.
In the Kojiki myth of the birth of Amaterasu Ōmikami, she is born when Izanaki washes his left eye, and Tsukuyomi is born when he washes his right eye. (Susano’o is born when he cleans out his nose.) Amaterasu Ōmikami is clearly a kami of the sun in some sense; her name means something like “Great Kami Illuminating the Heavens”, and there are numerous elements in the myths both explicitly and implicitly associating her with the sun. Tsukuyomi’s name is normally written with the character meaning “moon”, and he was born in a pair with Amaterasu Ōmikami, so it is natural to see him as the kami of the moon, and the main Nihonshoki version of their birth makes this explicit.
However, the most interesting thing about Tsukuyomi is just how unimportant he is in the myths and in Shinto practice. In all the myths of the birth of Amaterasu Ōmikami, she is the oldest of three siblings, and her youngest brother, Susano’o, is extremely important in the myths, and in Shinto practice. Tsukuyomi, however, almost completely disappears. There are two versions of a myth in which a kami of food is slain for perceived rudeness, and all the crops grow from her body: in one version, she is killed by Susano’o, and in the other by Tsukuyomi. In the version in which she is killed by Tsukuyomi, this provokes Amaterasu Ōmikami to exile him as an “evil kami”, thus separating day and night.
It is not just that he plays little role in the myths. After all, O-Inari-san plays little role in the myths, but is a very important kami. There are also very few jinja honouring him. (Tsukuyomi is conventionally regarded as male, but this is not clear. Nothing about his actions makes him clearly male, and Japanese language rarely makes gender explicit. The strongest argument is that he forms a contrasting pair with Amaterasu Ōmikami in many ways, and thus is likely to be male, as she is clearly female.)
On the other hand, there are important subsidiary jinja at Jingū that enshrine him, at both the Inner Sanctuary and the Outer Sanctuary, strongly suggesting that he was originally of considerable importance. He does not seem to have been casually added to the myths.
One possibility is that his original role was taken over by Susano’o. The only myth about him, as mentioned above, is about Susano’o in another version, and the various versions of his birth have him assigned rulership of the night or the sea, with the other one assigned to Susano’o. Similarly, his banishment from the presence of Amaterasu Ōmikami also matches an important part of the legend of Susano’o.
There is a theory that Amaterasu Ōmikami was originally a kami of the Yamato rulers, the ancestors of the Tennō, while Susano’o was a kami of the Izumo rulers, lords of a region in Western Japan that I am currently writing Patreon essays about, and that Susano’o was incorporated in the Yamato myths when Izumo was incorporated into the Yamato polity. In that case, he might have been given much of Tsukuyomi’s role, with Tsukuyomi pushed into the background. That is pure speculation, however.
One other thing worth noting is that the kami’s name is normally written with the characters for “moon” and “reading”, and this is normally taken to mean that the kami was originally associated with the lunar calendar. However, there is little trace of that in the rest of Shinto.
His name can also be written with the characters for “moon night seeing”, which is quite poetic, but also draws attention to just how little importance the lights in the sky have in Shinto. There are almost no myths about stars (there are some about the pole star, but they are later, and have a clear history going back to China), and even the sun and the moon are kept in the background. It is often said that Shinto is most concerned with this world, meaning that it has little to say about what happens after death. It seems that it is also, however, true in the sense that Shinto has little concern with worlds other than Earth, even when they are clearly visible in the sky.