What are the 24 solar terms based on?

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.