Solar terms

Meaning

The twenty-four solar terms divide the Sun’s year into twenty-four steps of 15° along its path, each about fifteen days long and each named for what the season does then: the Start of Spring, Rain Water, the Awakening of Insects, the Spring Equinox, and so round to Minor Cold and Major Cold. They are the solar side of the Chinese calendar, the one that farmers, almanacs and the arts of time have always counted by, while the festivals follow the Moon.

They come in two kinds, one after the other. The twelve jie, the even terms, open the solar months: Lichun opens the Tiger month, Jingzhe the Rabbit month, and so on. The twelve principal terms fall in the middle of those months, and they include the solstices and the equinoxes, which are mid-season in the Chinese year: spring begins at Lichun, about 4 February, and the Spring Equinox is its middle.

Their names are a small almanac of China’s north: rain, insects waking, grain swelling, dew turning white, frost, snow, cold. Each term is also split into three pentads of five days with their own signs, the east wind melting the ice, the swallows returning, the wild geese leaving: seventy-two images of a year watched closely.

In practice

• In BaZi the twelve jie set the month pillar: a birth belongs to the solar month that the last of them opened, and Lichun changes the year pillar too. The luck pillars start at an age counted from the days between the birth and the next or the last jie.
• In Chinese astrology the year animal changes at Lichun and the month animal at each jie, and the Cycles lens reads each month of the year ahead from its opening term.
• In Feng Shui the year’s stars change at Lichun and the month’s at each jie.
• The Calendar marks every term on its day, with the Sun’s degree and the month it opens or the middle it marks, and can draw them as a ring round the birth chart.

Across traditions

• In Western astrology the principal terms are the Sun’s entries into the signs, the Spring Equinox at 0° Aries, the Summer Solstice at 0° Cancer, and the jie its passages through their middles, 15° Aquarius for Lichun: the same circle, cut at the same places, read in two ways.
• In Japan, Korea and Vietnam the same twenty-four terms are kept as nijūshi sekki, jeolgi and tiết khí; Japan adapted the seventy-two pentads to its own climate in 1685.
• The Celtic year places its festivals close to the four Chinese starts of the seasons: Imbolc near Lichun, Beltane near Lixia, Lughnasadh near Liqiu and Samhain near Lidong.

Origin and history

The solstices and equinoxes were known in China long before the terms had names, measured with the gnomon’s noon shadow. The “Twelve Months” of the Lüshi Chunqiu, around 239 BC, already names the starts of the seasons and the mid-season days with their signs in nature, and the Huainanzi, in 139 BC, gives the complete list of twenty-four, the one still used. The pentads of five days come down in the Yi Zhou Shu and the “Monthly Ordinances” of the Book of Rites.

For most of their history the terms were spaced in equal times, a twenty-fourth of the year apart; since the Shixian calendar of 1645 they follow the Sun’s true motion, an equal 15° each, so that they are a little closer together in the northern winter, when the Earth moves faster. In 2016 UNESCO inscribed the twenty-four solar terms on its list of the intangible heritage of humanity.

How Ulune calculates it

Ulune finds each term as the instant the Sun’s apparent longitude reaches its degree, 315° for Lichun and 15° more for each after, from a table of the solar terms computed with the Swiss Ephemeris for 1800 to 2200, which the device reads. A term is an instant: the year, the month and their pillars change exactly then, whatever the clock or the time zone, and a birth within five minutes of one is marked uncertain. The Calendar shows each on the reader’s own date.

Correspondences