The autumnal equinox solar terms refer to the autumnal equinox solar terms.

1, autumnal equinox, the sixteenth solar term in the twenty-four solar terms of the lunar calendar, usually lasts from September 22nd to September 24th every year. The climate in the south began to decline from this solar term. First, on this day, the sun reached the longitude of the Yellow River 180 degrees, and directly hit the equator of the earth. Therefore, in most parts of the earth, 24 hours are divided equally, each being 12 hours. Near the North Pole (90 north latitude) and the South Pole (90 south latitude), the special phenomenon that "the sun circles on the horizon all day" can be observed on this day.

2. After the autumnal equinox, the northern hemisphere has shorter days and longer nights (shorter days than nights), while the southern hemisphere has longer days and shorter nights (longer days than nights). After the autumnal equinox, the polar night range near the North Pole and the polar sun range near the South Pole gradually expanded. According to the division of seasons in ancient China, which began with beginning of spring, Changxia, beginning of autumn and beginning of winter, the autumnal equinox falls within 90 days of autumn and is divided equally in autumn.

The autumnal equinox solar terms, not only in the north, but also in most parts of the south, began to enter autumn, and the summer heat quickly subsided. The so-called "half-combination of yin and yang" means that at the autumnal equinox, the qi and yin of heaven and earth flourish and yang decline, and they are close to each other. Compared with the lush summer, flowers and trees wither, plants and leaves fall, people's mood is also prone to produce a feeling of desolation and depression. The most typical representative of this "autumn depression" mentality is Song Yu's "autumn sadness is qi" in the Warring States period.

4. The meaning of the "twenty-four solar terms" in the autumnal equinox: "See the vernal equinox in mid-August". "Divide" means to divide day and night equally. Like the vernal equinox, the sun shines directly on the equator of the earth on this day, and day and night are equal. From then on, direct sunlight began to enter the southern hemisphere, and the northern hemisphere began to have shorter days and longer nights, and the temperature dropped, which was of universal significance throughout the country.