The autumnal equinox is the sixteenth of the twenty-four solar terms in the lunar calendar. It usually falls on September 22 or 23 every year. The climate in the south begins with autumn from this solar term. The autumnal equinox in 2012 came earlier than ever in the past 116 years. According to calculations from the Purple Mountain Observatory, the exact time of the autumnal equinox this year was 22:49 on September 22. This was also the earliest autumnal equinox in the past 116 years. The autumnal equinox earlier than this year dates back to 21:03 on September 22, 1896.

"Autumn Equinox" has two meanings:

First, the days and nights are equal, and the days are getting longer and the nights are shorter, and the days are getting shorter and the nights are longer. The sun reaches 180 degrees of ecliptic longitude on this day and shines directly on the earth's equator. Therefore, the 24 hours of day and night on this day are equally divided, with 12 hours each. The ancient Chinese book "Spring and Autumn Fanlu: The Entrance and Exit of Yin and Yang (Part 1 and 2)" says: "At the autumnal equinox, yin and yang are half in phase, so day and night are even and the cold and heat are equal." At the autumnal equinox, "fen" means "half". During the autumnal equinox, day and night are of equal length all over the world. After the autumnal equinox, the days get shorter and the nights get longer in the Northern Hemisphere, and the days get longer and the nights get shorter in the Southern Hemisphere. During the autumnal equinox, there are no extreme day and night phenomena around the world. After the autumnal equinox, the range of polar night near the North Pole becomes larger and the range of polar day near the South Pole becomes larger.

Second, the climate changes from hot to cool. According to the season division method of ancient my country with the beginning of spring, beginning of summer, beginning of autumn and beginning of winter as the beginning of the four seasons, the autumnal equinox is located in the 90 days of autumn and divides autumn equally. For example, Zuo Heshui's poem says: "The summer is gone, the autumn is clear and cool, and the daylight and night are both long." After this day, the direct sunlight position moves southward, and the days are shorter and the nights are longer in the northern hemisphere. It is called "the coolness of autumn".

The meaning of the autumnal equinox: The ancient Chinese book "The Collection of Seventy-Two Hours of the Moon Order": "In mid-August...the interpretation is the autumnal equinox." "Equinox" means the equinox of day and night. Just like the spring equinox, the sun shines directly on the earth's equator on this day, making day and night equal. Since then, the position of direct sunlight has moved further south, the days have become shorter and the nights have become longer in the northern hemisphere, and the temperature has dropped, which has universal significance across the country. In ancient my country, the autumnal equinox was divided into three periods: "The first period is when the thunder begins to cease; the second period is when the insects begin to live; and the third period is when the water begins to dry up." The meaning of Yihou is that thunder is caused by the strong Yang Qi. After the autumnal equinox, the Yin Qi becomes strong, so thunder no longer occurs. The word "璋" in the second period means fine soil, which means that as the weather gets colder, hibernating insects begin to hide in caves, and the entrances of the caves are sealed with fine soil to prevent the cold air from intruding. "The water begins to dry up in the third period" means that rainfall begins to decrease at this time. Due to dry weather and rapid evaporation of water vapor, the amount of water in lakes and rivers becomes less, and some swamps and puddles are drying up.

During the autumnal equinox, most areas of our country have entered a cool autumn. The cold air coming from the south meets the gradually attenuating warm and humid air, causing precipitation again and again, and the temperature also drops again and again. As people often say, it has come to the time of "one autumn rain and one cold", but the daily precipitation after the autumnal equinox will not be very large. At this time, field cultivation was different in the south and north. There is a farmer's proverb in North my country that says: "The white dew comes early and the cold dew comes late, so it is the right time to sow wheat at the autumnal equinox." The proverb clearly stipulates the time for sowing winter wheat in this area; and "When white clouds come at the autumnal equinox, good songs and good rice planting are everywhere" reflects It is the time to sow late rice in southern China. In addition, the working people's taboos about the autumnal equinox are also summarized in proverbs, such as "On the autumnal equinox, we are only afraid of thunder and lightning, but the extra rice will be more expensive."

During this period, many areas across the country began to enter a period of low precipitation. After the autumnal equinox, the number of rainfall days and rainfall in most areas of my country, including Jiangnan and South China (except for heavy rains caused by tropical cyclones), has entered a period of reduced precipitation. The water levels of rivers and lakes have begun to decline, and some seasonal rivers and lakes may even gradually dry up. During this period, individual tropical cyclones may also appear, but their impact locations are southerly, mostly affecting the coast of South China and Hainan Island. In addition to strong wind disasters, the rain brought by typhoons at this time is often beneficial to local soil moisture conservation, because After October, these areas enter the dry season.