The Japanese flag has the Sun on it because the imperial family is descended from the spirit of the Sun. We know this because the story has been handed down from the oral tradition, making the Japanese royal family not only one of the oldest but one of the few that has its own religion. The myths and legends surrounding the creation of and the early history of Japan are the subject of the first of a series on the history of Japan by Cameron Foster.
Cameron is an enthusiast and if you like your history to deal with the interesting details rather than the academic this is one for you. His robust Australian humour is a bonus.