The seventy-two microseasons: a calendar for noticing change

The solar terms divide the annual seasonal cycle into twenty-four parts; each is further divided into an initial, middle and final kō. The older calendar’s lunar months answer a different question. A month follows lunar phases, while these seasonal divisions follow the solar year.

Seasonal names differ between versions. In the Jōkyō list, the second kō of Risshun is ‘the plum blossoms become fragrant’; in the Hōreki-and-later list shown by the National Astronomical Observatory, the same position describes the bush warbler’s song. The arrival of swallows appears under Seimei in the latter list. Such names organise observations, while actual timing varies with region, year and local environment.

The imported Senmyō calendar

Long before the Jōkyō reform, Japan used the Chinese-derived Senmyō calendar, transmitted through Bohai, from 862 through 1684. It spanned the Heian period, medieval centuries and early Edo era. The National Astronomical Observatory’s account of its collections indicates that Chinese kō names were generally retained during this period. The reform of 1685 therefore revised an established calendrical tradition; it did not introduce the seventy-two kō to Japan for the first time.

Revisions after the Jōkyō reform

The system reached Japan with Chinese calendrical traditions. Shibukawa Shunkai’s work on the Jōkyō calendar, introduced in 1685, belongs to the history of adapting seasonal names to Japan. Subsequent calendars and almanacs changed further. A present-day list should therefore not be assigned indiscriminately to every older calendar.

The National Astronomical Observatory describes the Shichijūnikō-shō, printed in 1686, as an introduction to the Chinese system. Printing in Japan did not necessarily mean adopting Japanese revisions. Detailed guchūreki almanacs generally included the kō, while popular calendars written in kana often omitted them. Hangeshō, an agricultural marker, was one exception.

After the adoption of the solar calendar in 1873, seasonal terms continued to appear in calendars. Check the particular calendar for modern dates. The names offer a vocabulary for comparing local seasonal signs, not a substitute for a weather forecast or measured flowering records.

Equal time and equal angles

Equal divisions can mean equal time or equal angle. The heiki method divides the year into twenty-four equal time intervals, about 15.2 days apart. Through the Kansei calendar, the kō likewise divided the year into seventy-two equal periods. The teiki method instead marks successive 15-degree steps in the Sun’s apparent ecliptic longitude—its position along the ecliptic as seen from Earth. Earth’s orbital speed varies, so equal angles do not take equal numbers of days.

Changing dates for the kō

When the Tenpō calendar adopted teiki for solar terms, it stopped assigning a separate date to every kō. Almanacs could list all three under their solar term; the abbreviated official calendars of 1874–1884 used that arrangement too. Hangeshō retained a date. Today it is defined at solar longitude 100 degrees, around 2 July, but that particular rule cannot be applied indiscriminately to all seventy-two kō.

Some old names preserve explanations of nature that modern science does not accept. One surviving example describes fireflies as coming from decaying grass. Such wording belongs to the history of the calendar, rather than an account of how fireflies develop.