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.
Emperor Kōkaku
In 1817, Kōkaku abdicated in favor of his son, Emperor Ninkō. In the two centuries before Kōkaku's reign most emperors died young or were forced to abdicate. Kōkaku was the first Japanese monarch to remain on the throne past the age of 40 since the abdication of Emperor Ōgimachi in 1586. He was also the last Emperor to rule as a Jōkō (上皇), an emperor who abdicated in favor of a successor (it has been announced that the current Emperor, Akihito, has plans to abdicate in 2019, however, and would thus be the first Jōkō since then). Kōkaku travelled in procession to Sento Imperial Palace, a palace of an abdicated emperor. The Sento Palace at that time was called Sakura Machi Palace. It had been built by the Tokugawa shogunate for former-Emperor Go-Mizunoo.