What Are the 24 Solar Terms (Nijūshi Sekki)?
The 24 Solar Terms divide the year into 24 equal parts based on the sun's position along the ecliptic (solar longitude). They are widely used as a fundamental calendar system indicating seasonal changes, such as the Vernal Equinox, Summer Solstice, Autumnal Equinox, and Winter Solstice.
The 24 Solar Terms were established based on the climate of the Yellow River basin in ancient China. Starting from a solar longitude of 0° (the Vernal Equinox), they mark 24 seasonal turning points at 15° intervals.
Introduced to Japan during the Asuka period, they were adopted as an essential element in the operation of the lunisolar calendar. In the traditional calendar, the Zhongqi (mid-term) solar terms (Rain Water, Vernal Equinox, Grain Rain, etc.) determine month names and when to insert leap months.
The current 24 Solar Terms are calculated using the "Fixed Qi Method." Since a solar term is defined as the moment the sun reaches a specific angle on the ecliptic, the interval between each solar term varies from 14 to 16 days.
Setsu and Zhongqi
Setsu (Principal Term)
Solar terms that mark the beginning of each month. The 12 Setsu are: Beginning of Spring, Awakening of Insects, Clear and Bright, Beginning of Summer, Grain in Ear, Lesser Heat, Beginning of Autumn, White Dew, Cold Dew, Beginning of Winter, Heavy Snow, and Lesser Cold. They also serve as the basis for Jūnichoku transitions.
Zhongqi (Mid-term)
Solar terms positioned in the middle of each month. The 12 Zhongqi are: Rain Water, Vernal Equinox, Grain Rain, Xiaoman, Summer Solstice, Greater Heat, End of Heat, Autumnal Equinox, Frost Descent, Light Snow, Winter Solstice, and Greater Cold. They serve as the basis for determining lunar calendar month names.
List of the 24 Solar Terms
Spring
| Solar Term Name | Reading | Category | Solar ecliptic longitude | Period | Meaning |
|---|---|---|---|---|---|
| Risshun | りっしゅん | 節 | 315° | 2月4日頃 | 春の始まり。この日から立夏の前日までが暦の上の春。八十八夜・二百十日の起算日。 |
| 雨水 | うすい | Medium | 330° | 2月19日頃 | 雪が雨に変わり、氷が溶けて水になる頃。農耕の準備を始める目安。 |
| Keichitsu | けいちつ | 節 | 345° | 3月6日頃 | 冬ごもりの虫が地上に這い出る頃。大地が暖まり始める。 |
| 春分 | しゅんぶん | Medium | 0° | 3月21日頃 | 昼と夜がほぼ等しくなる。太陽が真東から昇り真西に沈む。彼岸の中日。 |
| Seimei | せいめい | 節 | 15° | 4月5日頃 | 万物が清らかで明るく美しい頃。花が咲き、鳥が歌い、風が心地よい。 |
| 穀雨 | こくう | Medium | 30° | 4月20日頃 | 穀物を潤す春の雨が降る頃。田畑の準備が整い、種まきの好期。 |
Summer
| Solar Term Name | Reading | Category | Solar ecliptic longitude | Period | Meaning |
|---|---|---|---|---|---|
| Rikka | りっか | 節 | 45° | 5月6日頃 | 夏の始まり。新緑が目に鮮やかで、風が爽やか。 |
| 小満 | しょうまん | Medium | 60° | 5月21日頃 | 草木が成長し、万物が次第に満ちてくる頃。麦が穂を出す。 |
| Boshu | ぼうしゅ | 節 | 75° | 6月6日頃 | 稲や麦など芒（のぎ）のある穀物の種をまく頃。田植えの目安。 |
| 夏至 | げし | Medium | 90° | 6月21日頃 | 1年で最も昼が長く夜が短い日。太陽が最も高く昇る。 |
| Shosho | しょうしょ | 節 | 105° | 7月7日頃 | 本格的な暑さが始まる頃。梅雨明けが近づく。 |
| 大暑 | たいしょ | Medium | 120° | 7月23日頃 | 1年で最も暑い頃。厳しい暑さがピークを迎える。 |
Autumn
| Solar Term Name | Reading | Category | Solar ecliptic longitude | Period | Meaning |
|---|---|---|---|---|---|
| Risshu | りっしゅう | 節 | 135° | 8月7日頃 | 秋の始まり。この日以降の暑さは「残暑」となる。 |
| 処暑 | しょしょ | Medium | 150° | 8月23日頃 | 暑さがおさまる頃。朝夕に涼しい風が吹き始める。 |
| Hakuro | はくろ | 節 | 165° | 9月8日頃 | 草花に朝露が宿り始める頃。本格的な秋の訪れ。 |
| 秋分 | しゅうぶん | Medium | 180° | 9月23日頃 | 昼と夜がほぼ等しくなる。彼岸の中日。 |
| Kanro | かんろ | 節 | 195° | 10月8日頃 | 露が冷たく感じられる頃。秋が深まり、紅葉が始まる。 |
| 霜降 | そうこう | Medium | 210° | 10月23日頃 | 霜が降り始める頃。朝晩の冷え込みが厳しくなる。 |
Winter
| Solar Term Name | Reading | Category | Solar ecliptic longitude | Period | Meaning |
|---|---|---|---|---|---|
| Ritto | りっとう | 節 | 225° | 11月7日頃 | 冬の始まり。木枯らしが吹き、冬の気配が強まる。 |
| 小雪 | しょうせつ | Medium | 240° | 11月22日頃 | わずかに雪が降り始める頃。北国では初雪の便りが届く。 |
| Heavy snow | たいせつ | 節 | 255° | 12月7日頃 | 本格的に雪が降り始める頃。山々は雪化粧。 |
| 冬至 | とうじ | Medium | 270° | 12月22日頃 | 1年で最も昼が短く夜が長い日。ゆず湯に入り南瓜を食べる風習がある。 |
| Shokan | しょうかん | 節 | 285° | 1月6日頃 | 寒さが厳しくなり始める頃。「寒の入り」。寒中見舞いの時期。 |
| 大寒 | だいかん | Medium | 300° | 1月20日頃 | 1年で最も寒い頃。寒稽古や寒仕込みの時期。 |
Fixed Qi Method (Current)
A method that defines a solar term as the moment the sun reaches a specific angle (ecliptic longitude) on the ecliptic. Because Earth's orbital speed is not constant, the interval between solar terms varies from 14 to 16 days. The current calendar uses this method.
Mean Qi Method (Traditional)
A method that divides the period from one Winter Solstice to the next into 24 equal parts. The interval between each solar term is a constant ~15.22 days. This method was used before the Tenpō calendar (1844).
- Lunar calendar — The Zhongqi (mid-term) solar terms (Rain Water, Vernal Equinox, etc.) determine the month names in the lunar calendar. A month without a Zhongqi becomes a leap month.
- Twelve Day Officers — The Jūnichoku assignments change at each Setsu (Beginning of Spring, Awakening of Insects, etc.).
- Nine Star Ki — Yōton and inton switch on the Kinoe-Ne (Kōshi) day near the Winter and Summer Solstices.
- 72 Microseasons — Each solar term subdivided into three pentads: first, second, and last.
- Seasonal Markers — Determined based on the solar terms, such as Setsubun (the day before the Beginning of Spring), Doyō (the end of each season), and Higan (around the Vernal and Autumnal Equinoxes).