A solar term is any of twenty-four periods in traditional Chinese

lunisolar calendar A lunisolar calendar is a calendar in many cultures, combining lunar calendars and solar calendars. The date of Lunisolar calendars therefore indicates both the Moon phase and the time of the solar year, that is the position of the Sun in the ...

s that matches a particular astronomical event or signifies some natural phenomenon. The points are spaced 15° apart along the

ecliptic The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic agai ...

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 ''Book of Documents'' (''Shūjīng'', earlier ''Shu King'') or ''Classic of History'', also known as the ''Shangshu'' (“Venerated Documents”), is one of the Five Classics of ancient Chinese literature. It is a collection of rhetoric ...

'', 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 The Western Zhou ( zh, c=, p=Xīzhōu; c. 1045 BC – 771 BC) was a royal dynasty of China and the first half of the Zhou dynasty. It began when King Wu of Zhou overthrew the Shang dynasty at the Battle of Muye and ended when the Quanrong no ...

dynasty, by measuring the length of the sun's shadow on an ancient timekeeper instrument named Tu Gui (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. Each solar term is divided into three (候 ''hòu''), 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 Korea ( ko, 한국, or , ) is a peninsular region in East Asia. Since 1945, it has been divided at or near the 38th parallel, with North Korea (Democratic People's Republic of Korea) comprising its northern half and South Korea (Republi ...

Vietnam Vietnam or Viet Nam ( vi, Việt Nam, ), officially the Socialist Republic of Vietnam,., group="n" is a country in Southeast Asia, at the eastern edge of mainland Southeast Asia, with an area of and population of 96 million, making it ...

, and Japan, countries in the

East Asian cultural sphere The East Asian cultural sphere, also known as the Sinosphere, the Sinic world, the Sinitic world, the Chinese cultural sphere, the Chinese character sphere encompasses multiple countries in East Asia and Southeast Asia that were historical ...

. Although each term was named based on the seasonal changes of climate in the

North China Plain The North China Plain or Huang-Huai-Hai Plain () is a large-scale downfaulted rift basin formed in the late Paleogene and Neogene and then modified by the deposits of the Yellow River. It is the largest alluvial plain of China. The plain is border ...

, peoples living in the different climates still use it without changes. This is exhibited by the fact that

traditional Chinese A tradition is a belief or behavior (folk custom) passed down within a group or society with symbolic meaning or special significance with origins in the past. A component of cultural expressions and folklore, common examples include holidays ...

characters for most of the solar terms are identical. On December 1, 2016, the solar terms were listed by

UNESCO The United Nations Educational, Scientific and Cultural Organization is a specialized agency of the United Nations (UN) aimed at promoting world peace and security through international cooperation in education, arts, sciences and culture. I ...

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 Traditional Chinese calendar divides a year into 24 solar terms. ''Lìchūn'', ''Risshun'', ''Ipchun'', or ''Lập xuân'' is the 1st solar term. It begins when the Sun reaches the celestial longitude of 315° and ends when it reaches the lon ...

(J1) and ends with Dahan (Z12).

Multilingual list

Chinese mnemonic song

The "Song of Solar Terms" () is used to ease the memorization of ''jiéqì'': ;Traditional Chinese:

;Simplified Chinese:

;Pinyin chūn yǔ jīng chūn qīng gǔ tiān,
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. Under this method, the determination of solar terms is similar to the astronomical determination of the special cases of

equinox A solar equinox is a moment in time when the Sun crosses the Earth's equator, which is to say, appears zenith, directly above the equator, rather than north or south of the equator. On the day of the equinox, the Sun appears to rise "due east" ...

and

solstice A solstice is an event that occurs when the Sun appears to reach its most northerly or southerly excursion relative to the celestial equator on the celestial sphere. Two solstices occur annually, around June 21 and December 21. In many count ...

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 JPL Horizons On-Line Ephemeris System provides access to key Solar System data and flexible production of highly accurate ephemerides for Solar System objects. Osculating elements at a given epoch (such as produced by the JPL Small-Body Dat ...

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.

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 The traditional Chinese calendar divides a year into 24 solar terms. ''Lìqiū'', ''Risshū'', ''Ipchu'', or ''Lập thu'' () is the 13th solar term. It begins when the Sun reaches the celestial longitude of 135° and ends when it reaches t ...

(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.

References

{{reflist

External links

24 Solar Terms calculator

Twenty Four Solar Terms and Their Customs in China

Chinese calendar with upcoming solar terms in current year

Date and time of Solar Terms

Intangible Cultural Heritage of Humanity