The origin of winter solstice.

Winter solstice is one of the "24 solar terms". "Twenty-four solar terms" and "December construction" are the basic contents of the calendar, which were concluded in ancient times (or ancient times). According to ancient records, Emperor Tiandi began to determine his age in the name of cadres and branches. "Twenty-four solar terms" is a special festival in the calendar, indicating the season, phenology, climate change and the establishment of "December construction". The ancient "Twenty-four solar terms" were determined by the direction of the top of the Beidou bucket handle. One-year-old is divided into "Twelve Chens" in the official calendar (built in December). "Building" represents the direction of the top of Beidou bucket handle, and bucket handle turns to "Twelve Chens" in turn. In traditional culture, the branch is linked with the time position of gossip, and the yin position is the "root position" of gossip the day after tomorrow. It is the position at the end of the year and the beginning of the year, representing the end and the beginning, such as Yi Shuo Gua Zhuan: "The root, the divination in the northeast, is also the beginning of everything." Therefore, bucket handle starts from the north of due east (Yin position, gossip position the day after tomorrow) and makes a circle through the south, west and north, which is a cycle. It began in the beginning of spring and ended in the great cold. When bucket handle points to the "lower" position due north, it is the solstice winter solstice.

In the Western Han Dynasty, the names of the twenty-four solar terms first appeared in the document "Tian Zi Xun in Huainan". During the period of Emperor Wu of the Han Dynasty, twenty-four solar terms were absorbed into the calendar as a supplement to the calendar guiding farming, and the division of solar terms adopted the method of "balancing qi" (that is, the average time method). The "leveling method" is to determine the shortest day in the Yellow River basin as the winter solstice by photogrammetry, divide the date between the winter solstice and the next winter solstice into twelve equal parts, which is called "Zhongqi", and then divide the dates between adjacent "Zhongqi" into equal parts, which is called "solar terms"; On average, there is a "neutral gas" and a "solar term" every month, which are collectively called "24 solar terms". "leveling method" is a time average method, and the interval of each solar term is about 15 days. The uneven motion of the sun on the ecliptic is not considered in the calculation. Using the method of "balancing qi" to divide the solar terms starts from the solstice of winter and ends with heavy snow.

The current "twenty-four solar terms" come from the "fixed gas law" concluded more than 300 years ago. The method of "fixing qi" is a method to determine solar terms according to the position of the sun on the regression ecliptic, that is, on a 360-degree circle "ecliptic" (the apparent path of the sun on the celestial sphere for one year), it is divided into 24 equal parts, each equal part is 15, with spring as the starting point of 0 degree (but beginning of spring still ranks first in the ranking), and arranged according to the degree of the ecliptic. Every time the sun moves 15 degrees on the ecliptic, it is a "solar term", and each "solar term" has the same degree and different time. Therefore, the 24 solar terms are 24 time points, and the specific date of the "point" is the natural result of celestial movement. The solar terms divided by the method of "fixing qi" began in the beginning of spring and ended in the great cold. The solstice in winter is when the sun moves to 270 degrees of the yellow meridian.