The sun passes through longitude 180 degrees, and the position where the sun passes through the equator is the autumnal equinox, which is a solar term indicating seasonal changes. The sun shines directly on the equator, and day and night are equal. this is

"The bucket refers to the autumnal equinox, and the northern and southern hemispheres are equally divided day and night, which is half autumn, hence the name." The longitude of the sun is 180 degrees, and the solar calendar is from September 22 to 24 every year. According to the old calendar, the autumnal equinox happens to be the midpoint of ninety days in autumn. Just like the vernal equinox, the sun is almost direct to the equator, and the length of day and night is equal again. It can be said that the autumnal equinox is a very special day. From this day on, the position of direct sunlight continued to move from the equator to the southern hemisphere, and the northern hemisphere began to have shorter days and longer nights.

It is recorded in the Spring and Autumn Annals: "The autumnal equinox is half cloudy and half sunny, so it is cold and hot day and night." In astronomy, the autumnal equinox is considered as late summer and early autumn.