At 8:38 on January 3, the earth reaches its orbital perihelion. This is the moment when the earth is closest to the sun this year. At that time, people will see the "maximum sun" in 2024. Why is the weather so cold when the sun is closer to the earth? Why is the sun sometimes "big" and sometimes "small"? Astronomy popularization experts from the Chinese Academy of Sciences reveal the secrets for you.
Wang Kechao, director of science popularization at the Purple Mountain Observatory of the Chinese Academy of Sciences, said that the earth revolves around the sun in an elliptical orbit. Because of this, the distance between the sun and the earth is not fixed. The position closest to the sun is called perihelion, and the position farthest from the sun is called perihelion. The aphelion and aphelion are about 5 million kilometers further than the perihelion. Every year, the Earth passes through perihelion in early January and aphelion in early July, corresponding to the times of the year when the apparent diameter of the sun is largest and smallest, respectively.
This year, the earth will reach perihelion at 8:38 on January 3. The distance between the sun and the earth is 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 solar surface that people can see is the largest this year. The sun at this time can be called the "largest sun of the year", and the apparent diameter of the sun is about 3.4% larger than when the earth passed the aphelion on July 5 this year.
Three days after the "biggest sun of the year" appears, January 6th is the Xiaohan solar term, and we will usher in the coldest period of the year. Why is the weather colder when the sun is closer to the earth? Wang Kechao explained that changes in the distance between the sun and the earth are not the main cause of seasonal changes. The earth's axis is tilted as it orbits the sun, causing the sun's direct point on the earth to move regularly between the Tropic of Cancer and the Tropic of Cancer, forming the four seasons. For the northern hemisphere, when the earth reaches perihelion, the direct point of the sun is in the southern hemisphere. At this time, the sunlight shining on the ground in the northern hemisphere has a large inclination angle, the sunshine time is short, and the overall solar heat received is less, so the temperature is lower and the weather is cold. When the earth reaches aphelion, the direct point of the sun is in the northern hemisphere. Although the distance between the sun and the earth is far away at this time, the ground can receive nearly direct sunlight, and the sunshine time is long. The overall solar heat received is more, and the temperature is also relatively high. high.
Interestingly, changes in the distance between the sun and the earth will affect the speed of the earth's revolution around the sun, which makes the length of different seasons on the earth different. When passing through perihelion, the Earth's revolution speed is faster than when passing through aphelion. In the Northern Hemisphere, summer is longer than winter.
Source: Xinhua News Agency
(Source: Xinhua News Agency)
