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Amaterasu and Tsukuyomi represented these Astral Objects, but other Sun and Moon Gods also belonged to the Pantheons of each one of them, and often times switched between leaders, as their powers were quite equal.
For example, Selene, the Titan Goddess of the Moon, would sometimes take over the Divine Moon Pantheon, or other times it would be the God Chandra and the Goddess Anumati, who often times worked on pairs. This time, however, Tsukuyomi was chosen due to his resourcefulness and abilities… And because Selene wanted to take a break as a Leader of such a massive Pantheon, Diana, her daughter, was not ready yet to take her role.