I knew that the sun represented Amaterasu, the Sun Goddess in Japanese mythology, but did not know that this sun-as-red-circle design was on folding fans in the Genpei Wars. I am guessing that Wikipedia is referring to the Battle of Yashima, famous for a warrior on a horse knocking a fan off of a ship’s mast in some awesome display of archery skill. I did not realize (or chose to forget) that it was a fan with a hinomaru. Maybe there were other scenes with the flag but I’ve never read the Heike Monogatari in full. [You medievalists out there, help!]
I also visited the the Japanese Wikipedia. This entry mentions that the Hinomaru had been used by Satsuma vessels, but then was later used as a sign of the Tokugawa Shogunate during the Boshin War(1868-9)! Supposedly the restorationists used a flag with the imperial Chrysanthemum crest.
I did not know all of this, and I’m surprised that the Hinomaru was used by Tokugawa Loyalists. Does this suggest that the red circle was not always associated with the imperial throne? I don’t know. Anyone know what the politics was of using the Hinomaru around the Meiji Restoration and how it came to be a symbol of Japan? Or any prior use of the flag? Or any bizarre trivia?