The origin of solar terms

In ancient China, the sundial was measured by earth gauge, and the longest sun shadow was defined as "sun to day" (also called long day to day, long day to day, winter to day), and the shortest sun shadow was "short day to day" (also called short day to day and summer to day). One day in spring and autumn is the same length of day and night, which is designated as "vernal equinox" and "autumnal equinox". There were only four solar terms in Shang dynasty, but it developed to eight in Zhou dynasty and was completely established in Qin and Han dynasties. In BC 104, the calendar edited by Luo and Deng officially set 24 solar terms in the calendar, and defined the astronomical position of the 24 solar terms.

The names of the twenty-four solar terms first appeared in Tian Zi Xun of Huainan, and the concepts of Yin and Yang, four seasons, eight places, twelve degrees and twenty-four solar terms were also mentioned in Preface of Historical Records Taishi Gong. During the period of Emperor Wu of the Han Dynasty, Luo compiled solar terms into the calendar, stipulating that the month without neutral gas was the last month of leap month.

Each of the 24 solar terms corresponds to a certain position reached by the sun every time it moves 15 on the ecliptic. The twenty-four solar terms are divided into 12 solar terms and 12 solar terms, which correspond to each other one by one. The twenty-four solar terms reflect the annual apparent motion of the sun, so their dates are relatively fixed in the Gregorian calendar. The solar terms in the first half of the year are on the 6th, the solar terms in the second half of the year are on the 8th and the solar terms are on the 23rd, with a difference of less than 1 ~ 2 days.

The naming of the twenty-four solar terms reflects three seasons, phenology and climate changes. Beginning of spring, Spring Equinox, Changxia, Lixia Solstice, beginning of autumn, Autumn Equinox, beginning of winter and Winter Solstice, also known as Eight; Phenological phenomena are amazing, clear, rich and complex. Climate change is manifested in rainfall, Grain Rain, slight summer heat, severe summer heat, intense heat, white dew, cold dew, first frost, light snow, heavy snow, slight cold and severe cold.