Tenmondo (ancient horoscopy) is an ancient study involving the observation and recording of unusual astronomical phenomena (extraordinary astronomical events/extraordinary natural phenomena) and investigation of their effects on the Earth. While it was equivalent to present-day astronomy, it was closer to astrology and was far from a science.
Originally, 'Tenmon' meant to record the changes that appeared in the sky. Astronomy as a science with the aim of seeking rational regularity was established in Japan only after Western astronomy was introduced in the Edo period. Because of this, in the Meiji period, astronomers who knew the difference between the two tried to translate the English or German word 'Astronomy' into terms other than 'tenmon (gaku/do)', such as "seigaku (study of stars)".
Rather, rekido (calendar studies) was thought to be more along the lines of some types of astronomy, such as rekisan (calendrical calculations) astronomy or positional astronomy.
Extraordinary Astronomical Events
Extraordinary astronomical events are astronomical phenomena which are seen only rarely. These include, for example, solar and lunar eclipses, shooting stars, comets, (the appearance of) abnormal closeness between the moon and stars, or between planets, low-latitude auroras, halation and white rainbows.
These phenomena were thought to significantly influence nations on earth and their rulers (kings and emperors). Therefore, advance warning was given for predictable phenomena, so that these days could be avoided when holding national events, while unforeseen incidents needed to be observed, interpreted by divination and reported to the rulers so that they could respond appropriately. What was required of tenmon hakase (masters of astrology), the authorities on Tenmondo, was to take measures concerning extraordinary astronomical events. Reporting the status of extraordinary astronomical events and the good omens they contained in sosho (reports to the Emperor) to the Emperor, conducted through the Onmyoryo (Bureau of Divination) or Kurododokoro (the Chamberlain's Office), was called tenmon misso.
Under the Ritsuryo system (a system of centralized government based on the ritsuryo code), Tenmondo was put under the supervision of the Onmyoryo, together with rekido and the Onmyoryo, and it consisted of one tenmon hakase (Shoshichiinoge (Senior Seventh Rank, Lower Grade)) and ten tenmon no sho (students of astronomy). As phenomena related to tenmon were considered to significantly affect the fate of the country, even students of tenmon were not allowed to read books on tenmon freely ('Yororitsuryo' zoryo Hisho gensho jo). It was also forbidden to talk about the results of observations.
Tenmon hakase observed the sky every night alongside students of tenmon, searching for signs of extraordinary astronomical events; whenever any sign was found, they sent a tenmon misso. Tenmon hakase also trained students of tenmon. The major textbooks used in Tenmon were the tenmonshi (astronomy records) "Kanjo (Historical records of the Han Dynasty)" and "Shin jo (History of the Jin Dynasty)", "Tenkanjo", "Sankehakusan (textbook for learners of astrology)", "Tenmon Yoshu (the summary of astrology)" and "Kanyo Yoshu (the summary of astrology)."
Meanwhile, tenmondo was not considered as important as onmyodo ('the way of Yin and Yang', an occult divination system based on the Taoist theory of the five elements) in the ritsuryo nation of Japan, and, as a rule, observations of the sky were conducted only at fixed times, such as from 8 PM to 10 PM and from 4 AM to 6 AM.
The Abe Family's Monopoly over Tenmondo
After the middle Heian period, during which ABE no Seimei, who succeeded to the art of tenmondo from KAMO no Yasunori, was active, the study became the hereditary learning of the Abe clan (later the Tsuchimikado family) and the clan showed a tendency to exclude people from other families. The Abe clan (安倍, also written as 阿倍), famed for its members ABE no Hirafu and ABE no Nakamaro, had been a line of local ruling families and nobility since the time preceding the Taika Reforms. Although some members of the family had been appointed as onmyo no kami (Director of Onmyoryo, or Bureau of Divination) as aristocratic bureaucrats, Seimei was the first to be assigned a job relating to onmyodo and tenmondo as an expert bureaucrat. At the age of 40 in 960, Seimei was still a Tenmon tokugyou no sho (researcher of astrology); in 972, he was promoted to tenmon hakase ("Chikanobu kyo ki") and was ranked as jushiinoge (Junior Fourth Rank, Lower Grade) Sakyo no gon no daibu (Provisional Master of the Eastern Capital Offices). However, he died in 1005 at the age of 85 without becoming onmyo no kami (director of the Onmyoryo); this fact is considered proof that the Abe clan was not a family which traditionally had a strong basis in Onmyoryo and tenmondo. Regardless, as he played an active role in onmyodo and tenmondo for a long time, enjoyed longevity, and was blessed with two sons, ABE no Yoshimasa and ABE no Yoshihira, a good foundation was established for the family. Yoshimasa was recommended by KAMO no Yasunori as tenmon tokugyou no sho in 970, was appointed tenmon hakase after Seimei in 986, and became onmyo no kami in 1004, also serving as tenmon hakase until his death. Meanwhile, Yoshihira is known to have been appointed as onmyo hakase (Doctor of onmyo) in 991; he was granted Jushiinoge (Junior Fourth Rank, Lower Grade) after serving as onmyo no suke (Assistant Director of Onmyoryo, or Bureau of Divination) and received an imperial decree of tenmon misso in 1019 following the death of Yoshimasa, who was the then tenmon hakase. There were two reasons for this emergency appointment: the first was that the then tenmongon no hakuji (master of astronomy), WAKE Hisakuni (? 和気久邦), was in Iyo Province, and the second was that the two people who received imperial decrees of tenmon misso were not sufficiently skilled. That year, Yoshihira recommended his own son, ABE no Akichika, who had been tenmon tokugyou no sho, as tenmon hakase. In addition, Akichika's younger brother, ABE no Norichika, was assigned to be tenmongon no hakuji in 1035, completing the Abe clan's monopoly of both hakase relating to tenmon (in 1031, their oldest brother ABE no Tokichika received an imperial decree of tenmon misso as well). Although there had been some precedent for new tenmon hakase to be appointed by a tenmon hakase's recommendation ("Choya gunsai (Collected Official and Unofficial Writings)"), from that point on, the tradition of tenmon hakase from the Abe family recommend their own kin for tenmon hakase or tenmongon no hakuji, keeping the positions in the family, was established. There were reasons for this: first, as tenmondo was then considered to be a special skill and was only passed down among family members (hereditary learning), the Imperial Court allowed it in order to preserve divination techniques, and second, a hereditary government office system had been established. (Meanwhile, the Kamo clan also claimed their position in rekido: in the Kamakura period, when a member of the Abe clan was onmyo no kami, the Kamo clan insisted that the ceremony of goryaku no so (the annual presentation of the calendar to the emperor), which had been conducted mainly by onmyo no kami, should be conducted by reki hakase. Saying, 'Tenmondo should deal with tenmon misso and rekido should handle goryaku no so', they excluded the Abe clan's onmyo no kami from the ceremony, changing the tradition.
In addition, in the book "Imakagami (The Mirror of the Present)", some connection was stated between the death of Shinzei, a member of the Southern House of the Fujiwara clan, in the Heiji War, and his deep knowledge of Tenmondo.)
The imperial decrees of tenmon misso to the Nakahara clan ended with the one for NAKAHARA no Moroyasu in 1118, and were subsequently issued only to members of the Abe clan. Furthermore, discord within the Abe clan over the position of tenmon hakase and decrees of tenmon misso increased as the family members established branch families. There was an incident quite contrary to the usual cases in which tenmon hakase conducted tenmon misso (Article on April 22, 1235 of "Meigetsuki (Chronicle of the Bright Moon) ") : in 1235, one member of the Abe clan who had not received an imperial decree was appointed as tenmon hakase for his distinguished service in reporting temporarily appearing stars. Additionally, like ABE no Yasuchika who later became the mainstream of the Abe clan, they came to interpret tenmondo with more importance given to onmyodo than to the astronomy which came from China. The divided Abe clan was reunited under ABE no Ariyo (a descendent of Yasuchika), who was backed by the Muromachi bakufu (a Japanese feudal government headed by a shogun); they named themselves the Tsuchimikado family (the Abe clan), and acquired a position from which they could supervise both tenmondo and onmyodo. Later in the Edo period, at the time of Yasutomi TSUCHIMIKADO, the soke (the head family or house) of the Kamo clan died out and the Abe clan took the position of rekido, over which they had fought with the Kotokui Family, a branch family of the Kamo Clan; in this way, they established the dominance of the Abe Clan's Tsuchimikado family in the Onmyoryo.
Transition to Modern Astronomy
Although it was only after the establishment of tenmonkata (officer in charge of astronomy) by the Edo bakufu that people started to conduct astronomical research in the current style, the first tenmonkata, Harumi SHIBUKAWA, discussed the theory on the relationship between astronomical bodies and divination, which followed the tradition of Tenmondo, as well as scientific observation of the sky in a book called "Tenmon Keito" (this may stem from the fact that Harumi learned Shinto from Yasutomi TSUCHIMIKADO). With resistance from Tsuchimikado family's Tenmondo, as well as Buddhist astronomy which came from Sukuyodo (astrology with its roots in esoteric Buddhism), the study was introduced in full after the Meiji period, when the Onmyoryo was abolished.