A solar term (or ''jiéqì'', zh, t=節氣, s=节气) is any of twenty-four periods in traditional Chinese

lunisolar calendar A lunisolar calendar is a calendar in many cultures, that combines monthly lunar cycles with the solar year. As with all calendars which divide the year into months, there is an additional requirement that the year have a whole number of mont ...

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 Earth's orbit, Earth around the Sun. It was a central concept in a number of ancient sciences, providing the framework for key measurements in astronomy, astrology and calendar-making. Fr ...

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 The Western Zhou ( zh, c=西周, p=Xīzhōu; 771 BC) was a period of Chinese history corresponding roughly to 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 in 77 ...

dynasty, by measuring the length of the sun's shadow on an ancient type of

sundial A sundial is a horology, horological device that tells the time of day (referred to as civil time in modern usage) when direct sunlight shines by the position of the Sun, apparent position of the Sun in the sky. In the narrowest sense of the ...

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 ( zh, c=候, p=hòu, labels=no), 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 China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...

, then spread to

Korea Korea is a peninsular region in East Asia consisting of the Korean Peninsula, Jeju Island, and smaller islands. Since the end of World War II in 1945, it has been politically Division of Korea, divided at or near the 38th parallel north, 3 ...

Vietnam Vietnam, officially the Socialist Republic of Vietnam (SRV), is a country at the eastern edge of mainland Southeast Asia, with an area of about and a population of over 100 million, making it the world's List of countries and depende ...

, and

Japan Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...

, countries in the East Asian cultural sphere. Although each term was named based on the seasonal changes of climate in the

North China Plain The North China 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 bordered to the north by th ...

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

traditional Chinese characters Traditional Chinese characters are a standard set of Chinese character forms used to written Chinese, write Chinese languages. In Taiwan, the set of traditional characters is regulated by the Ministry of Education (Taiwan), Ministry of Educat ...

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 (UNESCO ) is a List of specialized agencies of the United Nations, specialized agency of the United Nations (UN) with the aim of promoting world peace and International secur ...

as an Intangible Cultural Heritage.

Numbering

The solar terms used to mark the midpoint of the month (marked with "Z", for zh, t=中氣, p=zhōngqì) are considered the major terms, while the solar terms used to mark the start of the month (marked with "J", for zh, t=節氣, p=jiéqì) are deemed minor. The year starts with Lichun (J1) and ends with Dahan (Z12).

Multilingual list

Chinese mnemonic song

The "Song of Solar Terms" ( zh, t=節氣歌, s=节气歌, p=jiéqìgē) 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 A solar equinox is a moment in time when the Sun appears directly above the equator, rather than to its north or south. On the day of the equinox, the Sun appears to rise directly east and set directly west. This occurs twice each year, arou ...

and

solstice A solstice is the time when the Sun reaches its most northerly or southerly sun path, excursion relative to the celestial equator on the celestial sphere. Two solstices occur annually, around 20–22 June and 20–22 December. In many countries ...

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 In orbital mechanics, mean motion (represented by ''n'') is the angular speed required for a body to complete one orbit, assuming constant speed in a circular orbit which completes in the same time as the variable speed, elliptical orbit of the a ...

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 (, ) and simply divides the

tropical year A tropical year or solar year (or tropical period) is the time that the Sun takes to return to the same position in the sky – as viewed from the Earth or another celestial body of the Solar System – thus completing a full cycle of astronom ...

into 24 equal parts.

Regional note

In Japan, the term () originally referred to the eves of (, 315°, the beginning of Spring), (, 45°, the beginning of Summer), (, 135°, the beginning of Autumn), and (, 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.

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