Xinhua News Agency, Nanjing, September 22 (Reporter Qiu Bingqing) According to the calculations of the Purple Mountain Observatory of the Chinese Academy of Sciences, the autumnal equinox in the 24 solar terms will be ushered in at 3:21 on September 23.

  According to Wang Kechao, head of science popularization at the Purple Mountain Observatory of the Chinese Academy of Sciences, in the twenty-four solar terms, the beginning of spring, the beginning of summer, the beginning of autumn, and the beginning of winter are the beginnings of the four seasons.

The autumn is independent from autumn to the beginning of winter. It includes the six solar terms of Liqiu, Chushu, Bailu, Autumnal Equinox, Cold Dew, and Frostfall. The autumnal equinox is located in the middle of autumn.

  The autumnal equinox is one of the earliest solar terms identified in ancient my country.

Wang Kechao said that both solar terms and the Gregorian calendar embody the sun’s annual return movement. Therefore, the dates of the twenty-four solar terms in the Gregorian calendar are relatively fixed. The autumnal equinox of this century will all occur on September 23 or September 22, Beijing time.

  "The twenty-four solar terms change regularly in the Gregorian calendar." Wang Kechao said that from 2008 to 2096, the autumn equinox of each leap year will be on September 22, and the autumn equinox of each leap year will be more than the previous one. The autumn equinox of the leap year is earlier. It can be said that the autumn equinox of each leap year during this period is "the earliest year since 1896."

  Wang Kechao introduced that in astronomy, the autumnal equinox refers to the moment when the sun moves along the ecliptic from north to south and passes through the equator.

At this time, the sun hits the equator directly, and the day and night are equal in length everywhere on the earth.

In all parts of the northern hemisphere, the day is shorter than the night from the autumnal equinox, and after the autumnal equinox, as the sun gradually moves south, the weather becomes colder.

The southern hemisphere is just the opposite.

  “However, because the sun’s annual movement is uneven, it is the slowest in July in summer and the fastest in January in winter, so in fact the length of each season is not equal.” Wang Kechao said, the length of autumn is about 91.9 Days instead of 90 days, the autumnal equinox will be slightly "offset", about 46.5 days and 45.4 days before the beginning and end of autumn.

  In addition, due to the refraction of sunlight when passing through the atmosphere, in fact, the day of the autumnal equinox will be slightly longer than the night, and the length of day and night will also be slightly "offset".

Take Beijing as an example. On the autumnal equinox, day is about 20 minutes longer than night.