24 Solar Terms

Link between the 24 Solar Terms and Astronomy

The “24 solar terms” form an integral part of the Chinese calendar. It is a time knowledge system progressively built upon the long-term observation of periodic movement of the Sun, seasonal march and phenology, and evolves in time to meet the needs of social development. Originated from the Huanghe (Yellow River) basin in ancient times, the use of 24 Solar Terms has since been widely adopted by other places in China as well as its neighbouring countries.

Earth orbits around the Sun along an elliptical path. Its axis of rotation is not perpendicular to the plane of the orbit but is tilted at about 23 degrees. An observer on Earth, looking at the apparent movement of the Sun against remote stars, can define a path called the “ecliptic”. The solar terms, evenly spread conceptually along the ecliptic, mark the moments when the Sun reaches these 24 pre-defined positions and form a cycle that corresponds exactly to one year and more precisely one “sidereal year”.

The “24 solar terms” is a collective name of the system that comprises 12 “major solar terms” and 12 “minor solar terms”. Their names are listed in the table attached. “Vernal equinox” is the first “major solar term”, followed by the rest of the minor and major solar terms alternating with each other, with the cycle ending at “insects waken”.

History of Chinese Calendar

A complete set of “24 solar terms” first appeared in an ancient book “Huainan Zi” in early Han Dynasty in which a chapter gave a concise description about astronomy and phenology. These 24 solar terms were soon absorbed into “Tai Chu Li”, the first well documented calendar in the history of China. At that time, the “24 solar terms” began at “winter solstice”, and evenly marked on the “ecliptic” with a constant time interval of about 15 days.

Towards the end of the Ming Dynasty, traditional astronomy, mathematics and calendar were ameliorated with the use of western science and technology. In the early Qing Dynasty, when the authoritative organization for compiling the calendar was under the charge of the Jesuits, a major reform of the calendar was conducted. The official calendar “Shi Xian Li” was published in 1645 and the “vernal equinox” was adopted as the first solar term. Positions of all the “24 solar terms” were fixed at pre-defined longitudes in an “ecliptic” coordinate with a constant angular separation of 15 degrees, corresponding to a period of 14.71 to 15.74 days^[1]. Such a practice is still in use today.

The Chinese Agriculture Calendar is a combination of the lunar and solar calendar systems. There are 365.2422 days in one “sidereal year” while 12 lunar months add up to 354 days. To harmonize the lunar and solar calendars, a leap lunar month needs to be added every 2 to 3 years. Since the early Han Dynasty, a lunar month without major solar term has been adopted as a leap month. Such a practice remains generally valid today in most situations.

Related Applications and Customs

While calendar systems of other places only consist of winter solstice (longest nighttime), summer solstice (longest daytime[2]), vernal equinox and autumnal equinox (equal day and night), the Chinese calendar featuring “24 solar terms” provide detailed guidelines and timetable for agricultural activities, e.g. cultivating rice on “corn on ear”, barley on “cold dew” and bean on “frost”. The themes of “24 solar terms” are also popularly quoted in Chinese poetry, such as “Bright and Clear (Ching Ming)” by Du Mu, “Around Winter Solstice” by Du Fu, and “Severe Cold” by Lu You.

Nowadays, urban dwellers still retain some customs linked to the “24 solar terms”, such as family gathering on “winter solstice is more important than the New Year”, and the culinary practices of having dumpling on “winter solstice” and spring roll on “spring commences”. With more extensive cultural exchanges, the “24 solar terms” have been made known to people in other parts of the world and deservedly added to the “UNESCO World Intangible Cultural Heritage” list in 2016.

Note
[1]: Along the elliptical Earth’s orbit, different segments with the same angular distance correspond to different time spans.
[2]: In the northern hemisphere, daytime is longest at summer solstice and shortest at winter solstice.

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