What are the solar terms in China? Why are the solar terms in the lunar calendar not fixed?

The lunar calendar can directly reflect the phases of the moon according to the changes of the full and short periods of the moon, but it cannot reflect the changes of the four seasons due to the accumulation of deviations. The history of the sun is based on the rise and fall of the sun and the angle with Huang De, which can accurately reflect the changes of the four seasons. The current Gregorian calendar is one of the Gregorian calendars. 24 solar terms is calculated according to the lunar calendar and is relatively fixed on the Gregorian calendar. Because the moon revolves around the earth for about 29.5306 days, the average time between Zhongqi (24 solar terms has two days every month, the first day is called solar terms, and the second day is called Zhongqi) and Zhongqi is 30.4368 days, a difference of nearly one day, so that the date of Zhongqi in the lunar calendar (as well as in Qi Jie) is delayed by nearly one day every month.