Tenpō calendar

The Tenpō calendar (天保暦, Tenpō-reki), officially known as the Tenpō sexagenary unitary calendar (天保壬寅元暦 Tenpō jin'in genreki), was a Japanese lunisolar calendar.^[1] It was published in the Tenpō era (1830–1844). It remained in use throughout the late Edo period, from 1844 to 1872.^[2]

The Tenpō-reki system was developed by Shibukawa Kagesuke. It was the last calendar system devised by Japanese astronomers and mathematicians.^[2]

The Tenpō calendar is a lunisolar system which adopted Teiki-hō method, dividing solar terms by solar longitude instead of time, unlike the previous Heiki-hō method. It begins each lunar month on the day of the new moon and adds a leap month when necessary- specifically when three lunar months occurs between those including a solstice/equinox. the leap month lacks any chūki 中気 (one of the twelve solar terms that are used to determine the months of the year), and is inserted accordingly. Solstice and equinox consistently fall within the second, fifth, eighth and eleventh months. Observations from Kyoto^[3][4] dictates the time used for determining solar terms and lunar phases.

Unlike previous calendars with uniform hours lengths, the Tenpō calendars hour vary seasonally, posing the length of hours changed depending on the time of year.^[5] This made it extremely challenging to make Japanese mechanical clocks.

The Tenpō calendar is no longer officially maintained.

• Japanese calendar
• Sexagenary cycle
• Tenpō

• National Diet Library, "The Japanese Calendar"