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https://en.wikipedia.org/wiki/Imperial_House_of_Japan
The Imperial House of Japan (皇室 kōshitsu), also referred to as the Imperial Family or the Yamato Dynasty, comprises those members of the extended family of the reigning Emperor of Japan who undertake official and public duties. Under the present Constitution of Japan, the Emperor is "the symbol of the State and of the unity of the people". Other members of the Imperial Family perform ceremonial and social duties, but have no role in the affairs of government. The duties as an Emperor are passed down the line to their children.
The Japanese monarchy is claimed to be the oldest continuous hereditary monarchy in the world. The Imperial House recognizes 126 monarchs beginning with the legendary Emperor Jimmu (traditionally dated to 11 February 660 BC) and continuing up to the current emperor, Naruhito; see its family tree.
Historical evidence for the first 29 Emperors is marginal by modern standards, but there is firm evidence for the hereditary line since Emperor Kinmei ascended the throne 1,500 years ago.