How are the 24 solar terms formulated?

The twenty-four solar terms of the lunar calendar are supplementary calendars formulated by China to guide farming in ancient times, and they are the crystallization of the long-term experience and wisdom of the working people of the ancient Han nationality. The 24 solar terms are: beginning of spring, Rain, Sting, Equinox, Qingming, Grain Rain, Changxia, Xiaoman, Mangzhong, Summer Solstice, Xiaoxia, Daxia, beginning of autumn, Chushu, Bailu, Autumn Equinox, Cold Dew, First Frost, beginning of winter, Light Snow, Heavy Snow, Winter Solstice, Slight Cold and Severe Cold.

As early as the Spring and Autumn Period and the Warring States Period, the sundial was measured by earth gauge in ancient China (that is, a pole was erected on the plane to measure the length of the midday sun shadow) to determine the four solar terms: vernal equinox, summer solstice, autumnal equinox and winter solstice. In a year, the shortest day of the shadow of returning to the earth at noon is the summer solstice, the longest day is the winter solstice, and the shadow with moderate length is the vernal equinox or autumnal equinox. The Book of History in the Spring and Autumn Period recorded the solar terms. There were only four solar terms in Shang dynasty, while there were eight solar terms in Zhou dynasty. The names of the twenty-four solar terms first appeared in Liu An's "Instructions of Huainan Tian Zi" in the Western Han Dynasty, and the concepts of Yin and Yang, four seasons, eight places, twelve degrees and twenty-four solar terms were also mentioned in the preface to Historical Records Taishi Gong. In ancient China, the lunar calendar was used to record time, and the solar calendar was used to divide the 24 solar terms of spring, summer, autumn and winter. Our ancestors called five days 1, and three days were called solar terms. The whole year was divided into 72 days out of 24 solar terms.

The naming of the 24 solar terms reflects the seasons, climate phenomena and climate changes.

Beginning of spring, vernal equinox, long summer, summer solstice, beginning of autumn, autumnal equinox, beginning of winter and winter solstice are eight solar terms.

There are five solar terms symbolizing temperature change: slight summer heat, great summer heat, summer heat, slight cold and severe cold;

Reflecting precipitation are: rain, Grain Rain, Bailu, cold dew, first frost, light snow and heavy snow;

The solar terms reflecting phenological phenomena or agricultural activities are: fright, Qingming, Xiaoman and Mang.

The vernal equinox, autumnal equinox, summer solstice and winter solstice are divided from an astronomical point of view, reflecting the turning point of solar height change.

Beginning of spring, Changchun, beginning of autumn and beginning of winter reflect the beginning of the four seasons. Because China is a vast country with a very obvious monsoon and continental climate, the weather and climate vary greatly from place to place, so the four seasons in different regions are also very different.

The three solar terms, White Dew, Cold Dew and First Frost, on the surface reflect the phenomenon of water vapor condensation, but in essence they reflect the process and degree of gradual temperature drop.

Frightening and Qingming reflect natural phenological phenomena, especially stinging, which indicates that spring will return with the recovery of early thunder in the sky and underground stinging insects.