Japan’s old calendars: fitting months to seasons

A lunisolar month follows the Moon’s phases. The civil day containing the new moon begins the month; this first day is called sakujitsu. The next new moon starts another month. Because the interval averages about 29.5 days, calendar months have either 29 or 30 days; their sequence changes from year to year.

Twelve lunar months total roughly 354 days, about eleven days short of a solar year. An intercalary month brings the calendar back toward the seasons. It repeats the preceding month’s number: month three, intercalary month three, then month four. About seven insertions in nineteen years is an average frequency, rather than one timetable repeated by every historical system.

The twenty-four solar terms divide the year by the Sun’s position. They provide seasonal markers alongside the separate cycle of lunar dates. The Tenpō system used the twelve principal terms, including the equinoxes and solstices, when assigning month numbers and intercalary months.

The month beginning on Gregorian 24 September 2014 contained Frost Descent and was numbered nine. The next, starting 24 October, had no principal term and became intercalary month nine. Minor Snow placed the month beginning 22 November at ten; the winter solstice placed the one starting 22 December at eleven. The new moons supply boundaries, while solar terms help assign the numbers.

The month beginning 20 February 1985 contained no principal term, yet it was month one. The preceding winter-solstice month had to be eleven, and the following spring-equinox month had to be two. Those seasonal requirements also constrain the sequence: absence of a principal term alone does not settle the question.

Calendrical knowledge reached Japan through the Korean peninsula. The National Diet Library gives 604 as the traditional date of Japan’s first calendar. The court’s Bureau of Yin and Yang later handled calendrical and astronomical work. Calendars carried seasonal, ceremonial and auspiciousness annotations as well as dates.

Detailed annotated calendars, guchūreki, placed extensive notes beside the dates, and readers also used available space for diaries. A 1413 calendar shown by the National Diet Library has Mansai’s diary on its reverse. Simplified kana calendars made another form available; printed calendars appeared by the late Kamakura period.

The Senmyō system, used from 862 for more than eight centuries, increasingly diverged from observed eclipses. Jōkyō (1685) drew on Shibukawa Harumi’s observations and corrected Chinese Shoushi calculations for Japan’s longitude. Hōryaku (1755) later failed an eclipse prediction. Kansei (1798) and Tenpō (1844) incorporated Western astronomy transmitted through Chinese and Dutch texts.

Kansei calculations used elliptical orbits for the Sun and Moon while retaining epicycles for planets. Tenpō incorporated further astronomy from Dutch-language books. Production involved separate offices: the shogunate’s astronomers calculated dates, the Kyoto Bureau of Yin and Yang supplied annotations, and authorised publishers printed calendars locally. Changes in astronomy thus reached readers through an established production chain.

In 1872 the Meiji government announced the adoption of the Gregorian calendar. The day after the second day of the twelfth month of Meiji 5 became 1 January Meiji 6: Gregorian 1 January 1873. Today kyūreki commonly refers to the last pre-reform system, Tenpō, or to modern calculations modelled on it.

The change was announced on day nine of the eleventh lunar month of Meiji 5, after Tenpō calendars for the coming year were already circulating. Hastily produced solar calendars did not reach everywhere in sufficient numbers before year-end. Their design changed too: the old annotations disappeared and holidays occupied the upper margin. The abrupt reform affected both distribution and what readers found on the page.

The fifteenth night of the eighth lunar month marks the mid-autumn moon, while traditional Tanabata falls on the seventh day of month seven. These dates move through the Gregorian calendar. Keeping the same written month and day in July or August changes their seasonal position; observances may instead retain lunar dates or use a month-later convention.

Japan no longer has an institution issuing an official lunisolar calendar. For 2033–34, Tenpō month-numbering conditions conflict and allow different proposals for inserting an intercalary month. The National Astronomical Observatory sets out these alternatives. A modern table’s convention therefore matters, while historical dates can be checked against calendars actually issued at the time.

Tenpō requires the autumn-equinox month to be eight and the winter-solstice month to be eleven. In 2033 those months begin on Gregorian 23 September and 22 November. Only one lunar month, beginning 23 October, lies between them—too little room for both nine and ten. Giving different requirements priority produces different proposed intercalations.