The vernal equinox is summer to Sunday, autumn to Sunday, winter to Sunday, but the solar terms in China use the lunar calendar. Why is it the same every year?

The current Gregorian calendar in China is derived from the Roman Jules calendar, which is a solar calendar. There are 365 days every year, which is only about 6 hours away from the earth's revolution time of 365 1/4 days. So every four years, every day can basically coincide with the solar cycle.

However, China's summer calendar, that is, the lunar calendar, is a combination of yin and yang. The period of the moon is determined by the moon, and the day and year are determined by the rotation and revolution of the earth. In this way, the problem of the coincidence of months and years arises, and the summer calendar is solved by adding a few years to the moon.

The ancients observed the relationship between farming season and season and formulated 24 solar terms. Everything grows on the sun. Of course, the farming season is formulated according to the law of the earth's revolution, but it is impossible to formulate according to the law of the moon's revolution around the earth. Therefore, the 24 solar terms are evenly distributed on the ecliptic line and cannot be determined by the lunar month and day.