Records of heads of state
Officially, the current Emperor of Japan, Akihito is the 125th in line from the first emperor Jimmu, who is variously believed to have reigned in the 1st or 7th century BC. The earliest documentary evidence is only for the 29th emperor, Kinmei (AD 509–571); however, this is sufficient such that even the most conservative of estimates still places the Japanese imperial family as among the oldest lines in the world today.
Reserve power
Unlike most other constitutional monarchs, the Emperor of Japan has no reserve powers. Following Japan's defeat in World War II, the emperor's role is defined in Chapter I of the 1947 Constitution of Japan, as decided by the foreign powers that had defeated Japan in the war. It states that the sovereignty of Japan rests with the people, not the emperor, and that the emperor is merely the symbol of the State and of the unity of the people.