What are the solar terms based on?

It is reported that as early as the Spring and Autumn Period and the Warring States Period, China can already measure the length of the midday sun shadow with a soil gauge (erect a pole on the plane) to determine the four solar terms of winter solstice, summer solstice, vernal equinox and autumn equinox. 1 year, the shortest day at noon is the summer solstice, the longest day is the winter solstice, and the shadow length is the vernal equinox or autumnal equinox. By the Qin and Han Dynasties, the complete concept of the 24 solar terms had been formed.

In ancient times, 1 year was divided into 12 months, and each month had two solar terms. The former is solar terms, while the latter is neutral. Later generations collectively referred to solar terms and neutral gas as solar terms.

It can be seen from the naming of 24 solar terms that the division of solar terms fully considers the changes of natural phenomena such as seasons, climate and phenology. Among them, beginning of spring, Changxia, beginning of autumn, beginning of winter, vernal equinox, autumnal equinox, summer solstice and winter solstice are used to reflect the seasons. 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.

Five solar terms, such as slight summer heat, great summer heat, intense summer heat, slight cold and severe cold, reflect the change of temperature. The four solar terms of rain, Grain Rain, light snow and heavy snow reflect the phenomenon of precipitation, indicating the time and intensity of rainfall and snowfall. The three solar terms, White Dew, Cold Dew and First Frost, on the surface reflect the phenomenon of condensation of water vapor, but in essence reflect the process and degree of gradual temperature drop: when the temperature drops to a certain extent, water vapor condenses. The continuous drop in temperature not only increases condensation, but also gets colder and colder. When the temperature drops below zero degrees Celsius, water vapor condenses into frost.

Xiaoman and awn seeds reflect the maturity and harvest of related crops; 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.