Goddess Amaterasu was the Sun goddess, as Tyto mentioned. Amaterasu was a major character in religion and mythology, so it's no surprise that the battle flag (A variant of the current Hinomaru or Nisshoki, which both kinda mean 'rising sun' to an extent) also depicted the Sun for the sake of... Symbolism? (Japan/Nippon is/means "the land of the rising sun") That's basically all I know.
