What is the main basis for the division of the twenty-four 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.