A solar term (or jiéqì, ) is any of twenty-four moments in traditional Chinese lunisolar calendars that matches a particular astronomical event and signifies some natural phenomenon. The points are spaced 15° apart along the eclipticUntil 1644 (Chinese Empire and its tributary states) a period of time of the solar year itself had been equally divided instead of the spatial zodiac. and are used by Chinese lunisolar calendar to stay synchronized with the seasons, which is crucial for agrarian societies. The solar terms are also used to calculate intercalary months;When a lunar month's end does not reach a midpoint of the solar terms, it is regarded as the last month's intercalary one instead of the true "next" month. It is called lit. "midpoint intercalating system". which month is repeated depends on the position of the sun at the time.

According to the Book of Documents (書經; Shūjīng), the first determined term was Dongzhi (Winter Solstice) by Dan, the Duke of Zhou (周公旦; Zhōu Gōng Dàn), 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  (). Then four terms of seasons were set, which were soon evolved as eight terms; not until the Taichu Calendar of 104 BC were all twenty-four solar terms officially included in the Chinese calendar.

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, consisting of five, rarely six, days. Most of them are 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.なぜずれる？　二十四節気と季節感 (Why off-point? -solar terms and our real feeling of the seasons) Maritime Japan, for example, comparing the climate of Taiyuan with that of Tokyo and Kyoto: on Japanese islands differences in seasonal gap lengths are the main problem. In subtropical or tropical regions including southern China climate difference is more serious. 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.

Numbering The solar terms used to mark the midpoint of the month (marked with "Z", for ) are considered the major terms, while the solar terms used to mark the start of the month (marked with "J", for ) are deemed minor. The year starts with Lichun (J1) and ends with Dahan (Z12).

Multilingual list Season Chinese zodiac & Earthly Branch of Month Sun's eclipticlongitude ChinesenameSimplified Chinese characters are shown in parentheses if they differ from the Traditional Chinese characters. Korean nameHangul are shown in parentheses. For Hangul and romanisation, where the pronunciation differs between South Korea and North Korea, the South Korean pronunciation is given first before the slash, followed by the North Korean pronunciation. Vietnamesename Japanesename Ryukyuan(Okinawan) name Englishname (Hong Kong Observatory) Englishname (CCTV) GregorianDateDate can vary within a ±1 day range.(± 1 day) Reference forMonth Intercalating Corresponding Western astrological signSpring Tiger () () 315° Spring commences Beginning of Spring Feb 4 1st month initial Aquarius 330° the Yushui and Jingzhe have been exchanged by Liu Xin in Han dynasty. Rain water Rain Water Feb 19 1st month midpoint Pisces Rabbit () () 345° ()the Jingzhe and Yushui have been exchanged by Liu Xin in Han dynasty. ; ; Insects waken Awakening of Insects Mar 6 2nd month initial 0° Vernal equinox Spring Equinox Mar 21 2nd month midpoint Aries Dragon () () 15° the Qingming and Guyu have been exchanged by Liu Xin in Han dynasty. Bright and clear Pure Brightness Apr 5 3rd month initial 30° ()the Guyu and Qingming have been exchanged by Liu Xin in Han dynasty. Corn rain Grain Rain Apr 20 3rd month midpoint TaurusSummer Snake () () 45° Summer commences Beginning of Summer May 6 4th month initial 60° () Corn forms Grain Buds May 21 4th month midpoint Gemini Horse () () 75° () Corn on ear Grain in Ear Jun 6 5th month initial 90° Summer solstice Summer Solstice Jun 21 5th month midpoint Cancer Goat () () 105° Moderate heat Minor Heat Jul 7 6th month initial 120° Great heat Major Heat Jul 23 6th month midpoint LeoAutumn Monkey () () 135° Autumn commences Beginning of Autumn Aug 8 7th month initial 150° () End of heat End of Heat Aug 23 7th month midpoint Virgo Rooster () () 165° White dew White Dew Sep 8 8th month initial 180° Autumnal equinox Autumn Equinox Sep 23 8th month midpoint Libra Dog () () 195° Cold dew Cold Dew Oct 8 9th month initial 210° Frost Frost's Descent Oct 23 9th month midpoint ScorpioWinter Pig () () 225° Winter commences Beginning of Winter Nov 7 10th month initial 240° Light snow Minor Snow Nov 22 10th month midpoint Sagittarius Rat () () 255° Heavy snow Major Snow Dec 7 11th month initial 270° Winter solstice Winter Solstice Dec 22 11th month midpoint Capricorn Ox () () 285° Moderate cold Minor Cold Jan 6 12th month initial 300° Severe cold Major Cold Jan 20 12th month midpoint Aquarius

Chinese mnemonic song The "Song of Solar Terms" () is used to ease the memorization of jieqi:

Traditional Chinese

Simplified Chinese

Pinyin

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: Two jieqi per month; Gregorian dates are off by one or two days at most; In the first half of the year, jieqi happens around the 6th and 21st day of each (Gregorian) month; In the second half of the year, jieqi happens around the 8th and 23rd day of each (Gregorian) month.

Determination

The modern definition using ecliptic longitudes, introduced by the Shixian calendar, is known as . 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. The JPL Horizons On-Line Ephemeris System may be used to query for exact times of solar terms.

The older method, called the , simply divides the tropical year into 24 equal parts.

See also Chinese calendar Korean calendar Japanese calendar Vietnamese calendar Wheel of the Year

References

External links 24 Solar Terms calculator 24 Solar Terms – Hong Kong Observatory Twenty Four Solar Terms and Their Customs in China Chinese calendar with upcoming solar terms in current year 24 Sekki – NAOJ > ECO > Glossary > Twenty-four Solar Terms and meaning Date and time of Solar Terms The 24 Solar Terms