Why are beginning of spring and beginning of winter the solar calendar time of the solstice?

Twenty-four solar terms are based on the earth's rotation period, and both beginning of spring and Solstice are in the twenty-four solar terms.

Chinese civilization was first born in the Yellow River Basin, so the formulation of the 24 solar terms is based on the agricultural production, climate and phenology of the Yellow River Basin.

In Zuozhuan in the Spring and Autumn Period, there is a record about the solstice. In Lu's Spring and Autumn Annals at the end of the Warring States Period, beginning of spring, Spring Equinox, Long Summer, Summer Solstice, beginning of autumn, Autumn Equinox, beginning of winter and Winter Solstice are the eight most important solar terms among the 24 solar terms.

At the turn of Qin and Han dynasties, the 24 solar terms have been completely established. The names of the 24 solar terms in Huainanzi written by Liu An, King of Huainan in the early Western Han Dynasty are exactly the same as now. In BC 104, when Deng Ping made the taichu calendar Law, he put the twenty-four solar terms into the calendar and made clear the astronomical position of the twenty-four solar terms.

In ancient times, the astronomical position of the 24 solar terms was determined by the Big Dipper. With the movement of celestial bodies, the Big Dipper, Kuiheng and Beidou point to different directions and positions, and the positions they point to are the months they represent. The names of the 24 solar terms are named according to the place pointed by Dougang and the natural climate and landscape at that time.