"Annual Minimum Sun" Xiaoshu Appears in Orbit | Earth | Sun
On the 7th, we will welcome the Xiaoshu solar term. At 4:7 pm on the same day, the Earth will reach the aphelion of its orbit, which is the farthest time of the year for the Earth from the Sun. People will see a "minimum sun of the year". Why is the distance between the Earth and the Sun so close and so far? Will the weather become cooler when the sun is far away? Astronomical science popularization experts from the Chinese Academy of Sciences will reveal your secrets.
Wang Kechao, the science popularization director of the Purple Mountain Observatory of the Chinese Academy of Sciences, introduced that the orbit of the Earth around the sun is not a circle, but an ellipse with an eccentricity of 0.0167. On this elliptical orbit, the closest point to the sun is called the perihelion, and the farthest point is called the aphelion.
The distance between the sun and the earth is not fixed either. The average distance between the sun and the earth is about 149.6 million kilometers, with 152.1 million kilometers at aphelion and 147.1 million kilometers at perihelion, with a difference of 5 million kilometers between the two. The Earth passes through perihelion in early January and aphelion in early July. Therefore, in the summer of the Northern Hemisphere, the distance between the sun and the earth is relatively far.
This year, the time when the Earth moved to aphelion was 4:07 am on July 7th. At this time, the apparent diameter of the Sun was 31 arcminutes and 28 arcseconds, which is approximately 3.4% smaller than the visible diameter of the Sun when the Earth passed through perihelion on January 5th this year, making it the "smallest Sun of the year".
"Although the Earth is far from the Sun, it does not mean that the weather will become cooler. On the contrary, after a small summer heat, it will soon enter the hottest season of the year, which is about to enter the dog days." Wang Kechao said.
Wang Kechao explained that the change in distance between the sun and the earth is not the main reason for seasonal changes, and the changes in four seasons are mainly influenced by the surface receiving solar radiation. Before and after the summer solstice, the sun in the Northern Hemisphere has a large altitude angle and long sunshine duration during the day, receiving more radiation energy. "Considering the cumulative effect of surface heat balance, generally after the summer solstice, the heat absorbed by the surface is still greater than the heat released, and the surface temperature will continue to rise," said Wang Kechao.
