(Minghui.org) To achieve healthy crops and abundant harvests, farmers understand that the timing of sowing and harvesting is just as important as soil, fertilizer, sunlight, and water. In ancient China, by observing seasonal changes and the annual orbit of the Sun, people identified and summarized 24 solar terms (divisions of time) in each lunar year. Many believe that this system is not only a way of dividing time, but also the essence of the agricultural wisdom of the Chinese civilization.

The solar terms originated in the Yellow River Basin during the Warring States Period (475-221 B.C.). The 24 solar terms became official when documented in the Taichu Calendar, published during the Western Han Dynasty. The calendar was completed in the first year of the reign of Emperor Wu of the Han Dynasty (104 B.C.), and it is the first complete calendar preserved in China. From this calendar, we see how closely the ancient Chinese related to nature and how it affected them.

Astronomically, the 24 solar terms represent 24 specific positions of Earth in its yearly orbit around the Sun. Each period corresponds to a 15-degree division of the Sun’s ecliptic. It takes about 15 days for the Sun to travel from one solar term to the next, and the order of the solar terms reflects the changes in seasons, climate, and phenology. The various solar terms play significant roles in agriculture and have different impacts on human health.

The ecliptic plane refers to the plane of the Earth’s orbit around the Sun. There is a 23.4° angle between the ecliptic plane and Earth’s equatorial plane. The vernal equinox occurs when the ecliptic plane intersects the equatorial plane and the Sun shines directly on the equator moving from south to north (Figure 1). This is when the apparent ecliptic longitude is defined as 0°. Summer Solstice happens when the ecliptic intersects (and the Sun shines directly on) the Tropic of Cancer, a circular plane of latitude located at 23.4° north of the Equator, and the apparent ecliptic longitude is 90°. As the Earth continues to orbit around the Sun, and the Sun shines directly on the equator, again moving from north to south, it is the Autumn Equinox and the ecliptic longitude is 180°. When the Sun shines directly on the Tropic of Capricorn, it is the Winter Solstice with an ecliptic longitude of as 270°.

Figure 1: Seasonal configuration of the Earth and Sun: Earth’s orbit around the Sun, with the positions of solstices and equinoxes. (Source: Encyclopedia Britannica)

The solar terms represent four climate phenomena.

1. Seasonal changes (in the Northern Hemisphere): Beginning of Spring (Li-chun), Middle of Spring (Chun-fen, Vernal Equinox), Beginning of Summer (Li-xia), Middle of Summer (Xia-zhi, Summer Solstice), Beginning of Autumn (Li-qiu), Middle of Autumn (Qiu-fen, Autumn Equinox), Beginning of Winter (Li-dong), and Middle of Winter (Dong-zhi, Winter Solstice)

2. Temperature changes: Lesser Heat (Xiao-shu), Greater Heat (Da-shu), End of Heat (Chu-shu), Lesser Cold (Xiao-han), and Greater Cold (Da-han).

3. Forms of precipitation: Rainwater (Yu-shui), Grain Rain (Gu-yu), Winter Dew (Bai-lu), Cold Dew (Han-lu), First Frost (Shuang-jiang), Light Snow (Xiao-xue), and Heavy Snow (Da-xue).

4. Phenology and agriculture: Waking of Insects (Jing-zhi), Fresh Green (Qing-ming), Lesser Fullness of Grain (Xiao-man), Grain in Beard (Mang-zhong, and busy harvesting and seeding).

The first solar term is Beginning of Spring (Li-chun), which takes place on February 4 or 5. The date symbolizes the end of winter and the official start of spring. It is when people start farming activities for the year, as the temperature continues to rise from that day on, and the days become longer.

People follow the solar terms as they “plow in spring, weed in summer, harvest in autumn, store in winter.” They make the most of the resources available to them, and pray for a good harvest.

In the first half of the year (spring and summer), the two solar terms of each month mostly start on the sixth and twenty-first days of the Gregorian calendar. In the second half of the year (autumn and winter), the two solar terms of each month mostly occur on the eighth and twenty-third day of the Gregorian calendar. The specific dates of the solar terms fluctuate each year due to leap years or slight variations in the Earth’s revolution; they occur around these dates, with a maximum difference of one or two days.

The 24 solar terms were included in China’s “State-level Intangible Cultural Heritage List” in 2006, and in the “UNESCO Representative List of the Intangible Cultural Heritage of Humanity” in 2016. Their definition of Intangible Cultural Heritage is the non-physical elements of culture, knowledge, and skills that are passed down through the generations.

I have a different perspective on the terminology. Although the 24 solar terms do not exist in a physical way like a piece of furniture, we rely on them to grow crops, which are tangible and keep us alive. We would not have a bountiful harvest if we planted rice in November or wheat in December.

When something can’t be seen by the naked eye, it doesn’t mean that it’s intangible. For example, our eyes can’t see electromagnetic waves, a magnetic field, or DNA, but they do exist materially. We can observe and interact with these materials if we have the right tools. Similarly, we can interact with the 24 solar terms through farming tools and machinery, and thus put food on the table.

The term “heritage” refers to something passed down by our ancestors rather than a legal inheritance. However, the 24 solar terms do not belong to our ancestors; they are celestial movements that objectively exist. Our ancestors observed and documented celestial events to live in accordance with the celestial timing. The movements of celestial objects in the solar system are created for all of mankind and each generation, not for any specific one. Therefore, they should not be regarded as a heritage.

The solar terms are bestowed upon us by the divine. It may be more fitting to regard them as part of divine or traditional culture. After all, the culture from ancient China is not superficial, but divine, and it touches upon the intrinsic qualities of nature.