What are solar terms?

The twenty-four solar terms were originally formulated according to the changes of stars. The ancients judged the seasons according to the direction of the Big Dipper in the night sky, and instructed agricultural production not to miss the seasons.

Twenty-four solar terms originated in the Yellow River basin and were established on the basis of climate and phenology in this area. As early as the Spring and Autumn Period, four solar terms were set, namely, mid-spring, mid-summer, mid-autumn and mid-winter. By the end of the Warring States Period, solar terms had eight names: beginning of spring, Equinox, Long Summer, Summer Solstice, beginning of autumn, Equinox, beginning of winter and Winter Solstice. By the Qin and Han Dynasties, the 24 solar terms had been completely established. In BC 104, the taichu calendar written by Deng Ping officially set 24 solar terms in the calendar, and defined the astronomical position of the 24 solar terms.

There are two solar terms every month. The first solar term is called "Festival" and the middle solar term is called "Qi", which are collectively called solar terms.