There was no current solar calendar in ancient China, so why did the 24 solar terms happen to be those days in the solar calendar?

China used the lunar calendar in ancient times, also called the lunar calendar, which was calculated according to the period of the moon. Twenty-four solar terms are traditional festivals and meteorological solar terms that divide a year into twenty-four important solar terms in China, which are determined according to the running position of the sun on the ecliptic. Although the starting point and calculation method of lunar calendar and solar calendar are different, the determination of the 24 solar terms is based on observing the position of the sun on the ecliptic, so their dates in the solar calendar year are not fixed, but change with time. This is why the dates of the 24 solar terms fluctuate during the solar year. It is worth noting that according to the position of the sun and the arrangement of the lunar calendar, the 24 solar terms are usually close to the date of the solar calendar, but they are not always exactly the same.