The winter solstice sun is farthest from our China. Why is it not the coldest at this time, and why is it cold on March 9th?

First, correct: The winter solstice is not the farthest from us, but the shortest sunshine time.

the winter solstice is one of the twenty-four solar terms and one of the most important solar terms. The winter solstice is a solar term divided by astronomy, which is called "short day" and "short day" in ancient times. On the winter solstice, the sun is located at 27 degrees of the yellow meridian, and the sunlight is almost direct to the tropic of Capricorn, which is the shortest day in the northern hemisphere. Correspondingly, the daytime in the southern hemisphere is the longest in the whole year on the winter solstice.

Second, this day is not the coldest, because:

The orbit of the earth around the sun is not a perfect circle, but an ellipse with an approximate circle. The sun is at one focus of the ellipse (the ellipse has two focuses), so the position of the earth relative to the sun can be divided into perihelion (near the sun) and apohelion (far from the sun).

Because the earth axis (the central axis of the earth) is not perpendicular to the orbital plane (with an included angle of 23 degrees and 36 minutes), the angle between the earth and the sun is different during its revolution (one revolution is one year), so the temperature varies in different places in a year, which leads to the change of seasons and the length of days.

However, when the northern hemisphere (us) is in summer, the earth is at the apohelion; However, in winter, it is at the perihelion, so the summer solstice has the longest sunshine time, but it is not the hottest day, and the winter solstice has the shortest sunshine time, but it is not the coldest day.

Third, why is it cold on March 9th?

Before and after the winter solstice, the daytime is the shortest, the solar energy absorbed by the surface layer is the least, and the heat emitted at night is the most; But at this time, the heat accumulated in the surface layer in summer and autumn is still surplus, which can be compensated, so the temperature will not drop very low. By the "March 9th", the heat accumulated in the surface layer in summer and autumn has been exhausted, although it has increased during the day, but the heat absorption during the day can not offset the heat dissipation at night, and the income and expenditure are seriously upside down. In addition, before and after the "March 9th", the cold air in the north frequently went south, which made the weather particularly cold, so it was called "cold in the March 9th".