The Japanese term tenno is translated into English as emperor. I believe this term is very inaccurate. The correct English definition of this term should actually be king.
Why is the term Emperor used? Well, in Japan's first histories the term tenno was used to bolster national pride and to put the nation of Japan on the same level as that of the highly esteemed nation of China.
However, Japan has never had an empire and could never in any sense be considered akin to ancient Rome, China or Britain. A ruler, aided by numerous administrators over a large number of federated states cannot in any sense of the word be considered an emperor. At best he would be a king.
The chronicles of the Chinese Wei dynasty (3rd century C.E.) states that Japan - called Wa by the Chinese - consisted of numerous states ruled by numerous chieftains or priestesses. The paramount ruler was named Himiko and was recounted as being a crafty, calculating shaman priestess who ruled over a country called Yamataikoku. Yamataikoku is yet to be exactly geographically identified, but believed to be the area around modern day Okayama.
Himiko is reported to have unified many of the states of Wa and brought the numerous chieftains under her rule.
Japan's first history writers placed the first "emperor" at 660 B.C.E. He was given the posthumous title of Jimmu following the Chinese tradition and all dates of rule were borrowed from the Chinese calendar with years added or subtracted to allow his rule to coincide with that of China's great Emperors. Japan's first chroniclers quickly found that they had too many years using the Chinese system (one cycle equaled 60 years). They had not enough emperors to fit into the timeline they had created. The early writers made up for this by borrowing passages from Korean and Chinese histories - names were given, changed and simply invented to fill the blanks.
The entire Japanese "imperial" line right up to the mid-fourth century is completely fictitious, having been lifted from the histories of neighbouring Korea and China. The distortions of the eighth century writers have severely muddled Japan's history and this was followed by further meddling with history in the Meiji period (1868-1912) when the Japanese were attempting to reignite national interest. Even today many are unaware that a great deal of Japan's history is simply fiction and museums across Japan repeat the same stories as if they were actual.
Modern writers would do well to heed the incorrect use of the term tenno, and to look questioningly at the "history" of Japan.