Who is the earliest source of the 24 solar terms?

From the 5,000-year history, the main political, economic, cultural and agricultural activity centers in China are concentrated in the Central Plains of the Yellow River Basin [1], and the 24 solar terms were established on the basis of climate and phenology in this area. Due to the vast territory and changeable terrain of China, the 24 solar terms are only a reference for many regions. 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. In ancient times, a year was divided into twelve months, and each month had two solar terms. The former is a holiday calendar, and the latter is the spirit of China. For example, beginning of spring is the first month's festival, and it rains in the first month. Later generations called the festival calendar and China spirit solar terms.

The twenty-four solar terms are divided according to the position of the sun on the ecliptic (that is, the orbit of the earth 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. The dates of solar terms are relatively fixed in the solar calendar. For example, beginning of spring in the solar calendar always falls between February 3rd and 5th. However, in the lunar calendar, the date of solar terms is not easy to determine. Take beginning of spring as an example. It can be the first day of the lunar calendar in the previous year, 65438+February 15, and the latest is the first month, 15. The current lunar calendar is neither a lunar calendar nor a solar calendar, but a combination of lunar and solar calendars. There is a leap month in the lunar calendar. If one year is counted from the first day of the first month to the New Year's Eve of the twelfth month, the number of days in the lunar calendar varies greatly (leap year is 13 months). In order to standardize the number of years, the first day of the Lunar New Year (heavenly stems and earthly branches) is not the first day of the first month, but beginning of spring. In other words, a year in the lunar calendar is the day before from beginning of spring in that year to beginning of spring in the following year. For example, 2008 is the year of Wuzi, and the first day of Wuzi is not February 7, 2008 (the first day of the first lunar month), but February 5, 2008.