Rekido is the ancient Japanese study of calendar-making.
"Nihonshoki" (Chronicles of Japan) states that a reki hakase (master of reki (calendar)) had come to Japan from Paekche during the era of the Emperor Kinmei. However, the study of the calendar didn't take root in Japan until the twelfth year of the Empress Suiko (602), when the priest Kanroku came to Japan from Paekche. Therefore, Japan's rekido originates from Kanroku's visit.
Under the Ritsuryo system (a system of centralized government based on the ritsuryo code), the task of preparing the calendar based on the observations of the sun and moon was carried out by the Onmyoryo (a government office that had jurisdiction over calendar preparation, astronomy, divination, etc.) instead of the Daigaku-ryo (the Bureau of Education under the ritsuryo system). There were one or two reki hakase and ten reki no sho (students of the calendar). However, Japan had already adopted the Chinese rekiho (method of making calendars). This limited the application of rekido to a few tasks, such as the writing of rekichu (various information recorded in the almanac), the making of shichiyoreki (calendar of the seven luminaries) and chuseireki (chusei calendar), and the prediction of solar eclipses. Moreover, rekido covered the same area as tenmondo (ancient horoscopy) and sando (study of mathematics), which prevented it from gaining popularity. In a bid to reverse the situation, various programs got underway in 730, including the establishment of a scholarship program that offered financial aid to two reki tokugyo no sho (reki students) chosen from among the excellent reki no sho. The textbooks they used included 暦律志, such as "Kanjo"(historical records of the Han Dynasty) and "Jin shu" (History of the Jin Dynasty), commentaries for the adopted calendar ("Daienrekigi" (annotation book of the calendar), "Senmyorekikyo" (annotation book of the calendar)), and other books such as "Teitenron" (annotation book of the calendar) and "Shuhisankei" (a mathematics book from ancient China). Additionally, they used the textbooks of sando (study of mathematics) such as "Kyushosanjutsu" (the oldest mathematics book in China) and "Rokusho." It is believed that they referred to "Daito Onmyosho" (the book of yin and yang in the Tang era) and other books in order to write rekichu. During the Heian period, nobles seriously believed that there were taboo directions that varied from day to day. This boosted the popularity of guchureki (annotated calendar), which included rekichu showing a taboo direction for each day. Therefore, rekido, which was originally a scientific study of the calendar, began to take on the flavor of Onmyodo (way of Yin and Yang, an occult divination system based on the Taoist theory of the five elements).
In the mid-Heian period, rekido became the hereditary learning of the Kamo clan since the astrologer, KAMO no Yasunori, initiated his son, KAMO no Mitsuyoshi. Concerned about a decline in the success rate of solar eclipse prediction, the KAMO no Yasunori asked Nichien, a priest of the Tendai sect who was studying in Wu-yueh, to bring back the futenreki (the Futian calendar table) and then tried to use it. However, the court did not allow him to use it. Consequently, after KAMO no Mitsuyoshi, the Kamo clan pretended to adopt only the Senmyo calendar in the eyes of the public, while in secret they also used the futenreki. Meanwhile, rekido was overshadowed by sukuyodo, the emerging astrology that had been established by combining the futenreki and Esoteric Buddhism. Initially, scholars of rekido and sukuyodo collaborated to make a calendar (according to an entry in "Shoyuki" (the diary of FUJIWARA no Sanesuke) dated August 30, 1015). However, in about 1038 a conflict arose between them, and the scholars of sukuyodo began criticizing those of rekido (according to an entry in "Shunki (Spring Tale)" dated December 31, 1038).
It was the custom of the day to manipulate the calendar in order to pretend that 'sakutan toji' came or did not come regardless of the cycle of celestial bodies (which is called 'kaireki').
(When the winter solstice happens to be the first day of November (in the old lunar calendar) once in 20 years, it is called sakuten toji.)
(Sauten toji was considered auspicious, and a huge and very expensive ceremony was held on this day.)
People believed superstitions such as the following:
An intercalary month is added on seven occasions during a 19-year period.'
If the first intercalary month becomes intercalary August, it is unlucky.'
If a month that consists of 30 days continues four times in a row, it is unlucky.'
These superstitions prompted the manipulation of the calendar (which is called kaireki). In some cases, the scholars of rekido changed the calendar just to cover up mistakes in rekizan (calculation of the calendar). Cunningly cheating the system of the futenreki (the Futian calendar table), they concocted a fake Chijun-ho (a calendar technique of setting a leap month and leap day) while insisting that they were following kuden (oral instructions) or secret teachings they had inherited. Thus they manipulated the calendar in secret (which raised the possibility that solar eclipses and summer and winter solstices would not occur according to the forecasts). These acts of the rekido scholars came under fire from the scholars of sando (study of mathematics) and sukuyodo (the astrology brought as part of Esoteric Buddhism). After the Kamakura era, the status of rekido further declined through the advent of the Kana-goyomi calendar (a calendar written in the Japanese syllabaries, or kana) and the Minkan-reki calendar. In the Muromachi era, reki hakase occasionally carried out tenmon misso (reporting unusual astronomical phenomena to the emperor). However, after the extinction of the Kadenokoji family line, the direct descendant of the Kamo clan, the Tsuchimikado family (the main branch of the Abe clan), became the leading figure in rekido and forced the Kotokui family, a branch family of the Kamo clan, to come under their control.
They also gained control over Onmyoji (Master of Yin yang) and koyomi-shi (calendar makers) throughout the country, which enabled them to acquire the right to charge myogakin (money to dedicate) for the sales of their calendars. After the Meiji Restoration, however, the Onmyoryo was abolished and the Tokyo astronomical observatory replaced rekido in all matters relating to the calendar.