Behind elliptical orbits lies gravity. “All the planets tend to jostle each other around,” pulling their orbits from perfect circles, Runyon said. “It’s literally this chaotic tug of war between small amounts of gravitational influence that the planets have on each other.”
Earth reaches aphelion every July. This year it did so on Friday.
That Earth has an aphelion is a result of its orbit being elliptical, rather than circular. According to Kirby Runyon, a geologist at the Planetary Science Institute, all planets in the solar system travel in elongated circles around the Sun, rather than perfect ones. This is most likely true for worlds around other stars, too.
Behind elliptical orbits lies gravity. “All the planets tend to jostle each other around,” pulling their orbits from perfect circles, Runyon said. “It’s literally this chaotic tug of war between small amounts of gravitational influence that the planets have on each other.”
Jupiter exerts the most influence because it is the most massive planet in our solar system, he added.
How much an orbit deviates from a perfect circle is measured by its eccentricity. The higher the eccentricity, the more elliptical the orbit. For some bodies in the solar system, eccentricity is quite pronounced: Mars has an eccentricity of 0.094. Pluto is even more eccentric at 0.244. On the other hand, Earth’s eccentricity is 0.017.
How far is the Earth from the Sun at aphelion?
At aphelion, Earth’s distance from the Sun is about 152.1 million km.
Six months later, in early January, Earth reaches perihelion — the point at which it is closest to the Sun. At perihelion, the distance between Earth and the Sun is roughly 147.1 million km (see illustration).
Does aphelion affect temperatures on Earth?
A common misconception is that Earth’s varying distance from the Sun is what gives rise to the seasons.
We get 7% less sunlight at aphelion compared to what we are exposed to at perihelion, leading to slightly milder summers and winters in the Northern Hemisphere. But this effect is offset by Earth’s tilt on its axis — at different points along its orbit the hemispheres slant either toward or away from the Sun.
What would happen if there were no aphelion?
If our planetary orbit were a perfect circle, the seasons’ lengths would be exactly the same — right now, spring and summer are a few days longer than fall and winter in the Northern Hemisphere. But not much else would change.
“If, somehow, we snapped our magic fingers and Earth’s orbit became more circular, it’d probably be fine,” according to Runyon.
But if something made Earth’s orbit grow more eccentric, the consequences could be catastrophic. Seasons in the Southern Hemisphere would become too extreme — summers would be unbearably hot, and winters would be intolerably cold. This could lead to crop failures and freezes.
“If it got really bad,” Dr Runyon said, “advanced civilisation would not be possible.” For now, be thankful our planet is in a sweet spot.