It’s 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 need to know about this celestial event that happens every year as summer gets underway.
What causes aphelion and when does it occur?
Earth reaches aphelion every July, and this year it happens on Friday at 1:06 a.m. ET.
The fact that the Earth has an aphelion is a result of its elliptical rather than circular orbit. According to Kirby Runyon, a geologist at the Institute of Planetary Sciences, all the planets in the solar system travel in elongated circles around the sun, rather than perfect ones. And it is most likely true of worlds around other stars as well.
The culprit for all these elliptical orbits is gravity.
“All the planets tend to rock each other around,” pulling their orbits out of perfect circles, Dr. Runyon. “It’s literally this chaotic tug of war between the tiny amounts of gravitational influence that the planets have on each other.”
Jupiter exerts the greatest 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 pronounced: Mars, with an eccentricity of 0.094, ranges from 129 to 155 million miles from the sun. Pluto, whose distance from the sun varies from 2.8 to 4.5 billion miles, is even stranger at 0.244.
On the other hand, our home planet has an eccentricity of only 0.017. “Earth’s orbit is quite circular,” said Larry Wasserman, an astronomer at the Lowell Observatory in Flagstaff, Ariz. “If you drew it on a piece of paper to scale, you probably wouldn’t notice that it was slightly flattened.”
How far are we from the sun at aphelion?
At aphelion, Earth’s distance from the sun is about 94.5 million miles. Six months later, in early January in the winter, Earth is at its closest point to the sun at 91.5 million miles. This place is known as perihelion.
From the ground, three million miles may seem like a lot, but it’s not much on an astronomical scale. The size of the sun in the sky appears about 4 percent smaller at aphelion than at perihelion, an effect that is too small to notice without precise instruments, said Dr. Wasserman.
Does aphelion affect temperatures on Earth?
A common misconception is that Earth’s varying distance from the sun is what creates the seasons. It has a small impact: we receive 7 percent less sunlight at aphelion compared to the amount we are exposed to at perihelion, leading to slightly milder summers and winters in the Northern Hemisphere.
But this effect is offset by the Earth’s tilt on its axis, which means that at various points along its orbit the hemispheres tilt either toward or away from the sun.
At aphelion, which occurs just a few weeks after a solstice, the northern half of the planet is tilted toward the sun, resulting in the longest and hottest summer days even though Earth is further away.
And at perihelion in January, the northern hemisphere tilts away from the sun, making the days shorter and temperatures cooler.
In the southern hemisphere, this influence 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 hemisphere tilts toward the sun, making southern summers slightly warmer.
For planets with more exaggerated eccentricities, the change in distance can have a greater impact. Sunlight on Mars, for example, can vary by as much as 31 percent along its orbit.
It is a coincidence that the Earth reaches aphelion near the time when its tilt towards the sun is greatest. And that will eventually change, as other planets in the solar system complain and gravitationally squeeze Earth’s orbit in the future. Its eccentricity is currently decreasing, meaning its path around the sun is becoming more circular.
What would happen if there was no aphelion?
If our planetary orbit were a perfect circle, the seasons would be exactly the same length—right now, spring and summer are a few days longer than fall and winter in the Northern Hemisphere—but not much else would. to move. “If, somehow, we snapped our magic fingers and the Earth’s orbit became more circular, maybe that would be good,” said Dr. Runyon.
But if something causes Earth’s orbit to become more eccentric, the consequences could be catastrophic. Seasons in the southern hemisphere would become very extreme – summers would be extremely hot and winters would be extremely cold. This can lead to crop failures and freezing.
“If it got really bad,” said Dr. Runyon, “advanced civilization would not be possible.”
For now, be thankful that our planet is in a sweet spot.