Hello, teacher. I looked at the right ascension of the sun with Stellarium, and found that solar terms do not follow the astronomical time setting, and the sky can be lost for up to two days. Why?

The setting of solar terms is not determined by the right ascension of the sun, but by the yellow longitude of the sun. The error you observed is caused by the intersection angle between right ascension and right ascension (the moving speed of the sun projected on the equatorial plane is not consistent with the actual moving speed).

The vernal equinox is close, the sun is on the ecliptic, and the meridian of the sun is equal to the right ascension of the sun, so there is no error. The solstice is located at the midpoint of the bisector, and the uneven movement trend is also averaged, which neutralizes the errors brought by the above counties and towns to some extent.

How to determine the beginning of the current Gregorian calendar every year may be more appropriate in the historical part. But as far as I know, when Gregory was concluded, the Pope stipulated that the vernal equinox of the following year would be changed from March 1 1 to March 2 1, thus implementing new legislation. I dare not say that the vernal equinox is the basic point of the calendar, but when it comes to important influence, it is quite appropriate.