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二十四節気
Japan’s traditional calendar uses twenty-four solar terms to mark positions in the Sun’s apparent annual movement. Risshun, Shunbun, Geshi and Tōji are four of them. Astronomical calculation determines when they occur; their Gregorian dates are approximate rather than permanently fixed.
Adjacent terms are separated by 15° of solar longitude.
Each traditional season contains six terms.
Setsubun, Higan and Doyō are supplementary seasonal markers, not members of the twenty-four.
Solar longitude expresses the Sun’s position along the ecliptic, measured from the spring equinox at 0°. The modern terms occur at multiples of 15°: Shunbun at 0°, Geshi at 90°, Shūbun at 180° and Tōji at 270°. These are positions within the same annual cycle.
Earth’s orbital speed varies, so the Sun does not pass between successive term positions in identical lengths of time. Under the current method, calendar-date intervals can be fourteen, fifteen or sixteen days. A familiar “around February 4” is therefore a useful approximation, not a permanent date.
In Japan Standard Time, the National Astronomical Observatory gives Risshun as February 4, 2026 at 05:02, and Shunbun as March 20 at 23:46. An exact time needs its year and time zone: the same astronomical instant can fall on another calendar date elsewhere.
Solar longitude: 360° in a full cycle; 24 positions, 15° apart
0° → 15° → 30° → … → 345° → 360° (back to the equinox).
The six are Risshun, Usui, Keichitsu, Shunbun, Seimei and Kokuu: 立春、雨水、啓蟄、春分、清明、穀雨. Their names refer to seasonal weather, living things and farming. Keichitsu uses the Japanese form 啓蟄 where Chinese uses 驚蟄. They do not predict identical weather across Japan.
Rikka, Shōman, Bōshu, Geshi, Shōsho and Taisho are 立夏、小満、芒種、夏至、小暑、大暑. Geshi marks the summer solstice, around the longest daylight of the Northern Hemisphere’s year. The hottest weather need not coincide with that astronomical turning point.
Risshū, Shosho, Hakuro, Shūbun, Kanro and Sōkō are 立秋、処暑、白露、秋分、寒露、霜降. Risshū begins traditional autumn while much of Japan can still be hot. The calendar’s seasonal division and a place’s observed temperature describe different things.
Rittō, Shōsetsu, Taisetsu, Tōji, Shōkan and Daikan are 立冬、小雪、大雪、冬至、小寒、大寒. Tōji marks the winter solstice near the Northern Hemisphere’s shortest daylight. Shōkan and Daikan follow before Risshun begins another traditional spring. Actual snowfall and cold vary locally.
The system developed within Chinese calendrical traditions and reached Japan with calendar knowledge. In a lunisolar calendar, lunar phases organize months while solar markers help keep them aligned with the seasons. Twelve sectional terms (sekki) alternate with twelve principal terms (chūki). The principal terms help assign month names and extra leap months; for example, the month containing Shunbun is ordinarily the second lunar month.
Through the Kansei calendar, Japan divided the year into twenty-four equal time intervals, using heikihō. The Tenpō calendar, first used in 1844, adopted teikihō, based on actual solar positions. A modern calculated instant should therefore not be projected unchanged into every older calendar.
Zassetsu supplement the solar terms. Setsubun now usually means the day before Risshun; Higan surrounds the equinoxes, and Doyō precedes the beginnings of the traditional seasons. Hachijūhachi-ya and Nyūbai also belong to this supplementary group, rather than extending the list of twenty-four.
Japan adopted the Gregorian calendar in 1873. The terms no longer organize the official months, but their names remain seasonal markers in calendars. Gregorian dates, traditional lunar month names and solar terms therefore describe different aspects of the calendar.
They are based on the Sun’s position. A lunisolar calendar uses both lunar months and solar seasonal markers.
No. Their solar longitudes differ by 15°, but the time intervals vary.
No. It is a traditional calendar boundary, while local temperatures follow their own seasonal patterns.
No. It is a supplementary marker, or zassetsu.