Beijing is about 40 degrees north latitude. During the summer solstice in the northern hemisphere, the sun shines directly on the Tropic of Cancer. At this time, the height of the noon sun here is the highest, about 73.5; On the solstice of winter, the sun shines directly on the tropic of Capricorn. At this time, the noon sun height here is the smallest, about 26.5.
Because of the revolution of the earth, the sun travels between the Tropic of Cancer and the Tropic of Cancer at the direct point of the earth within one year. Take the area where the Tropic of Cancer passes in the northern hemisphere as an example: when the sun shines directly on the Tropic of Cancer in a year, it is the summer solstice solar term in northern China. At noon, the sun is directly above the head, that is, the solar altitude angle is 90, which is the maximum value in a year.
When the sun shines directly on the tropic of Capricorn, it coincides with the solar term of winter solstice in northern China. At this time, the solar altitude angle is 43, which is the minimum value in a year. Due to the rotation of the earth, the apparent motion of the sun (rising in the east and setting in the west), that is, the daily variation of the height of the sun.
Because of the revolution of the earth, the direct point of the sun moves north and south, which leads to the latitude change and seasonal change of the height of the sun at noon.