How did the ancients judge the arrival of solar terms?

How did the ancients judge the arrival of solar terms?

The twenty-four solar terms of the lunar calendar include: beginning of spring, rain, fright, vernal equinox, Qingming and Grain Rain; Long summer, xiaoman, mangzhong, summer solstice, summer heat, summer heat; Beginning of autumn, first ripe, white dew, autumnal equinox, cold dew, early autumn frost; Beginning of winter, light snow, heavy snow, winter solstice, slight cold, cold winter.

The months in the whole year are set according to the cycle of the moon phase change. The big moon has 30 days and the small moon has 29 days. The first day of each month in the lunar calendar is called "New Moon" and the fifteenth day of each month is called "Hope". Because the lunar year is about 355 days, which is about 1 1 day different from the earth year, there should be a leap month every 3 years and 7 leap months every 19 years.

A year with a leap month is also called a leap year. The lunar calendar is stipulated according to the time of the year of the return of the earth and the change of the moon phase, and refers to the surrounding meteorological and phenological conditions and agricultural activities, which is more practical, so it has become a calendar with four distinct seasons and easy memory. In China, the lunar calendar is especially suitable for the vast areas of the Central Plains.