Winter Solstice is the 22nd solar term, and the most important one. It was set according to astronomy, called “short day (ri duan)” or “the shortest day” in ancient times. On that day, the sun reaches 270 degrees of longitude, and shines directly on the Tropic of Capricorn. The day time turns shortest on that day. The ancient people recorded their knowledge of the day in some books. In the Collection of the Seventy-two Hou, it says, “on the 15th day of the Eleventh month, the hidden qi reaches its peak.” “The Winter Solstice comes on the fifteenth day after the Major Snow. It is in the middle of the eleventh month. The yin goes comes to an end and the yang begins. The sun goes to the south. From then on, the days become longer.” (Tong Wei of Qing Dynasty)

There are three hous of the Winter Solstice: “in the first hou, earthworms curl.” It is said the earthworms curl in the yin and extend in the yang. Though the yang has begun, the yin is still very powerful. Thus the earthworm is still curling. “In the second hou, the horns of Mi disappear.” Mi are like deer, but they belong to yin, while deer belong to yang. The ancient Chinese believed that because the horns of Mi grow backward, it belongs to the yin. During the Winter Solstice, the yang begins, so the horns gradually disappear. “In the third hou, the spring flows.” As the yang begins, the water of springs in mountains begins to flow and become warm.

On the day of the Winter Solstice, the duration of sunshine is shortest and the heat dissipated from the ground is more than the heat it receives. Starting from the Winter Solstice, the coldest month of the winter begins in China. The Chinese make nine days into a clump, and around the third nine-day, it is the coldest.

Astronomically, the Winter Solstice is the beginning of winter. During this period, the average temperature in the northwest areas of China is below zero degrees Celsius, and it is about 6 or 8 degrees Celsius. In the low-altitude valley of the southwest, however, the average temperature is above 10 degrees Celsius even in the coldest month. In that area, the spring comes after autumn and there is no winter at all.