"Hour" is a basic time unit in ancient climatology, and it is also the smallest time unit among the twenty-four solar terms. In the twenty-four solar terms, "five days are one period, and three periods are one qi." That is to say, one period is five days, and three "one periods" are exactly 15 days, which is exactly equal to the length of one solar term. Because of this, a solar term is called "three seasons". Therefore, there are different sayings such as the three seasons of Beginning of Spring, the three seasons of Grain Grain, the three seasons of Beginning of Autumn, and the three seasons of Autumnal Equinox.

Each solar term can be connected with the three seasons to form a new term, such as the three periods of Beginning of Summer and the three periods of Beginning of Winter. This is a bit like seven days in a week. The ancients replaced seven days with three seasons, and replaced one week with specific solar term names. Since one solar term has three periods, there are 72 periods in the twenty-four solar terms in a year, and each period corresponds to a phenological phenomenon.

Extended information

About the three seasons of the Beginning of Autumn

"The cool breeze arrives at the beginning of autumn" refers to "The cold wind in the West is called cool breeze, and the temperature changes. The cold air begins to subside." In other words, the wind at the beginning of autumn is no longer the hot wind in summer, and the weather has begun to turn cooler.

"The second period of white dew falls" means "after the heavy rain, a cool breeze comes, and the weather drops are vast and white, but the beads have not yet condensed, so it is said that the white dew falls, indicating the white color of autumn gold." To put it simply, it means that there is fog on the ground in the morning.

"The three-season chilling cicada chirps" refers to "the chilling cicadas that feel the gloom in autumn also begin to chirp." When the cicada starts to chirp, it means that the weather is really getting colder. The cicada is to autumn what the cuckoo is to summer, it is a signal.