How is the date of the 24 solar terms determined? Is there a calculation formula?

The twenty-four solar terms are divided according to the position of the earth on the ecliptic (that is, the earth's orbit around the sun). Depending on the sun starting from the vernal equinox (longitude zero, at this moment the sun shines vertically on the equator), every advance of 15 degrees is a solar term; After a week's operation, I returned to the vernal equinox, which is a tropical year with 360 degrees, so it is divided into 24 solar terms.

Historical origin

In ancient China, the sundial was measured by earth gauge, and the longest sun shadow was defined as "sun to day" (also called long day to day, long day to day, winter to day), and the shortest sun shadow was "short day to day" (also called short day to day and summer to day). One day in spring and autumn is the same length of day and night, which is designated as "vernal equinox" and "autumnal equinox". There were only four solar terms in Shang dynasty, but it developed to eight in Zhou dynasty and was completely established in Qin and Han dynasties. 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 names of the twenty-four solar terms first appeared in Tian Zi Xun of Huainan in the Western Han Dynasty, and the concepts of Yin and Yang, four seasons, eight places, twelve degrees and twenty-four solar terms were also mentioned in the Preface of Historical Records Taishi Gong. During the period of Emperor Wu of the Han Dynasty, Luo compiled solar terms into the calendar, stipulating that the month without neutral gas was the last month of leap month.