A solar term (or jieqi,) is any of twenty-four periods in traditional Chinese lunisolar calendars that matches a particular astronomical event or signifies some natural phenomenon.[1] The points are spaced 15° apart along the ecliptic[2] and are used by lunisolar calendars to stay synchronized with the seasons, which is crucial for agrarian societies. The solar terms are also used to calculate intercalary months;[3] which month is repeated depends on the position of the sun at the time.
According to the Book of Documents, the first determined term was Dongzhi (Winter Solstice) by Dan, the Duke of Zhou, while he was trying to locate the geological center of the Western Zhou dynasty, by measuring the length of the sun's shadow on an ancient type of sundial called (Chinese: 土圭).[4] Then four terms of seasons were set, which were soon evolved as eight terms; until 104 BC in the book Taichu Calendar, the entire twenty-four solar terms were officially included in the Chinese calendar.[5]
Because the Sun's speed along the ecliptic varies depending on the Earth-Sun distance, the number of days that it takes the Sun to travel between each pair of solar terms varies slightly throughout the year, but it is always between 15 and 16 days. Each solar term is divided into three, so there are 72 pentads in a year. Each pentad consists of five, rarely six, days, and are mostly named after phenological (biological or botanical) phenomena corresponding to the pentad.
Solar terms originated in China, then spread to Korea, Vietnam, and Japan, countries in the East Asian cultural sphere. Although each term was named based on the seasonal changes of climate in the North China Plain, peoples living in the different climates still use it without changes.[6] This is exhibited by the fact that traditional Chinese characters for most of the solar terms are identical.
On December 1, 2016, the solar terms were listed by UNESCO as an Intangible Cultural Heritage.[7]
The even solar terms (marked with "Z", for) are considered the major terms, while the odd solar terms (marked with "J", for) are deemed minor. The year starts with Lichun (J1) and ends with Dahan (Z12).[8]
| Sun's ecliptic longitude | Chinese name[9] | Korean name[10] | Vietnamese name | Japanese name | Ryukyuan (Okinawan) name | English name (Hong Kong Observatory)[11] | English name (Beijing 2022 Winter Olympics)[12] | Gregorian Date[13] (± 1 day) | Reference for Month Intercalating | Chinese zodiac & Earthly Branch of Month | Corresponding Western astrological sign |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 315° | Chinese: [[Lichun|立春]] | Korean: 立春 (입춘/립춘) | Vietnamese: Lập xuân|italic=no | Japanese: 立春（りっしゅん） | 立春（りっしゅん） | Spring commences | Beginning of Spring | Feb 4 | 1st month initial | Tiger (Chinese: 虎) (Chinese: 寅) | Aquarius |
| 330° | Chinese: [[Yushui (solar term)|雨水]] [14] | Korean: 雨水 (우수) | Vietnamese: Vũ thủy|italic=no | Japanese: 雨水（うすい） | 雨水（うしー） | Rain water | Rain Water | Feb 19 | 1st month midpoint | Pisces | |
| 345° | Chinese: [[Jingzhe|驚蟄]] (Chinese: 惊蛰) [15] | Korean: 驚蟄 (경칩) | Vietnamese: Kinh trập|italic=no | Japanese: 啓蟄（けいちつ） | 驚く（うどぅるく）
虫驚（むしうどぅるく） | Insects waken | Awakening of Insects | Mar 6 | 2nd month initial | Rabbit (Chinese: 兔) (Chinese: 卯) | |
| 0° | Chinese: [[Chunfen|春分]] | Korean: 春分 (춘분) | Vietnamese: Xuân phân|italic=no | Japanese: 春分（しゅんぶん） | 春分（すんぶん） | Vernal equinox | Spring Equinox | Mar 21 | 2nd month midpoint | Aries | |
| 15° | Chinese: [[Qingming (solar term)|清明]] [16] | Korean: 淸明 (청명) | Vietnamese: Thanh minh|italic=no | Japanese: 清明（せいめい） | 清明（しーみー） | Bright and clear | Pure Brightness | Apr 5 | 3rd month initial | Dragon (Chinese: 龍) (Chinese: 辰) | |
| 30° | Chinese: [[Guyu|穀雨]] (Chinese: 谷雨) [17] | Korean: 穀雨 (곡우) | Vietnamese: Cốc vũ|italic=no | Japanese: 穀雨（こくう） | 穀雨（くくー） | Corn rain | Grain Rain | Apr 20 | 3rd month midpoint | Taurus | |
| 45° | Chinese: [[Lixia|立夏]] | Korean: 立夏 (입하/립하) | Vietnamese: Lập hạ|italic=no | Japanese: 立夏（りっか） | 立夏（りっかー） | Summer commences | Beginning of Summer | May 6 | 4th month initial | Snake (Chinese: 蛇) (Chinese: 巳) | |
| 60° | Chinese: [[Xiaoman|小滿]] (Chinese: 小满) | Korean: 小滿 (소만) | Vietnamese: Tiểu mãn|italic=no | Japanese: 小満（しょうまん） | 小満（すーまん） | Corn forms | Grain Buds | May 21 | 4th month midpoint | Gemini | |
| 75° | Chinese: [[Mangzhong|芒種]] (Chinese: 芒种) | Korean: 芒種 (망종) | Vietnamese: Mang chủng|italic=no | Japanese: 芒種（ぼうしゅ） | 芒種（ぼーすー） | Corn on ear | Grain in Ear | Jun 6 | 5th month initial | Horse(Chinese: 馬) (Chinese: 午) | |
| 90° | Chinese: [[Xiazhi|夏至]] | Korean: 夏至 (하지) | Vietnamese: Hạ chí|italic=no | Japanese: 夏至（げし） | 夏至（かーちー） | Summer solstice | Summer Solstice | Jun 21 | 5th month midpoint | Cancer | |
| 105° | Chinese: [[Xiaoshu|小暑]] | Korean: 小暑 (소서) | Vietnamese: Tiểu thử|italic=no | Japanese: 小暑（しょうしょ） | 小暑（くーあちさ） | Moderate heat | Minor Heat | Jul 7 | 6th month initial | Goat (Chinese: 羊) (Chinese: 未) | |
| 120° | Chinese: [[Dashu (solar term)|大暑]] | Korean: 大暑 (대서) | Vietnamese: Đại thử|italic=no | Japanese: 大暑（たいしょ） | 大暑（うーあちさ） | Great heat | Major heat | Jul 23 | 6th month midpoint | Leo | |
| 135° | Chinese: [[Liqiu|立秋]] | Korean: 立秋 (입추/립추) | Vietnamese: Lập thu|italic=no | Japanese: 立秋（りっしゅう） | 立秋（りっすー） | Autumn commences | Beginning of Autumn | Aug 8 | 7th month initial | Monkey (Chinese: 猴) (Chinese: 申) | |
| 150° | Chinese: [[Chushu|處暑]] (Chinese: 处暑) | Korean: 處暑 (처서) | Vietnamese: Xử thử|italic=no | Japanese: 処暑（しょしょ） | 処暑（とぅくるあちさ） | End of heat | End of Heat | Aug 23 | 7th month midpoint | Virgo | |
| 165° | Chinese: [[Bailu (solar term)|白露]] | Korean: 白露 (백로) | Vietnamese: Bạch lộ|italic=no | Japanese: 白露（はくろ） | 白露（ふぁくるー） | White dew | White Dew | Sep 8 | 8th month initial | Rooster (Chinese: 雞) (Chinese: 酉) | |
| 180° | Chinese: [[Qiufen|秋分]] | Korean: 秋分 (추분) | Vietnamese: Thu phân|italic=no | Japanese: 秋分（しゅうぶん） | 秋分（すーぶん） | Autumnal equinox | Autumn Equinox | Sep 23 | 8th month midpoint | Libra | |
| 195° | Chinese: [[Hanlu|寒露]] | Korean: 寒露 (한로) | Vietnamese: Hàn lộ|italic=no | Japanese: 寒露（かんろ） | 寒露（かんるー） | Cold dew | Cold Dew | Oct 8 | 9th month initial | Dog (Chinese: 狗) (Chinese: 戌) | |
| 210° | Chinese: [[Shuangjiang (solar term)|霜降]] | Korean: 霜降 (상강) | Vietnamese: Sương giáng|italic=no | Japanese: 霜降（そうこう） | 霜降（しむくだり） | Frost | Frost's Descent | Oct 23 | 9th month midpoint | Scorpio | |
| 225° | Chinese: [[Lidong|立冬]] | Korean: 立冬 (입동/립동) | Vietnamese: Lập đông|italic=no | Japanese: 立冬（りっとう） | 立冬（りっとぅー） | Winter commences | Beginning of Winter | Nov 7 | 10th month initial | Pig (Chinese: 豬) (Chinese: 亥) | |
| 240° | Chinese: [[Xiaoxue|小雪]] | Korean: 小雪 (소설) | Vietnamese: Tiểu tuyết|italic=no | Japanese: 小雪（しょうせつ） | 小雪（くーゆち） | Light snow | Minor Snow | Nov 22 | 10th month midpoint | Sagittarius | |
| 255° | Chinese: [[Daxue (solar term)|大雪]] | Korean: 大雪 (대설) | Vietnamese: Đại tuyết|italic=no | Japanese: 大雪（たいせつ） | 大雪（うーゆち） | Heavy snow | Major Snow | Dec 7 | 11th month initial | Rat (Chinese: 鼠) (Chinese: 子) | |
| 270° | Chinese: [[Dongzhi (solar term)|冬至]] | Korean: 冬至 (동지) | Vietnamese: Đông chí|italic=no | Japanese: 冬至（とうじ） | 冬至（とぅんじー） | Winter solstice | Winter Solstice | Dec 22 | 11th month midpoint | Capricorn | |
| 285° | Chinese: [[Xiaohan|小寒]] | Korean: 小寒 (소한) | Vietnamese: Tiểu hàn|italic=no | Japanese: 小寒（しょうかん） | 小寒（すーかん） | Moderate cold | Minor Cold | Jan 6 | 12th month initial | Ox (Chinese: 牛) (Chinese: 丑) | |
| 300° | Chinese: [[Dahan (solar term)|大寒]] | Korean: 大寒 (대한) | Vietnamese: Đại hàn|italic=no | Japanese: 大寒（だいかん） | 大寒（でーかん） | Severe cold | Major Cold | Jan 20 | 12th month midpoint | Aquarius |
The "Song of Solar Terms" is used to ease the memorization of jieqi:
The first four lines provides a concise version of the names of the 24 jieqi. The last four lines provide some rules of thumb about the Gregorian dates of jieqi, namely:
The modern definition using ecliptic longitudes, introduced by the Shixian calendar, is known as Chinese: 定气法 . Under this method, the determination of solar terms is similar to the astronomical determination of the special cases of equinox and solstice dates, with different ecliptic longitudes to solve for. One can start with an approximation and then perform a correction using the anomalies and mean motion of the sun.[18] The JPL Horizons On-Line Ephemeris System may be used to query for exact times of solar terms.
The older method is known as Chinese: 平气法 and simply divides the tropical year into 24 equal parts.
In Japan, the term (Japanese: 節分) originally referred to the eves of (Japanese: 立春, 315°, the beginning of Spring), (Japanese: 立夏, 45°, the beginning of Summer), (Japanese: 立秋, 135°, the beginning of Autumn), and (Japanese: 立冬, 225°, the beginning of Winter), but currently mostly refers to the day before . The name of each solar term also refers to the period of time between that day and the next solar term, or 1/24th of a year.