The 24 solar terms: China's calendar of the sun

The twenty-four solar terms (二十四节气, èrshísì jiéqì) divide the year by the sun, not by the moon. According to the China Meteorological Administration, you start from 0° of the sun's longitude on the ecliptic, the point of the spring equinox (春分), and each time the sun moves on by 15°, a new term begins: 360° split into twenty-four parts, or two terms a month. The first term of each month is called a 节气 in the narrow sense, the second a 中气, a "middle breath".

Because the sun's apparent speed changes over the year, the gap between two terms is not always the same: it lasts between 15 and 16 days.

The terms are part of the traditional Chinese calendar, often called the "lunar calendar"; yet Chinese Wikipedia points out that they belong to the solar calendar. So they fall on almost the same Gregorian dates every year: Lichun (立春, "start of spring"), the first term on the list, comes between 3 and 5 February, while Lunar New Year moves around between 21 January and 20 February.

This double system is what makes the Chinese calendar lunisolar: the months follow the moon, the terms follow the sun. According to Wikipedia, the terms keep the calendar in step with the seasons, which is crucial for a farming society.

The solar terms come from the Yellow River (黄河) basin: according to UNESCO, their criteria were worked out by observing the seasons, the sky and other natural phenomena in that region.

It all starts with a shadow. As early as the Spring and Autumn and Warring States periods, says the China Meteorological Administration, people measured the noon shadow of a gnomon, the tugui (土圭). The shortest shadow marks the summer solstice, the longest the winter solstice, and the equinoxes fall halfway: this gives the "two solstices and two equinoxes" (二至二分).

The "Annals of Lü Buwei" (吕氏春秋), compiled by the scholars gathered by the Qin chancellor Lü Buwei (吕不韦), then mention eight terms: the two solstices, the two equinoxes and the "four starts" (四立) of the seasons.

The full list of twenty-four first appears in the Huainanzi (淮南子), a work compiled in the 2nd century BCE under the patronage of Liu An (刘安), king of Huainan, in its chapter on the heavens (天文训). The names and their order are already today's. In 104 BCE, under Emperor Wu of Han (汉武帝), the Taichu calendar (太初历), drawn up by Deng Ping (邓平) and others, officially wrote all twenty-four terms into the calendar.

The table follows the usual Chinese order, which begins at Lichun, the "start of spring". Four families stand out: the four starts of the seasons (立春, 立夏, 立秋, 立冬), the two equinoxes (春分, 秋分) and the two solstices (夏至, 冬至), then names about rain, dew, frost, snow, heat and cold, or about the grain in the fields.

The sun's longitude grows by 15° from one term to the next, starting at 0° at the spring equinox. The dates given, taken from the tables in the English and Chinese Wikipedia, can shift by a day from year to year (Lichun falls between 3 and 5 February). The rhythm is the thing to remember: one term in the first half of each month, another in the second half.

The 15° definition is not the oldest one. Before 1645, the terms were worked out by the "equal breaths" method (平气): the tropical year was simply divided into twenty-four equal lengths of time.

But the sun does not move along the ecliptic at a constant speed. The "fixed breaths" method (定气) therefore puts each term at an exact position of the sun, every 15° of longitude from the spring equinox: this is today's definition, and it is why the gap between two terms now varies from 15 to 16 days. It was officially adopted with the Shixian calendar (时宪历), in use from 1645 to 1913, although the idea had been put forward as early as the Sui (隋朝) by Liu Zhuo (刘焯).

This calendar was the result of a project of the late Ming (明朝): the scholar Xu Guangqi (徐光启) had prepared it from 1624 to 1644 with the help of the Jesuits Johann Schreck and Johann Adam Schall von Bell. After the death of the Chongzhen Emperor (崇祯), it was the Shunzhi Emperor (顺治), at the start of the Qing (清朝), who put it into use under the name Shixian. According to Wikipedia, it was the first systematic introduction of European mathematics and astronomy into China.

In 2016, UNESCO inscribed the solar terms on the Representative List of the Intangible Cultural Heritage of Humanity as "The Twenty-Four Solar Terms, knowledge in China of time and practices developed through observation of the sun's annual motion". The listing notes that they have been passed down from generation to generation and are traditionally used as a timeframe for production and daily routines; they remain particularly important to farmers, who use them to guide their work.

They shape other calendars too. Fortune-tellers who cast the "eight characters" (八字) of a birth count in solar months that begin with the terms: their first month, 寅月, opens at Lichun, and that is where they change zodiac animal.

Finally, the terms are the starting point of some countdowns. In 2024, the Xinhua news agency announced the start of the "fu days" (伏), the dog days of summer, quoting the saying 夏至三庚数头伏: literally, "after the summer solstice, at the third geng, count the first fu". 庚 (gēng) is one of the ten heavenly stems, which are also used to count days. That year the first fu began on 15 July, four days later than the year before.