What does "first class, second class and third class" mean in the 24 solar terms?

Seventy-two Seasons is the earliest calendar in China that combines astronomy, meteorology and phenology to guide agricultural activities. It originated in the Yellow River valley and was completely recorded in the Book of Yizhou Shi Xun Jie in the 2nd century BC.

Take five days as days, three days as qi, six days as hours, four days as years, and twenty-four solar terms * * * 72 days a year. Each climate corresponds to a phenological phenomenon, which is called climate response. Among them, the plant climate should be germination, flowering, fruiting, etc. Animals should start vibrating, singing, mating, migrating, etc. Abiotic climate should include initial freezing, thawing and thunder. The time series changes of 72 climate events reflect the overall situation of climate change in a year.

Extended data

The weather lasts for five days, 365 days a year (72 days on average). In order to correspond to the 24 solar terms, it is stipulated that three days are one day (gas) and a year is 72 days. Each climate corresponds to a phenological phenomenon called "waiting".

The "weather" of weather should include abiotic and biotic. The former includes "the water begins to dry up", "the east wind thaws", "the rainbow begins to see" and "the ground begins to freeze". The latter includes animals and plants, such as "the swan goose comes", "the tiger begins to make love", "Ping begins to live", "Bitter vegetable show" and "the peach blossom begins to bloom".

The seventy-two changes originated very early and played a certain role in agricultural activities. Although some phenological descriptions are not so accurate and some are unscientific, they still have certain reference value for understanding the climate and its changes in ancient North China. Because the initial unit of phenology at that time was small and the actual climate and region were quite different, it was difficult to be widely used.

It is said that it is based on the time series changes of astronomical phenomena, temperature, precipitation and phenology in the middle and lower reaches of the Yellow River. For other parts of China, the situation described by the same solar term may be quite different.

But in fact, although "Li Si" can reflect the climate characteristics of the four distinct seasons in the middle and lower reaches of the Yellow River, the specific climate significance of "Li Si" is not significant. In climatology, beginning of spring is usually defined as a day when the daily average temperature is stable above 10℃ every five days, which is inconsistent with the meaning of beginning of spring in the middle and lower reaches of the Yellow River. Only South China really enters spring.

Reference source

Baidu encyclopedia-seventy-two changes.