Tenpō calendar (天保暦 Tenpō-reki?), officially the Tenpō sexagenary unitary calendar (天保甲戌元暦 Tenpō jin'in genreki), was a Japanese lunisolar calendar (genka reki). It was published in the Tenpō era (1830-1844). It was in use in the late Edo period, from 1844 to 1872.
The calendar was a lunisolar calendar with 355 days in a year. The length of months was either 29 or 30 days and calculated to match the actual lunar cycle. A leap month was added seven times in a nineteen-year Metonic cycle. Each month was synchronized with the sexagenary cycle of Ten Celestial Stems and Twelve Branches, the Twenty-Eight Lunar Mansions, the Eight Warrior Deities, the Five Phases, and the Nine Irregular Days, or Zassetsu (雑節). The only Zassetsu still celebrated in Japan today is setsubun.
In previous calendars, hours were of uniform lengths. In the Tenpō calendar, the length of hours changed depending on the time of year. This made it extremely difficult to make Japanese mechanical clocks.
Unlike the Chongzhen calendar which will continue to be accurate into the long-term future, the Tenpō calendar does not perfectly synchronize the lunar and solar cycles, and has started to drift backwards over the years. Also, in the years 2033 and 2034 there will be several months when the system breaks down and there is no legally defined month under this calendar.
- Nussbaum, Louis-Frédéric. (2005). "Calendar" in Japan Encyclopedia, p. 98.
- Orchiston, Wayne et al. (2011). Highlighting the History of Astronomy in the Asia-Pacific Region, p. 155.
- Jessica Kennett Cork. The Lunisolar Calendar: A Sociology of Japanese Time.