Why are the winter solstice, summer solstice, vernal equinox and autumn equinox fixed every year?

The current Gregorian calendar in China is derived from the Roman Jules calendar, which is a solar calendar. There are 365 days in a year, which is only about 6 hours away from the earth's revolution time of 36514 days. Therefore, a leap day every four years can basically coincide with the solar-terrestrial cycle.

The summer calendar in China, that is, the lunar calendar, is a combination of yin and yang. The period of the moon is determined by the moon, while the day and year are determined by the rotation and revolution of the earth. In this way, there will be a problem of overlapping months and years, which can be solved by adding leap months in the years of the summer 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 basically coincides with the date of the Gregorian calendar.