The summer solstice is one of the twenty-four solar terms. When the Sun runs to 90 ° of the Yellow meridian, it is the intersection point, which is generally from June 21 to 22 of the Gregorian calendar. On the summer solstice, the position where the sun shines directly on the ground reaches the northernmost end of the year, almost directly on the Tropic of cancer. At this time, the daytime time in all parts of the northern hemisphere reaches the longest in the whole year. For the Tropic of cancer and its northern regions, the summer solstice is also the highest day of the year at noon. On this day, the northern hemisphere received the most solar radiation, nearly twice as much as the southern hemisphere.
　　The summer solstice is the northernmost day of the year and the extreme of the northward movement of the sun. The summer solstice is the turning point of the sun. After this day, it will go back. The direct sunlight point begins to move southward from the Tropic of cancer (23 ° 26 north latitude), and the days in the northern hemisphere will be shortened day by day. After the summer solstice, the height angle of the noon sun also began to decrease day by day in the Tropic of cancer and its north. At the same time, after the summer solstice, the night sky astrology gradually becomes the summer sky. As the saying goes, "the sundial in the day has become extremely cloudy, and the night drain has been long since then".