How did the ancients calculate the 24 solar terms?

In essence, solar terms belong to the solar calendar category, that is, they are also based on the cycle of the earth's orbit around the sun (tropic year). So in the solar calendar, the date range of solar terms is very small.

The twenty-four solar terms do have a fixed position in the solar calendar. The so-called solar calendar is based on the relative position of the sun and the earth as the standard for dividing dates. But the lunar calendar is a kind of lunar calendar, that is to say, the lunar calendar also takes into account the position of the sun. People often say that the lunar calendar has two meanings, one is to refer to the lunar calendar verbally; The other is a calendar that divides dates only according to the relative position of the moon and the earth. In short, the 24 solar terms of the lunar calendar are the embodiment of the lunar calendar as a yin-yang calendar.

In ancient times, there were probably several ways to divide it: for example, about 3,000 years ago, one way was to watch the Big Dipper at dusk and set the solar terms with bucket handle. Bucket handle points to the east as the vernal equinox, to the south as the summer solstice, to the west as the autumnal equinox, and to the north as the winter solstice. There is also a painting method, that is, at dusk, look at what constellation the zenith is, such as the star at the zenith at dusk, which is called "fire" in ancient books, that is, "antares", that is, summer has arrived.