The shortest day for the shadow of flagpole at noon in Tiananmen Square, Beijing.

Summer solstice: on June 2 1 or 22, the sun reaches 90 degrees of the yellow meridian, which is the "summer solstice" solar term. According to "Zunxian Manuscript", "The sun arrives in the north, the days are long and the shadows are short, so it is called the summer solstice. At best, it is excellent. " From summer to the sun, the sun directly reaches the northernmost point of the year, almost directly to the Tropic of Cancer (23 27' north latitude), and the daytime in the northern hemisphere is the longest, and the farther north it is, the longer it is. For example, the daily length of Haikou in Hainan is a little more than 13 hours, Hangzhou is 14 hours, Beijing is about 15 hours, and Mohe in Heilongjiang can reach 17 hours.