Why are there different days in the 24 solar terms of the lunar calendar?

The lunar calendar (that is, what we usually say) belongs to the combination of yin and yang, and the month depends on the moon, and the sun is seen in the 24 solar terms. The periods of the sun and the moon are different. In order to achieve harmony between Yin and Yang, it is necessary to set up large and small months and leap months, so the lunar calendar date of solar terms is not fixed. However, because the Gregorian calendar that everyone often talks about is also based on the sun, the Gregorian calendar date of solar terms is relatively fixed, with a difference of a few days at most.

As for the number of days in each solar term, it used to be a flat calendar, and each solar term was allocated an average of one year, and the number of days in each solar term was equal. Later, people found that the number of days in each solar term is not exactly the same (the earth's orbit is elliptical, and the change of the direct point of the sun is uneven), so they invented a fixed calendar, and the number of days in each solar term is different.