It is summer in the northern hemisphere. But while you’re enjoying long, balmy days at the beach or elsewhere in nature, you might be surprised to learn that our planet is creeping toward its greatest distance from the Sun, a point known as aphelion.
Here’s what you should know about this celestial event that happens every year when summer is in full swing.
What causes aphelion and when does it occur?
Earth reaches aphelion every July and this year on Thursday at 10:06 PM Pacific Time.
This aphelion of the Earth is a result of its orbit being elliptical rather than circular. According to Kirby Runyon, a geologist at the Planetary Science Institute, all the planets in the solar system travel in elongated circles around the sun, rather than perfect ones. And most likely this also applies to worlds around other stars.
The culprit behind all these elliptical orbits is gravity.
“All the planets tend to push each other,” Runyon said. “It’s literally a chaotic tug-of-war between the small amount of gravitational influence that the planets have on each other.”
Jupiter has the biggest 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, this is quite significant: Mars, with an eccentricity of 0.094, is 129 to 155 million miles from the Sun. Pluto, which ranges from 2.8 billion to 4.5 billion miles from the Sun, is even more eccentric to 0.244.
On the other hand, our home planet has an eccentricity of 0.017. “Earth’s orbit is quite circular,” said Larry Wasserman, an astronomer at the Lowell Observatory in Flagstaff, Arizona. “If you drew it on a piece of paper to scale, you probably wouldn’t notice that it’s slightly flattened.”
How far are we from the Sun at aphelion?
At aphelion, the Earth’s distance from the Sun is about 94.5 million miles. Six months later, in early January in winter, the Earth is at its closest point to the Sun at a distance of 91.5 million miles. This location is known as perihelion.
From the ground, 3 million miles may seem like a lot, but by astronomical standards, it’s not much. The size of the sun in the sky appears about 4% smaller at aphelion than at perihelion, an effect too small to be detected without precise instruments, Wasserman said.
Does aphelion affect temperatures on Earth?
A common misconception is that the changing distance of the Earth from the Sun is what causes the seasons. It has little impact: We get 7% less sunlight at aphelion compared to the amount we get at perihelion, leading to slightly milder Northern Hemisphere summers and winters.
However, this effect is offset by the tilt of the Earth on its axis, meaning that at various points in its orbit the hemispheres are tilted either towards or away from the Sun.
At aphelion, which occurs just a few weeks after the solstice, the northern half of the planet tilts toward the Sun, resulting in longer and warmer summer days even though Earth is further away.
And at perihelion in January, the Northern Hemisphere turns away from the Sun, making days shorter and temperatures cooler.
In the Southern Hemisphere, this impact is reversed. Because the hemisphere tilts away from the Sun when Earth is at aphelion, southern winters are slightly colder than they would be if our orbit were perfectly circular. Then, as the planet approaches perihelion in January, the tilt of the hemisphere toward the Sun causes the southern summers to be slightly warmer.
For planets with larger eccentricities, changing distance can have a greater impact. For example, sunlight on Mars can vary by as much as 31% along its orbit.
It is a coincidence that the Earth reaches aphelion near when its inclination to the Sun is greatest. And that will eventually change as the other planets in the solar system in the future gravitationally tug and compress Earth’s orbit. Its eccentricity is currently decreasing, meaning its orbit around the Sun is becoming more circular.
What would happen if there was no aphelion?
If our planetary orbit were a perfect circle, the lengths of the seasons would be exactly the same—right now, spring and summer are several days longer than fall and winter in the Northern Hemisphere—but not much else would change. “If we somehow snapped our magic fingers and the Earth’s orbit became more circular, it would probably be fine,” Runyon said.
But if something were to cause Earth’s orbit to become more eccentric, the consequences could be catastrophic. Seasons in the southern hemisphere would become too extreme – summers would be unbearably hot and winters unbearably cold. This could lead to crop failure and freezing.
“If it got really bad,” Runyon said, “advanced civilization wouldn’t be possible.”
For now, be thankful that our planet is in a sweet spot.