Six Chinese Systems, Then One Japanese One

Japan adopted Chinese lunisolar calendrical science as a matter of state administration beginning in the late seventh century, with the Gembu calendar introduced via the kingdom of Baekje in 690 CE, and over the following centuries the imperial court successively adopted a run of further Chinese systems, Genka, Giho and, from 862, the Senmyo calendar, which remained officially unrevised for more than eight hundred years, an unusually long span attributable as much to institutional inertia as to the calendar's own accuracy. The first calendar computed by a Japanese astronomer rather than imported from China arrived only in 1684, when Shibukawa Shunkai's Jokyo calendar replaced the by then badly drifted Senmyo system, and further Japanese-computed reforms followed, Horyaku and Kansei among them, ending with the Tenpo calendar of 1844, the last of Japan's traditional lunisolar calendars and the one still meant by kyureki, "the old calendar," today. All of these systems shared the same lunisolar mechanics inherited ultimately from China: twelve lunar months with an intercalary thirteenth added periodically to keep the count aligned with the solar year, months reckoned by the moon's phase rather than a fixed day count.