At 8:38 a.m. on January 3, the Earth will orbit to perihelion, the closest time of the year to the Sun, when people will see the "maximum Sun" of 2024. Why is the sun close to the earth, but the weather is very cold? Why is the sun "big" and sometimes "small"? Experts from the Chinese Academy of Sciences in astronomy will reveal the secret for you.
Just today, the "biggest sun of the year" came →
Wang Kechao, director of science popularization at the Purple Mountain Observatory of the Chinese Academy of Sciences, introduced that the earth revolves around the sun in an elliptical orbit, and because of this, the distance between the sun and the earth is not fixed, the closest position to the sun is called perihelion, and the farthest position from the sun is called aphelion, which is about 5 million kilometers farther than perihelion. The Earth passes through perihelion in early January and aphelion in early July, corresponding to the time of year when the apparent diameter of the Sun is the largest and the smallest, respectively.
This year, the Earth orbited to perihelion at 8:38 on January 3, with a distance of about 0.983 astronomical units, or about 147 million kilometers. At this time, the apparent diameter of the Sun is 32 arc minutes and 31 arc seconds, and the sun surface is the largest of the year. At this time, the Sun is the "largest Sun of the year", and the apparent diameter of the Sun is about 3.4% larger than when the Earth passed aphelion on July 5 this year.
Three days after the "biggest sun of the year" appears, January 6 is the Xiaohan solar term, and we will usher in the coldest period of the year. Why is the sun closer to the earth, but the weather is cold? Wang Kechao explained that the change in the distance between the sun and the earth is not the main reason for the seasonal change. The Earth's axis is tilted as the Earth revolves around the Sun, which allows the Sun's direct point on the Earth to move regularly between the Tropic of Cancer, forming the seasons. For the Northern Hemisphere, when the Earth reaches perihelion, the direct point of the sun is in the southern hemisphere, and the sunlight on the ground in the northern hemisphere has a large inclination, short sunshine time, and less solar heat received overall, so the temperature is low and the weather is cold. When the earth travels to aphelion, the direct point of the sun is in the northern hemisphere, although the distance between the sun and the earth is far, the ground can receive close to direct sunlight, and the sunshine time is long, the overall reception of solar heat is more, and the temperature is also higher.
Interestingly, the change in the distance between the Sun and the Earth affects the speed at which the Earth orbits the Sun, which makes the length of the different seasons on the Earth different. When passing perihelion, the Earth revolves faster than when it passes aphelion. In the Northern Hemisphere, summers are longer than winters.
