How is the time of solar terms determined? Why can it be accurate to what time?

24 solar terms has a special formula for calculating time, that is, multiplying the last two digits of the current year by a d value belonging to solar terms minus the number of leap years, and adding the obtained value to the c value of the century, thus the specific time of each solar term can be obtained.

Twenty-four Qi generally starts from beginning of spring, but in modern astronomy, the solar calendar is calculated from the vernal equinox. Solar term is a special festival in the trunk and branch calendar that expresses the change of natural rhythm and establishes "December construction" (monthly order).

The 24 solar terms are: beginning of spring, Rain, Sting, Equinox, Qingming, Grain Rain, Changxia, Xiaoman, Mangzhong, Summer Solstice, Xiaoxia, Daxia, beginning of autumn, Early Summer, Bailu, Autumn Equinox, Cold Dew, Early Frost, beginning of winter, Light Snow, Heavy Snow, Winter Solstice, Slight Cold and Severe Cold. The twenty-four solar terms are related to cadres and gossip and have a long history. In the historical development, the 24 solar terms have been incorporated into the lunar calendar and become an important part of the lunar calendar.

Solar term month:

"Twenty-four solar terms" and "Twelfth Construction" are the basic contents of the cadre calendar, and the Twelfth Construction is a solar month based on the twenty-four solar terms. Taking the direction pointed by the Big Dipper bucket handle as the standard for determining the month is called sword fighting. Bucket handle rotates clockwise once, which is called "one year old". Every year at the age of 18, bucket handle rotates, pointing to the twelve days in turn, which is called "December Construction".

The buildings in December are: Yin in the first month, Mao in February, Chen in March, Si in April, Wu in May, Wei in June, Shen in July, You in August, Xu in September, Hai in October, Zi in November and Ugly in December.

The division of "December Construction" is based on solar terms, namely, beginning of spring, Jingzhe, Qingming, Changxia, Mangzhong, Xiaoshu, beginning of autumn, Bailu, Hanlu, beginning of winter, Daxue, and Xiaohan are regarded as silver moon, Uzuki, Chen Yue, Mitsuki, Noon, No Bud, Shen Yue, Moon and Xu Yue respectively.