Why are there only summer solstice and winter solstice, but not spring solstice and autumn solstice? Then why divide them into four grades?

It is not divided into four levels. Summer solstice and winter solstice are two of the 24 solar terms, and there are no spring solstice and autumn solstice in the 24 solar terms.

Twenty-four solar terms refer to the special festivals in the dry calendar that indicate the season, phenology, climate change and the establishment of "December construction". Initially determined by the bucket handle direction of the Big Dipper, bucket handle rotates around the east, south, west and north for a period of time, which is called one of the "years old" (photo); Every rotation cycle starts from beginning of spring and ends with the Great Cold.

The current "twenty-four solar terms" are divided by "constant gas method", that is, each solar term corresponds to a certain position that the earth reaches every time it runs 15 on the ecliptic; The method of "fixing qi" divides the sun's annual movement track into 24 equal parts, each equal part is 15, and each equal part is a solar term, which starts from beginning of spring and ends with severe cold. "Twenty-four solar terms" express the unique concept of time between human beings and the natural universe, and contain the long-standing cultural connotation and historical accumulation of the Chinese nation.