Solar term


A solar term 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 ecliptic 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; 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 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  . 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 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. 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 are considered the major terms, while the solar terms used to mark the start of the month are deemed minor. The year starts with Lichun and ends with Dahan.

Multilingual list

Chinese mnemonic song

The "Song of Solar Terms" is used to ease the memorization of jieqi:
;Traditional Chinese:
春雨驚春清穀天

夏滿芒夏暑相連

秋處露秋寒霜降

冬雪雪冬小大寒

每月兩節不變更

最多相差一兩天

上半年來六、廿一

下半年是八、廿三

;Simplified Chinese:
春雨惊春清谷天

夏满芒夏暑相连

秋处露秋寒霜降

冬雪雪冬小大寒

每月两节不变更

最多相差一两天

上半年来六、廿一

下半年是八、廿三

;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 month;
  • In the second half of the year, jieqi happens around the 8th and 23rd day of each month.

    Determination

The modern definition using ecliptic longitudes, introduced by the Shixian calendar, is known as 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. The JPL Horizons On-Line Ephemeris System may be used to query for exact times of solar terms.
The older method is known as 平气法 and simply divides the tropical year into 24 equal parts.