In Mandarin, what was used to determine solar terms in ancient China?

The lunar calendar in China is based on the movement of the sun and the moon, and farming is entirely based on the sun. Therefore, the "twenty-four solar terms" that reflect the solar cycle alone are added to the calendar as the standard for determining leap months.

Twenty-four solar terms originated in the Yellow River Basin. As early as the Spring and Autumn Period, four solar terms were set, namely, mid-spring, mid-summer, mid-autumn and mid-winter. After continuous improvement and perfection, 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.

The above is for reference only, thank you!