In the 24 solar terms, there are fifteen or sixteen days every other day.

In ancient times, the period from winter solstice to winter solstice (about 365.25 days), that is, the tropic of cancer, was usually divided into 24 segments (each segment was about 65438+5 days at 2: 05), and each segment started from a solar term, namely: winter solstice, slight cold, severe cold, beginning of spring, rain, fright, vernal equinox and Qingming.

Among the 24 solar terms, winter solstice, severe cold, rain, vernal equinox, Grain Rain, Xiaoman, summer solstice, summer heat, summer heat, autumn equinox, first frost and light snow are neutral, which are usually used to determine the month. The winter month of solstice is winter month, the month of severe cold is the twelfth month, the month of rainy season is the first month, the month of vernal equinox is February, ... the month of light snow is October, and it returns to the month of winter solstice.

The interval between gas days is about 5.25 hours on the 30th, so the interval between gas days (including before and after) is 30 or 3 1 day. And a month's time is 29 or 30, and a month is either neutral or not. So neutral gas can directly determine the month and leap, and the month without neutral gas is the leap month of the previous month.

Reference: 24 solar terms Baidu Encyclopedia

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