Solar shadows and 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 taichu calendar written by Deng Ping 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 Huainanzi Astronomical Training, and the concepts of Yin and Yang, four seasons, eight places, twelve degrees and twenty-four solar terms were also mentioned in Historical Records Taishi Gongxu. 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. The reaction seasons are beginning of spring, Spring Equinox, Long Summer, Summer Solstice, beginning of autumn, Autumn Equinox, beginning of winter and Winter Solstice, also known as Eight; Phenological phenomena include fright, Qingming, Xiaoman and Miscanthus. Rain, Grain Rain, little summer, big summer, early summer, white dew, cold dew, first frost, light snow, heavy snow, slight cold and severe cold are the responses to climate change. According to the position of the sun on the ecliptic, modern people have accurately determined the specific time of the twenty-four solar terms: in spring, the sun in beginning of spring is located at 3 15 degrees of the ecliptic, on February 2-5, the sun in rainy days is located at 330 degrees of the ecliptic, on February 18-20, and the sun in thunder is located at 345 degrees of the ecliptic, on March 5-7. The Qingming Sun is located at 0/5 degrees of the Yellow Meridian, April 4-6, Grain Rain Sun at 30 degrees of the Yellow Meridian, April 19-2 1, Long Summer Sun at 45 degrees of the Yellow Meridian, May 5-7, Xiaoman Sun at 60 degrees of the Yellow Meridian, and May 20-22. On June 5-7, the summer solstice sun is located at longitude 90 degrees, June 2 1-22, summer sun at longitude 105 degrees, July 6-8, summer sun at longitude 120 degrees, and July 22-24, beginning of autumn autumn sun at longitude/kloc. On August 7-9, summer sun is at longitude 150, August 22-24, dew sun is at longitude 165, September 7-9, autumnal equinox sun is at longitude 180, September 22-24, and dew sun is at longitude/kloc-0. The first frost sun is at 2 10 degrees on1October 8-9, the beginning of winter sun is at 225 degrees on1October 23-24, and the Xiaoxue sun is at 240 degrees1October 7-8. 65438+February 6-8, winter solstice sun at meridian 270 degrees, 65438+February 2 1-23, slight cold sun at meridian 285 degrees, 65438+1October 5-7, great cold sun at meridian 300 degrees, 65438+. The C value of beginning of spring in this century is 4.475, and the date of beginning of spring in 20 17 is as follows: [2017× 0.2422+4.475]-[2017/4-15] = 492-489.