- Born: 1639
- Died: 1715
- Titles: Tenmongata
- Other Names: 保井算哲 (Yasui Santetsu, Motoi Santetsu)
- Japanese: 澁川 春海 (Shibukawa Harumi, Shibukawa Shunkai)
He was born into a family of go-players to the shogunate, but was also interested in mathematics and astronomy. At that time Japan was still calculating the calendar using the Tang calendar the Senmyô calendar 宣明暦, which it had adopted in 8612, and inaccuracies in the calendar were obvious, especially that the winter solstice was calculated almost two days late. Also, it was not very accurate with eclipses, in particular predicted far too many. Harumi like some other scholars of the time believed that the Mongol-period Juji calendar授時暦, which was the apex of the Chinese calendar tradition,should be adopted in Japan. Through his professional connections as a go-player he was able to interest several officials in the project, especially Hoshina Masanori 保科正之 of Aizu, the shogun's guardian, and Mito Mitsukuni. He made a table of eclipses as predicted by the Senmyô and Juji calendars to prove the superiority of the later. However, on 1675/5/1 an eclipse that was predicted by the Senmyô calendar but not by the Juji calendar did occur, and so the idea of changing calendars was rejected. Harumi managed to get hold of a (forbidden) Chinese work on western astronomy, and "localized" the 13th-century Chinese calendar for 17th century Japan, and in 1683 petitioned the imperial court to adopt the "Yamato" calendar. However, the next year the court decided to adopt the Ming-period Daitô calendar 大統暦, a very slight revision of the Juji calendar. Harumi again petitioned, saying the Daitou calendar was not suitable for Japan, and finally on 1684/10/29 the Yamato calendar was accepted, and it went into effect the next year as the Jôkyô calendar 貞享暦.
After that, the shogunate established the office of the Tenmon gata 天文方, and Harumi became the first holder of that post. He had an observatory on his property and built some astronomical instruments.
A 2009 novel Tenchimeisatsu 天地明察 by Ubukata Tô 冲方丁 about him won the Yoshikawa Eiji New Authors Prize 吉川英治文学新人賞. The book was made into a movie in 2012, directed by 滝田洋二郎, who directed Okuribito.
- Hirose Yoshio, Koyomi (Nihon-shi Shôhyakka), 1978.
- Japan Chronik