Why are the spring equinox and autumn equinox in Gregorian calendar?

Solar terms represent the position of the earth in the orbit of revolution. Starting from the vernal equinox, every 15 degree is a solar term, and there are 24 solar terms in the tropic of cancer. According to the order of the four seasons:

Spring: beginning of spring, rain, fright, vernal equinox, Qingming, Grain Rain;

Summer: Long summer, Xiaoman, Mangzhong, Summer Solstice, Xiaoshu, Dashu;

Autumn: beginning of autumn, early summer, Bailu, autumnal equinox, cold dew and early frost;

Winter: beginning of winter, light snow, heavy snow, winter solstice, slight cold and severe cold.

So solar terms represent the solar calendar, which is basically the same as the date of the Gregorian calendar.

Because the solar day 1 tropic year =365.2422 is not an integer, it cannot be fixed.

Winter solstice, vernal equinox, summer solstice and autumn equinox are generally in February, March, June and September.

Generally, it deviates from 1.

Some people think that the 24 solar terms are subordinate to the lunar calendar, but they are actually defined according to the solar calendar. That is, according to the position of the sun on the ecliptic, a year is divided into 24 equal parts.