Tenpo-reki (Tenpo calendar) (天保暦)

Tenpo-reki (or Tenpo calendar) is a Japanese calendar based on the lunar solar calendar used in old Japan. Officially, it is called Tenpo jiningenreki. This is because the calendar of the same name Tenpo-reki was previously used in China.

In a wide sense, Japanese old calendar means all the calendars and eras adopted in Japan before the Gregorian calendar was introduced, but in a narrow sense, it means Tenpo-reki.

Attention
The Christian Era and dates after 1873 are in the Gregorian calendar system.

The period when the Tenpo calendar was used

On February 18, 1844, it was changed from the Kansei calendar and used for about twenty nine years until December 31, 1872.

On January 1, 1873, it was changed to the Gregorian calendar (solar calendar).

Summary

Kagesuke SHIBUKAWA completed this calendar after he introduced western astronomy to Japan, and the calendar was considered the most accurate lunar solar calendar which had been practically used.
According to the calculation of Kiyotsugu HIRAYAMA,a solar year of Tenpo-reki is 365.24223 days and a synodic month is 29.530588 days, whereas the average solar year is 365.24219 and the average synodic month is 29.530589 days; it is said the difference is smaller than the solar year of 365.2425 days of the Gregorian calendar ("Rekiho oyobi Jiho" (calendar and timetable method).)

Before the Kansei-reki (Kansei calendar) was introduced, ' Heikiho' (a way of expressing 24 divisions of the old calendar) was used and was calculated by dividing one year equally (divided by time), into 24 divisions of the old calendar. In Tenpo-reki, Teikiho (a way of expressing 24 divisions of the old calendar) was used, which is the method to calculate the position of the sun and divide the orbit of the sun on the celestial sphere by twenty-four (divide in the space) to obtain 24 divisions of the old calendar.

However there was some criticism about introducing Teikiho, since it made Chijun-ho (a calendar system of setting the leap month) more complicated. Because of this, it is reported that the date will be confused in 2033 (if it is used in accordance with its definition, the month of November will come after September) (the issue of the old calendar 2033). Also there is a strong criticism for officially introducing the Japanese classical Timetable Method in the calendar, going against the influence of China and Europe.

Haruo TSUCHIMIKADO of the Bureau of Divination was against introducing the solar calendar and suggested a plan to change the calendar of the lunar solar calendar during the period of the Meiji Restoration, however after he died suddenly, this plan was cancelled and the Tenpo-reki became the last lunar solar calendar in Japan.

The transition into the Gregorian calendar

On December 9, 1872, the Imperial edict of the Emperor Meiji and the edict of the Grand Council of State No. 339 were suddenly issued to change the existing calendar to a Gregorian calendar.

There was confusion among people since they were told that there were only two days in December, the next month of the announcement, and the next day was going to be January 1, 1873. (Gregorian calendar) due to the change.

For the details of the confusion due to the change of the calendar, refer to the section 'introducing the Gregorian calendar to Japan.'

The old calendar after the change of the calendar

In Japan, after the change to the new calendar up to the present, the lunar-solar calendars called 'the old calendar' were described in calendars and expository books about calendar. Strictly speaking, they are not Tenpo-reki; the, first day of the month and 24 divisions of the old calendar were calculated from the movement of the moon and the sun of modern astronomy, and to the result of which, only the Chijun-ho was applied like Tenpo ho.