The direction of solar irradiation in different solar terms is

The vernal equinox and autumnal equinox point directly to the equator, the summer solstice points directly to the Tropic of Cancer, and the winter solstice points directly to the Tropic of Cancer. The vernal equinox is the fourth solar term in the "24 solar terms". "The vernal equinox, Yin and Yang. So it's hot and cold day and night. " A word "fen" tells the boundary between day and night, cold and heat. At this time, the longitude of the sun is 0 degrees and the position of the sun is above the equator. According to the lunar calendar, "it refers to the vernal equinox, which is about Sunday. The northern and southern hemispheres are equally divided day and night, which is also half of spring, so it is called the vernal equinox."

The summer solstice is the tenth of the twenty-four solar terms. The "Twenty-four solar terms" were originally determined by the bucket handle direction of Beidou. Bucket handle starts from the north of due east and turns south, west and north, which is one of the "years old". The battle refers to noon, which is the summer solstice. The current "twenty-four solar terms" are divided by the method of "constant gas", that is, each solar term corresponds to a certain position reached by the earth every time it runs 15 on the ecliptic, and the summer solstice is when the sun runs to 90 degrees on the ecliptic.