The Japanese flag, kokki, has a well-known design.  It shows a crimson disc centred on a white background.  The disc represents the sun, and refers to the Japanese myth concerning the first emperor.  The earliest Japanese historical chronicles state that the first Japanese Emperor was a descendant of Shinto’s most important goddess, the Sun Goddess, and that he was crowned in 660 BC.   The flag is known as the hinomaru which literally means “sun disc”.  When Kubilai Khan sought to conquer Japan in 1274 and again in 1281, the priest Nichiren presented this sun flag to the shogun.  The "rising-sun" flag with 16 rays used by the former Japanese navy and by the present Maritime Self Defense Forces is a military service flag and should not be confused with the national flag.

 

In 1853, the shogun decreed that Japanese ships should fly the hinomaru.  In the aftermath of the Richardson Affair there were naval engagements between the (British) Royal Navy and the Satsuma and Choshu fleets.  The British thought the hinomaru was the national flag of Japan.  Japan realised that this had become the international belief, and subsequently adopted the flag as the country’s national flag.  The proclamation (as if you were interested) read as follows:

 

The Prime Minister's Proclamation No. 57 issued on January 27 in the 3rd Year
of Meiji (1870):
Regulations of Merchant Ship (abridged)
 
1. The national flag:
   This shall not be removed and even a ferryboat shall keep it hoisted.
   Hoist it at 8 a.m. every morning and haul it down at sunset every evening.
   In case of non-hoisting of the national flag it is customary of the
international law that no plea is justified if treated as pirates.
2. The dimensions of the national flag:
   The ratio of the hoist and fly 7:10.
   The diameter of the disc: three fifths of the hoist length of the flag
locating in the center.
 
Regulated as above for strict observances.
 
                                        Ministry of Home Affairs
                                        Ministry of Foreign Affairs