The origin of solar terms from summer to sun.

1. In the 7th century BC, the shadow of the sun was measured by civil soil gauge, and the summer solstice was determined. The summer solstice is the northernmost day of the sun in a year, the culmination of the sun's northward movement, the longest sunshine day in the northern hemisphere, and the day that exceeds the night time the most. However, the long days in various places show a decreasing trend from north to south. This is because the days are long and the nights are short caused by the tilt of the earth's rotation axis, and the closer it is to the two levels, the more obvious it is.

2. On the day of summer solstice, the direct sunlight reaches the northernmost point of the year, almost directly on the Tropic of Cancer, which runs through China's four provinces of Yunnan, Guangxi, Guangdong and Taiwan from west to east. These four provinces are the areas where the sun is in the most central position in the sky, with the shortest shadow, the longest day and the shortest night.

The solstice in summer is the turning point of the sun. After that, the sun will return to the past. After the summer solstice, the direct point of the sun gradually moved southward, and the days in the northern hemisphere began to shorten gradually. For the Tropic of Cancer (23 26 north latitude) and its northern area, after the summer solstice, the noon sun height began to decrease day by day. Summer is the longest sunshine season in the northern hemisphere, and there will be extreme days in the Arctic Circle, and the sun will not set all day.