Solar term



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. 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 (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 tugui (土圭). 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.

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 Pentad (calendar), 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. 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 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).

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

春雨驚春清穀天
 * Traditional Chinese:

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

春雨惊春清谷天
 * Simplified Chinese:

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

chūn yǔ jīng chūn qīng gǔ tiān,
 * Pinyin

xià mǎn máng xià shǔ xiāng lián,

qiū chù lù qiū hán shuāng jiàng,

dōng xuě xuě dōng xiǎo dà hán.

měi yuè liǎng jié bù biàn gēng,

zùi duō xiāng chā yī liǎng tiān

shàng bàn nián lái liù, niàn yī

xià bàn nián shì bā, niàn sān

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 定气法 (dìng qì fǎ, steady term method). 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 (píng qì fǎ, equal term method) and simply divides the tropical year into 24 equal parts.

Regional note
In Japan, the term Setsubun (節分) originally referred to the eves of Risshun (立春, 315°, the beginning of Spring), Rikka (立夏, 45°, the beginning of Summer), Risshū (立秋, 135°, the beginning of Autumn), and Rittō (立冬, 225°, the beginning of Winter), but currently mostly refers to the day before Risshun. 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.