Are the 24 solar terms determined by the solar calendar or the lunar calendar? Why is the lunar calendar date of a solar term different every year, and the solar calendar date is only one or two days apart?

Twenty-four solar terms originated in the Yellow River Basin. As early as the Spring and Autumn Period, four solar terms were set, namely, mid-spring, mid-summer, mid-autumn and mid-winter. After continuous improvement and perfection, by the Qin and Han Dynasties, the 24 solar terms had been completely established. In BC 104, the taichu calendar written by Deng Ping officially set 24 solar terms in the calendar, and defined the astronomical position of the 24 solar terms. In other words, the 24 solar terms were decided by the ancient people of China.

Since ancient times, the vernal equinox has been set at 0 degrees, and every advance 15 degrees is a solar term, and the running week is divided into 24 solar terms. Therefore, the date of solar terms in the solar calendar is basically fixed, with a difference of only one or two days, but the date in the lunar calendar changes greatly, so the date of a solar term is different every year.

Twenty-four solar terms are 360 degrees on Sunday in a tropical year, which is divided into 24 equal parts on average, so the apparent position of the sun in each solar term is very fixed. The time corresponding to the 24 solar terms in the Gregorian calendar is also relatively fixed, with a maximum difference of one or two days. If you look it up in the lunar calendar, it will be complicated. The lunar calendar is calculated by the profit and loss of the moon, with 30 days in the big month and 29 days in the small month. In a normal year, the lunar year is 354 days, while the tropic year in the solar calendar is 365 days. Lunar year is less than solar calendar 1 1 day.

So there will be an extra month every three years, and this month is a leap month. Take 2000 and 200 1 for example. In Chen Geng in 2000, there was no beginning of spring, and it rained on the 15th day of the first month. 1 1 26th of the month was the winter solstice. By 200 1 leap year, the rainfall was delayed to the 26th day of the first month, which was 1 1 day later than in 2000, because there was a leap April in the middle.

In the lunar calendar, the vernal equinox can be on February 1 day at the earliest, on February 30th at the latest, and on August 1 day at the latest, with a difference of one month. Therefore, the date of solar terms in the solar calendar is basically fixed, with a difference of only one or two days, but the date in the lunar calendar changes greatly, so the date of a solar term is different every year.