Why do you eat jiaozi on the winter solstice? Which dynasty can the solar terms from the winter solstice date back to? When did the custom of eating jiaozi begin?

Many festivals in China are about eating jiaozi, so jiaozi became the representative of China's holiday food, and then became the representative of China's food culture. 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. During the Spring and Autumn Period, China observed the sun with soil gauge and determined the winter solstice, which was the earliest of the 24 solar terms. Jiaozi originated in ancient jiaozi, formerly known as Joule. According to legend, it was initiated by Zhang Zhongjing, a Chinese doctor, with a history of 1800 years.