This Friday, Earth will move more slowly than the rest of the year, as it will be at aphelion, the farthest point in its orbit around the Sun. Earth passes through aphelion every year, and to complete its orbit of about 940 million kilometers, the planet slows down compared to the annual average. Scientist Alfred Rosenberg, from the Canary Islands Institute of Astrophysics (IAC) in Spain, explains why this phenomenon occurs and whether it impacts the seasons and climate.
The Earth completes an orbit of about 940 million kilometers at an average speed of 30 km/s. This motion is known as translation. The Earth’s orbit is not circular; it is elliptical, which causes variations in orbital speed: sometimes slower, sometimes faster. At perihelion (which in 2024 occurred on January 3), the distance from the Sun is 147 million kilometers.
At aphelion, the distance from the Sun increases to 152 million kilometers. According to Kepler’s second law, the Earth moves more slowly when it is farther from the Sun (aphelion) and more quickly when it is closer (perihelion). At aphelion, the Earth’s speed will be just over 29 km/s, or 1 km/s slower than at perihelion. What is this change in speed equivalent to? For example, the approximate distance of 500 km between Santiago and Concepción would be covered in 17 seconds at Earth’s speed. It would take half a second longer to cover this distance at aphelion than at perihelion. This effect means that, for example, summer in the Northern Hemisphere is five days longer than winter due to differences in orbital speeds.
What about other planets like Mercury? Mercury has a more noticeable variation in its orbit: from 70 million kilometers at aphelion to 46 million kilometers at perihelion. Mercury’s speed varies from nearly 40 km/s at aphelion to nearly 60 km/s at perihelion. The apparent size of the Sun on Mercury changes considerably due to this orbital variation. While the Sun normally appears four times larger from Mercury’s surface, it appears ten times larger during aphelion.
What about the seasons?
Two factors are key: the distance from the star and the tilt of the planet’s axis of rotation. In Mercury’s case, the tilt is zero, so the distance from the Sun primarily defines the temperature. However, on Earth, the tilt of the axis of 23.5 degrees is the main factor in defining the seasons, not the distance from the Sun. This is why the seasons are reversed in the two hemispheres.
Does this make the seasons in one hemisphere more extreme than the other? Although one might think that being closer to the Sun, the seasons in the southern hemisphere would be more extreme than those in the northern, since the minimum distance from the sun coincides with its summer season. C
However, this is not the case. Weather and climate are complex systems influenced by a variety of factors. An additional important factor is land and ocean cover, which has a large influence on climate. The land surface is about twice as large in the Northern Hemisphere as in the Southern Hemisphere, causing greater warming in the north. During aphelion, regions between 20 and 25 degrees north latitude will receive more direct solar radiation, favoring maximum warming in these areas.
Earth will move more slowly this Friday: Why does this phenomenon occur and what impact does it have (emol.com)
Similar articles