How are the 24 solar terms determined?

Twenty-four solar terms are based on some latitudes where the sun shines directly on the earth. Therefore, it belongs to the solar calendar, and the beginning of each solar term can basically correspond to a certain day in the solar calendar, with a difference of at most one or two days.

The earth revolves around the sun 1 circle at 5: 48: 46 every 365 days and rotates 1 circle every 23 hours, 56 minutes and 4 seconds. Because the orbital plane of the earth's rotation around the sun is not the same as the equatorial plane, but keeps a certain inclination, so the position of direct sunlight on the earth is different all year round. In the northern hemisphere, when the direct sun is 23.5 north latitude, it is called summer solstice in astronomy; The direct sunlight at 23.5 south latitude is called the winter solstice; The solstice in summer and the solstice in winter mean that they are halfway between summer and winter. When the sun shines directly on the equator twice a year, it is called the vernal equinox and the autumnal equinox respectively. The vernal equinox and autumnal equinox refer to the middle of spring and autumn; The days of these two days are as long as the nights. In this way, a year can be divided into four parts: vernal equinox, summer solstice, autumn equinox and winter solstice. If each section is divided into six small sections, each small section is about 15 days, and the whole year can be divided into 24 small sections, thus producing 24 solar terms.

During the Qin and Han Dynasties in China, the names of the 24 solar terms were beginning of spring, Rain, Creeping, Vernal Equinox, Qingming, Grain Rain, Long Summer, Xiaoman, Mang, Summer Solstice, Slight Summer, Great Summer, beginning of autumn, Early Summer, Bailu, Autumn Equinox, Cold Dew, Early Frost, beginning of winter, Light Snow, Heavy Snow, Winter Solstice, Slight Cold and Great Cold.

From the meaning of the names of the twenty-four solar terms, we can see that there are eight solar terms reflecting the changes of the four seasons: beginning of spring, vernal equinox, long summer, summer solstice, beginning of autumn, autumn equinox, beginning of winter and winter solstice; Beginning of spring, Changsha, beginning of autumn and beginning of winter are called "Li Si", which means the beginning of the four seasons. However, due to different climates, the time and length of the four seasons are not exactly the same. There are five solar terms reflecting temperature changes: slight summer heat, great summer heat, summer heat, slight cold and severe cold. There are seven solar terms reflecting weather phenomena: rain, Grain Rain, white dew, cold dew, first frost, light snow and heavy snow. Phenological phenomena are reflected in four solar terms: fright, Qingming, Xiaoman and Mangzhong.

Phenomena related to climate, weather and phenology often have regional color. For example, the "first frost festival" in the solar calendar is around 10 on June 23rd, which means that with the arrival of this solar term, frost will appear on the earth for the first time, but there is no frost in southern Fujian, Taiwan Province and Hainan provinces all year round. On the contrary, the first frost in northern areas such as Heilongjiang Province and Inner Mongolia Autonomous Region in the northeast was earlier than this period. This is because people in China lived in the Yellow River valley during the Qin and Han Dynasties, and the 24 solar terms were mainly determined according to the relationship between climate and agricultural activities in the Yellow River valley at that time. Due to the great climate change in China from Qin and Han Dynasties to modern times, the Yangtze River basin can also be applied to a certain extent at present. At the same time, 15 days is more convenient in arranging agricultural production, so it is used in all parts of rural areas in China. However, China has a vast territory and a different climate. In some places, agricultural activities are arranged not only according to the literal meaning of solar terms, but as a time period. Specifically, the arrangement of agricultural activities in different places should be decided according to the differences of climate, crop characteristics and soil. For example, it is totally inappropriate for Heilongjiang Province and Guangdong Province to arrange agricultural activities according to the literal meaning of solar terms.