How to determine the exact date of the ancient solar terms in China?

24 solar terms divided the year into 24 equal parts according to the position of the sun on the ecliptic. That is to say, the ecliptic is divided into 24 equal parts, each equal part accounts for 15 of the ecliptic.

Because the time required for the sun to pass through each bisection is almost equal, and the Gregorian calendar dates of the 24 solar terms are roughly the same every year, the Gregorian calendar time of each solar term is relatively fixed.

China's traditional calendar is a combination of Yin and Yang, and the criterion is the movement of the sun (day, year and solar terms) and the movement of the moon (month).

In order to let our ancestors know the climate change in cold and summer, decide the progress of farming or serve as a reference for daily life, ancient historians stipulated that the time from the winter solstice of each year to the winter solstice of the next year in a tropical year should be divided into twelve equal parts, which is called Zhongqi; Then, each gas division is called solar terms, which is the source of 24 solar terms. The method of making this solar term is called "flat gas method".

However, because the earth's orbit around the sun is elliptical. Near the sun, the speed of the earth's revolution is faster, but far away, it is slower. For the Central Plains region where this calendar was made at that time, some solar terms could not reflect the real climate. Therefore, since the Qing Dynasty, the vernal equinox has been set at 0 degrees, and every time the sun runs 15 degrees on the ecliptic, it is set as a solar term or neutral atmosphere, so the 24 solar terms have 24 specific moments instead of 24 specific days. This method of making solar terms is called "fixing gas method".

Because the length of a year is determined by the movement of the sun, and the 24 solar terms are also determined by the position of the sun, basically the date of each solar term will not change much every year.

However, the actual length of a tropical year is 365.2422 days, and the Gregorian calendar year is 365 days, so there will be 0.2422 days (equivalent to 5.8 hours) every year, and the specific time of solar terms will be "delayed" by 0.2422 days every year; As a result, it accumulated to 0.9688 days in four years, almost one day. In order to correct it, there is a "leap year" system in the Gregorian calendar, and there will be one more day on February 29 every four years. If the date of a solar term is delayed from midnight to the next day or advanced to the previous day due to leap year, the date of the solar term will change, but the date difference will only be within three days.

Therefore, it is because there is a slight gap between the length of the tropic year and the actual calendar year, and the date of the Gregorian calendar year needs to be corrected by the actual length of the tropic year, so the solar term is a relatively fixed day.