What is behind the solar terms in Grain Rain?

This is part of the 24th solar term song. Refers to the twenty-four solar terms.

Twenty-four solar terms: beginning of spring, rain, fright, vernal equinox, Qingming, Grain Rain, Changxia, Xiaoman, Mangzhong, Summer Solstice, Xiaoxia, Daxia, beginning of autumn, Chushu, Bailu, Autumnal Equinox, Cold Dew, First Frost, beginning of winter, Light Snow, Heavy Snow, Winter Solstice, Slight Cold and Severe Cold.

Twenty-four solar terms were divided by the wise people in ancient China according to the position of the sun on the ecliptic through careful observation of nature and life, and the interval between two adjacent solar terms was about 15 days. In order to facilitate memory, people compiled 24 solar terms songs:

The full text is:

The spring rain shakes the spring and clears the valley, and the summer is full of mountains and summers.

Autumn dew, autumn frost, little snow in winter and cold in winter.

The first half of the year is June 2 1 day, and the second half is August 23.

The two festivals remain unchanged every month, with a maximum difference of one or two days.