The Relationship between the Twenty-four Solar Terms and Yin and Yang

In our daily discourse, the solar calendar is often called "solar calendar" and the lunar calendar is often called "lunar calendar". There is nothing wrong with calling the Gregorian calendar "solar calendar", but it is wrong to call the lunar calendar "lunar calendar".

There are three kinds of calendars in the world: the first is the solar calendar, which only considers the annual apparent movement of the sun-in fact, it is produced by the earth's annual revolution around the sun. Now the Gregorian calendar is popular in the world, with a length of 365.2422 days. The second is the lunar calendar-the real lunar calendar only considers the periodic movement of the moon. This calendar has been used in today's Islamic world, the so-called hijri calendar. The average length of a year is 354 days, and leap years are 355 days.

The third is the combination of yin and yang. Relatively speaking, the solar calendar and the lunar calendar are relatively simple, but our traditional calendar in China is more complicated than them, because the lunar calendar in China is a combination of yin and yang.

Twenty-four solar terms first appeared in Huainan Xun in Han Dynasty, and some of their names have appeared in pre-Qin classics. The twenty-four solar terms are probably developed from the observation records of phenology by primitive people, because the annual phenological change is caused by the annual revolution of the earth around the sun, so the phenological change corresponds to the annual apparent movement of the sun.

In the calendar handed down from ancient times, whenever there is a list of twenty-four solar terms, each solar term is often accompanied by "seventy-two phases" and each qi has three phases, which obviously implies that the source of the twenty-four solar terms is closely related to the phenological observation of our ancestors.

After the establishment of the twenty-four solar terms system, the ancients pursued the solar terms diligently and kept improving. At first, we simply divided the year into 24 equal parts, each equal part is a solar term, and the time of each solar term is exactly equal. The solar terms thus divided are called "flat gas".