"Autumn dew and frost fall in autumn" is a sentence in the Song Dynasty of the 24 solar terms, and the second "autumn" refers to

The second autumn refers to the autumnal equinox. Autumn dew and frost autumnal equinox: beginning of autumn, Chushu, Bailu, vernal equinox, cold dew and first frost respectively.

The autumnal equinox is the sixteenth of the twenty-four solar terms, and the time is generally from September 22nd to September 24th in the Gregorian calendar every year. On the day of the autumnal equinox, the sun reaches the longitude 180 (autumnal equinox), almost reaching the equator of the earth, and the length of day and night is equal all over the world (regardless of the refraction of the sun light by the atmosphere and the shadow in the morning).

In the autumn equinox, most parts of China have entered a cool autumn. The cold air from the south meets the gradually decaying warm and humid air, which causes repeated precipitation and the temperature drops again and again. As people often say, it's time for "an autumn rain and a cold", but the daily precipitation after the autumnal equinox will not be great. At this time, the field farming in the south and the north is different.

In northern China, there is an agricultural proverb that says: "The white dew is early, the cold dew is late, and it is the right time to plant wheat in the autumn equinox." Proverbs clearly stipulate the sowing time of winter wheat in this area; In addition, the working people's taboos on the autumnal equinox solar terms are also summarized as proverbs, such as "The autumnal equinox is only afraid of lightning, how much is it?"