The "smallest sun of the year" appeared in the sky today, at 15:11!

At 15:11 on the 4th, the Earth reaches its orbital aphelion, and people will see the "smallest sun of the year" at this time.

Why is the sun sometimes "big" and sometimes "small"? Why is the weather hotter when the sun is farther away? Astronomy experts from the Chinese Academy of Sciences reveal the secrets.

Wang Kechao, the director of science popularization at the Purple Mountain Observatory of the Chinese Academy of Sciences, said that the orbit of the Earth around the Sun is not a perfect circle, but an ellipse with an eccentricity of 0.0167. Because of this, the distance between the Sun and the Earth is not fixed, and the aphelion is about 5 million kilometers farther than the perihelion.

Every year, the Earth passes through perihelion in early January and aphelion in early July, which correspond to the times when the sun's apparent diameter is the largest and smallest in the year respectively.

This year, the Earth reached its aphelion at 15:11 on July 4. At this time, the apparent diameter of the Sun was 31 minutes and 28 seconds, which can be called the "smallest Sun of the year", about 3.4% smaller than the apparent diameter of the Sun when the Earth passed its perihelion on January 4 this year.

The elliptical orbit also causes the unevenness of the annual apparent motion of the sun. The earth's speed is slowest when it passes through the aphelion and fastest when it passes through the perihelion. The lengths of the intervals between seasons are also unequal. For example, it takes 178 days, 20 hours and 12 minutes from the autumnal equinox in 2021 to the spring equinox in 2022, while it takes 186 days, 9 hours and 31 minutes from the spring equinox in 2022 to the autumnal equinox in 2022.

This year, the hottest period of the year is approaching on July 16. Why is the weather the hottest when the sun is farther away?

Wang Kechao explained that the change in the distance between the sun and the earth is not the main reason for the change of seasons. The change of seasons is mainly affected by the obliquity of the ecliptic. Due to the existence of the obliquity of the ecliptic, the direct point of the sun on the earth moves between 23 degrees and 26 minutes north and south latitude. In summer, the northern hemisphere has a high angle of sunlight, a long illumination time, and receives more radiation energy, so it is the hottest.

Source: Jointly produced by Xinhua News Agency Domestic Department and Xinhua News Agency Jiangsu Branch, produced by Starry Sky Studio