Why is the longitude 22.5 pointed by the sun designated as beginning of winter?

Beginning of winter is between165438+1October 7-8, that is, the sun is located at 225 degrees of the yellow longitude.

In ancient China, the sundial was measured by earth gauge, and the longest sun shadow was defined as "solstice" (also known as long day to day, that is, four images (lunar calendar), long day to day, and winter to day), and the shortest sun shadow was "short day to day" (also known as 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 (four images, namely four seasons) in Shang Dynasty, but it developed to eight (eight diagrams) in Zhou Dynasty, and 24 solar terms were completely established in Qin and Han Dynasties.

In this way, each of the 24 solar terms corresponds to a certain position where the sun reaches every movement 15 on the ecliptic. The ecliptic is the path of the sun through the celestial sphere in a year, and it crosses the celestial equator twice a year, once at the vernal equinox and once at the autumnal equinox. Because there is no obvious point on the ecliptic that can be regarded as the longitude of 0 degrees, the vernal equinox is arbitrarily designated as the position of 0 degrees of the ecliptic, and the longitude of the celestial body is the angular distance from the celestial body to the vernal equinox. The sun is located around the longitude of the Yellow River 225, and the precipitation in most parts of China has obviously decreased, and the weather has turned cold. This solar term was named beginning of winter, indicating the beginning of winter.