How to calculate beginning of spring in ancient times?

The 24 solar terms are not calculated according to the solar calendar, and the interval between them is almost 15 days at 5: 05. This is obtained by observing the ancient astrology, and has nothing to do with the solar calendar. Take beginning of spring as an example, it may be February 4th or 5th. The lunar month must contain a festival. If there is only one festival or one festival, it is necessary to add Runyue. The cycle of solar terms is different from that of lunar month. Lunar calendar is the cycle of the moon's revolution around the earth, and solar terms are ecliptic cycles, that is, the cycle of the earth's revolution around the sun, so this is consistent with the Gregorian calendar, so you will have the misunderstanding that solar terms come from the Gregorian calendar.

For example, if the hornbill in the green dragon appeared somewhere in the southern sky, the ancients would know that it was the beginning of spring. This is the so-called Zhan Hou, which is usually called night astrology. Its source has little to do with the Gregorian calendar, but the results are consistent because they are all based on the calculation of the earth's revolution.