The Agricultural Calendar and the 24 Solar Terms

The agricultural calendar is the traditional Chinese way of calculating the passage of the days, also known as Xia or lunar calendar. The name “Xia” comes from the fact this calendar has been used since the Xia Dynasty (2070 BC-1600 BC); at the same time it is a “lunar calendar”, because it is associated with the cycles of the Moon and people in ancient times treated its waxing and waning as a single month. The calendar is used to guide the annual arrangement of agriculture and has always been popular among farmers.

In the traditional Chinese calendar, there are 24 solar terms, which are the Beginning of Spring, Rain Water, Waking of Insects, Spring Equinox, Pure Brightness, Grain Rain, Beginning of Summer, Grain Full, Grain in Ear, Summer Solstice, the Slight Heat, the Great Heat, the Beginning of Autumn, the Limit of Heat, the White Dew, the Autumnal Equinox, the Cold Dew, the Frost’s Descent, the Beginning of Winter, the Small Snow, the Great Snow, the Winter Solstice, the Slight Cold, and the Great Cold. To certain extent, the 24 solar terms objectively reflect the changes in temperature throughout the year, rainfall, phenomenology, and so on, so it was an important tool for the people of ancient times in organizing farming activities.

The Agricultural Calendar and the 24 Solar Terms

The 24 solar terms are divided based on the positions of the Sun as it travels along the ecliptic longitude (e.g. the orbit in which the Earth revolves around the Sun). When the Sun lies at zero degree of celestial longitude, where its direct rays shine on the equator, the Spring Equinox is formed. Starting from the point of Spring Equinox, a solar term will be formed every time the Sun travels 15 degrees, and a tropical year will be formed when it returns to the same point after making one revolution (360 degrees). In this way, there are 2 solar terms in a month and 24 solar terms in a year. The calendar dates of the 24 solar terms are approximately the same each year: around the 6th and the 21st in the first half of the year, and around the 8th and the 23rd in the second half of the year.

In China, a large agricultural country, agriculture has very close relationship with weather, so the ancients began observing the relationship between the farming season and climate, and accumulated rich experience.

From the title point of view, the 24 solar terms mostly reflect the change of climate, while the others indicate phenomenology and farming activities.

The Beginning of Spring, the Beginning of Summer, the Beginning of Autumn and the Beginning of Winter respectively indicate the beginning of the four seasons.

The Spring Equinox and the Autumnal Equinox mean the time of equal length between day and night, reflecting the change of the Sun’s path. On the day of the Spring Equinox, the direct rays of the sun shine on the northern part, so that the northern hemisphere begins to have longer days and shorter nights. On the day of the Autumnal Equinox, the direct rays of the sun shine on the southern part, so that the northern hemisphere begins to have shorter days and longer nights.

The Summer Solst ice and Winter Solstice mark the seasonal apex. On the day of the Summer Solstice, the northern hemisphere has the longest day - called “Ribeizhi” by the ancients - meaning the day when the sun travels to its northernmost point. After the Summer Solstice, as the sun gradually moves south, the northern hemisphere has shorter and shorter days and longer and longer nights. On the day of the Winter Solstice, the northern hemisphere has the longest night - known as “Rinanzhi” by the ancients - meaning the day when the sun reaches its southernmost point. After the Winter Solstice, the direct rays of the sun again gradually shine on the northern hemisphere so that the days there gradually grow longer.

The Slight Heat, the Great Heat, the Limit of Heat, the Slight Cold and the Great Cold reflect the change of temperature. “Heat” means burning hot. The Slight Heat means it is very hot, but not yet the hottest, which, of course, is the Great Heat; the Limit of Heat marks the end of the hot summer, and the onset of cooler weather. The Slight Cold means it is becoming cold, but not at the intensity of the Great Cold that follows.

The Rain Water, the Grain Rain, the Small Snow and the Great Snow reflect the phenomenon of precipitation. The Rain Water means the beginning of precipitation; the Grain Rain means the arrival of sufficient rainfall for growth of the grain to begin, hence the saying that “after the rain, the grain begins to grow”; the Small Snow means the temperature drops and snowfall begins, but not in a large amount; the Great Snow means the snowfall is increasing with accumulated snow on the road.

The White Dew, the Cold Dew and the Frost’s Descent seemingly reflect the condensation and sublimation of water vapor. The White Dew means cooler weather is coming with the emergence of dew. The Cold Dew means there is more dew and lower temperatures. The Frost’s Descent means the weather has become cold and begun to freeze.

The Grain Full and the Grain in Ear reflect the maturity and harvesting of crops. The Grain Full indicates that the seeds of summer crops are becoming plump but not yet mature enough for harvesting. The Grain in Ear indicates wheat and other such crops have mature and can be harvested. In northern China, there is a saying that “it is unsuitable to plant after the Grain in Ear”, which means that if the dry crops such as corn are planted after the Grain in Ear, the harvest will not be so good.

The Waking of Insects and the Pure Brightness reflect natural phenomena. The Waking of Insects means the spring thunder wakens hibernating animals and insects in the soil. The Pure Brightness means the weather and lush trees and grass.

The 24 solar terms originated in the Yellow River basin, so there will be a few days gap in reflecting changes in climate or phenomenology in other regions, however, the 24 solar terms are generally accepted and some have become common festivals of the Chinese people, such as the Pure Brightness Festival and the Winter Solstice Festival.