Emperor of Japan
Currently, the Emperor of Japan is the only head of state in the world with the English title of "Emperor". The Imperial House of Japan is the oldest continuing monarchical house in the world. The historical origins of the Emperors lie in the late Kofun period of the 3rd–7th centuries AD, but according to the traditional account of the "Kojiki" (finished 712) and "Nihon Shoki" (finished 720), Japan was founded in 660 BC by Emperor Jimmu, who was said to be a direct descendant of the sun-goddess Amaterasu. The current Emperor is Akihito. He acceded to the Chrysanthemum Throne upon the death of his father, Emperor Shōwa (Hirohito), in 1989. The Japanese government announced in December 2017 that Akihito will abdicate on 30 April 2019.
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.