Nanjing, June 20 (Xinhua) -- According to the "Chinese Astronomical Almanac", at 11:32 Beijing time on June 21, the "summer solstice" of the 24 solar terms of the lunar calendar was ushered in. On the summer solstice, the northern hemisphere has the longest daytime, the midday sun is the highest, and the shadow of vertical objects is the shortest.
Wang Kechao, director of science popularization at the Purple Mountain Observatory of the Chinese Academy of Sciences, introduced that the summer solstice in astronomy refers to the moment when the sun runs to the northernmost point from the equator and directly hits the Tropic of Cancer. After this day, the sun will "turn" south, so the area through which the Tropic of Cancer passes is also called "the place where the sun turns".
In Shantou, Conghua, Guiping, Mojiang, Mengzi and other places in China, there will be an interesting phenomenon of "no shadow" at noon on the summer solstice, because the sunlight is directly emitted from the zenith. On the evening of the summer solstice, residents of the Northern Hemisphere witness the longest twilight of the year.
Since the northern hemisphere has the longest daylight on the summer solstice, will it have the earliest sunrise and the latest sunset? Usually not, this is related to the time system we adopt. The time determined by sunrise and sunset is called true solar time. In the true solar time system, the summer solstice day is indeed the earliest sunrise and the latest sunset, but due to the heterogeneity of the true solar time, we use the flat solar hour based on the hypothetical average motion of the sun. The difference between true solar time and normal solar time is called time difference, and due to the existence of time difference, in real life, the phenomenon of the earliest sunrise and the latest sunset usually does not occur on the day of the summer solstice. Of course, this is for most parts of the planet. In the Arctic Circle, the summer solstice day does not set in the whole region, and there is no sooner or later.
While the summer solstice day falls on June 21 in most years, it is also possible to be june 22 or June 20. For example, the summer solstice in 2015 is June 22, while the summer solstice in 2048 is June 20.
Many people may think that the longest daylight in the northern hemisphere on the summer solstice means that the sun is close to us, but this is not the case. About two weeks after the summer solstice, the Earth moves to the aphelion. So, around the summer solstice, the sun is almost the farthest away from us.
Producer: Zhao Cheng
Producer: Hu Qinghai
Planner: Liu Xinhui
Text: Wang Jueyue
Poster Maker: Xu Zhan
Edit: Qi Leijie
It was co-produced by Xinhua News Agency and Jiangsu Branch of Xinhua News Agency
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