Nanjing, 21 Sep (Xinhua) -- "Cold and summer are peaceful day and night, and yin and yang are half at the autumn equinox. "At 21:31 on the 22nd, the autumn equinox of the twenty-four solar terms will be ushered in. This is also one of the earliest established solar terms in ancient China.
For a long time, people have believed that the autumn equinox is divided equally and equally: it refers to both the equal division of autumn and the equal division of day and night. However, experts from the Purple Mountain Observatory of the Chinese Academy of Sciences, which specializes in almanac astronomy research, said that the points of the autumn equinox are only roughly equal, not accurately divided.
Cheng Zhuo, an associate researcher in the calendar room of the Purple Mountain Observatory, introduced that in ancient China, the autumn equinox was considered to be the solar term of equal autumn. The "Explanation of the Seventy-two Waiting Collections of the Moon Order" says, "The divider is half, and this is the half of the ninety days, so it is called the division." "In the twenty-four solar terms, Spring, Summer, Autumn, and Winter are the beginning of the four seasons. Autumn from the beginning of autumn to the end of the winter, including the autumn, summer, white dew, autumn equinox, cold dew, frost fall of these six solar terms, the autumn equinox is located in the middle of autumn. However, since the current twenty-four solar terms are formulated according to the annual movement of the sun at an unequal rate, in practice, the length of the seasons is not equal, and there will be some deviations in the equal equinox of the autumn equinox.
According to modern almanac calculations, the actual length of autumn is not 90 days, but about 91.9 days. The autumn equinox is about 46.5 days from the beginning and end of autumn and 45.4 days respectively. And because both the solar terms and the Gregorian calendar represent the annual return movement of the sun, the dates of the twenty-four solar terms in the Gregorian calendar are relatively fixed, and within this century, the autumn equinox occurs on September 22 or 23.
Zhang Yang, director of the science popularization department of the Purple Mountain Observatory, said that astronomically speaking, the autumn equinox refers to the moment when the sun moves along the ecliptic from north to south through the equator. At this time, the Sun is located at zero degrees of the Polaris Longitude in the center of the Earth, directly hitting the equator, and the days and nights are equal in length throughout the earth. The English word for autumn equinox is also a combination of "autumn" and "day and night equality".
However, due to reasons such as refraction of sunlight as it passes through the atmosphere, the autumn equinox day will actually be slightly longer than the night. Taking the Beijing area as an example, on the day of the autumn equinox, the "day" is about 20 minutes longer than the "night", and the length of the day and night is also slightly "partial".
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