The formula of nine is circulated among the people.

The full text of Jiujiu Song is: 1929 without guns, walking on the ice in 3949, watching willows along the river in 59 and 69, freezing the river in 79, swallows coming in 89, adding 19 in 99, and plowing cattle everywhere.

The ninth day after the winter begins.

Jiujiu Song is the crystallization of the wisdom of working people in China. As for its origin and who did it, there is no way to verify it. The earliest historical written record is in the Chronicle of Jingchu Years, written by Zong Gu in the Liang Dynasty: "The number of days from winter to day, 998 1 day, is cold." People count 998 1 day from the winter solstice to calculate the degree of cold in winter. )

Extended data:

During the winter solstice, the daytime is the shortest, the angle of direct sunlight on the ground is the smallest, and the sunshine intensity per unit area on the ground is also the smallest, so the ground gets the least solar heat every day, and the daily heat budget deficit on the ground has not reached the maximum.

On the solstice after winter, although the day is growing day by day, the growth rate is very slow. From the winter solstice to the "March 9th" period, the average daily growth rate is only about one minute, and the direct angle between the sun and the ground is slowly increasing. The daily heat budget of the ground is still in a deficit state, so it continues to lose heat and the ground temperature continues to drop.

The deficit of total heat budget on the ground only reached the maximum in April and September, and the temperature was mainly affected by the ground temperature, resulting in the lowest temperature in March and April. In addition, around March 9, the cold air mass in Siberia also developed to a very strong stage, and the frequent invasion of strong cold air is also one of the reasons for the low temperature.