Solar term length

The earth's orbit around the sun is called the "tropic year". The actual length of a tropical year is 365.2422 days, and the length of a Gregorian calendar year (tropical year) is 365 days, so there will be 0.2422 days more each year, and the specific time of solar terms will be "delayed" by 0.2422 days each year.

The twenty-four solar terms are divided according to the position of the sun on the ecliptic (that is, the orbit of the earth around the sun). Depending on the sun starting from the vernal equinox (longitude zero, at this moment the sun shines vertically on the equator), every advance of 15 degrees is a solar term; After a week's operation, I returned to the vernal equinox, which is a tropical year with 360 degrees, so it is divided into 24 solar terms. The dates of solar terms are relatively fixed in the solar calendar. For example, beginning of spring in the solar calendar always falls between February 3rd and 5th.

In ancient times, a year was divided into twelve months, and each month had two solar terms. The former is solar terms, while the latter is neutral. For example, beginning of spring is the first month's festival, and it rains in the first month. Later generations called solar terms and neutral gas solar terms.

So 1 "solar term" time 15 days is just a general statement. More precisely, the difference between adjacent solar terms is:

365.2422/24 =15.2184 days.

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