Why do the 24 solar terms correspond to the solar calendar date?

The ancient calendar in China is not a pure lunar calendar, that is, it is not entirely based on the changes of the moon phases, but a calendar combining yin and yang. According to the movement of the sun, especially the cold and hot changes caused by it, and the growth of living things, especially crops, the time of solar terms is determined, such as the awakening of insects, which is related to the recovery of hibernating animals during thunder. According to the length of day and night and the length of shadow, it is divided into two parts (vernal equinox, autumnal equinox, winter solstice and summer solstice), which are as long as day and night. The night in winter solstice is the longest, the day when the sun is perpendicular to the tropic of cancer is the summer solstice, the day perpendicular to the tropic of cancer is the east, and the day perpendicular to the equator is the vernal equinox and autumnal equinox. Therefore, solar terms are related to the movement of the sun, that is, to the solar calendar, and thus have a relatively fixed corresponding relationship with the time of the current solar calendar.

However, the first half of this correspondence will be on the 6 th and 2 1, and the second half will be on the 8 th and 23 rd, with no more than two days before and after.