Reki Hakase (Chief Court Calendar-Maker) (暦博士)

The Reki Hakase was one of the government posts established under the Ritsuryo system (a system of centralized government based on the ritsuryo code) in Japan. The Reki Hakase belonged to the Bureau of Onmyo in the Nakatsukasa Department, and was responsible for the preparation and revision of the annual calendar, forecasting solar eclipses and educating Reki no sho (students of calendar preparation). Initially, the post was held by one official, and later one Kenkan (powerful official) was added, and their rank was equivalent to Jushichiinojo (Junior Seventh Rank, Upper Grade); however, in later times it became customary to give the post a Goi (Fifth Rank) or higher ranking.
The Japanese reading of the Chinese characters is 'Koyomi no Hakase.'
The Tang name is Shireki, Shirekiseiho.

The earliest mention of the post is found in the record of the fourteenth year of the reign of the Emperor Kinmei in "Nihonshoki" (Chronicles of Japan), but it was only in later ages that a government-regulated organization was actually established. In 791, 7.35 acres of shikiden (office-rice fields) were provided. In ancient times, toraijin (foreigners, particularly from China and Korea, who settled in early Japan and introduced the culture of the Asian continent to the Japanese) who were well versed in Rekido (study of the calendar) or Sando (study of mathematics) were often appointed, but later only members of the Okasuga clan, Iehara clan, Onakatomi clan or Kamo clan were appointed to the post. The following two figures are well-known as Reki Hakase: OKASUGA no Manomaro, who in the reign of the Emperor Seiwa advocated the introduction of Goki reki (Wuji calendar, a lunar-solar calendar developed in the Tang by Guo Xianzhi and used in China during the years 762 to 821) and Senmyo reki (a variation of the lunar calendar that was created in ancient China); and KAMO no Tadayuki, believed to be the sixth generation descendant of KIBI no Makibi, who brought the Taien calendar to Japan. KAMO no Yasunori, who was the son of Tadayuki, held Reki Hakase concurrently with Tenmon Hakase (master of astrology); however, after his death the descendants of KAMO no Mitsuyoshi, the son of Yasunori, took over rekido and the descendants of ABE no Seimei, a follower, took over tenmondo (ancient horoscopy). Because the private possession of rekisho (explanatory books concerning calendars) was strictly regulated by Hishogenjo-rei edicts, which were classified as Zoryo (Law on Miscellaneous Matters), it was difficult to recruit personnel outside the Imperial Court; therefore, when TAIRA no Masakado proclaimed himself as a 'new emperor' and nominated Bunbu Hyakkan (all the officials, both military and civil), he was unable to appoint Reki Hakase, and in the same manner the Yoshino Imperial Court (the Southern Court (Japan)) was unable to appoint Reki Hakase, and consequently there is no Guchu-reki (annotated calendar) that includes its gengo (era name).

In a later period, once all the direct descendants of the Kamo family had died out, Tomokage KOTOKUI, a member of a branch line of the Kamo clan, became Onmyo no kami (Director of Onmyoryo) and after Tomosuke KOTOKUI, who was the grandson of Tomokage, Reki Hakase became a hereditary post for descendants of the Kotokui family. However, the Kotokui family were suppressed both in Onmyodo (the way of Yin and Yang, an occult divination system based on the Taoist theory of the five elements) by the Tsuchimikado family (Abe clan), and in rekigaku by the Tenmonkata (Astronomical Institute) of the Edo bakufu (Japanese feudal government headed by a shogun); therefore, the Kotokui family was, in later ages, limited to the preparation of rekichu (assorted information recorded in the almanac). After the Meiji Restoration, along with the abolition of Onmyoryo (a government office that had jurisdiction over calendar preparation, astronomy, divination, etc.), the post of Reki Hakase was abolished, and soon the official calendar was changed to a solar calendar, signaling the end of the role of Reki Hakase.