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What Is Aphelion? Earth Reaches Its Greatest Distance From the Sun on Friday

It’s summer in the Northern Hemisphere. But while you indulge in long, balmy days at the beach or elsewhere in nature, you may be surprised to learn that our planet is creeping toward its greatest distance from the sun, a point known as aphelion.

Here’s what to know about this celestial event that happens every year as the summer is underway.

Earth reaches aphelion every July, and this year it occurs on Friday at 1:06 a.m. Eastern time.

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. And it’s most likely true for worlds around other stars, too.

The culprit for all of these elliptical orbits is gravity.

“All the planets tend to jostle each other around,” pulling their orbits from perfect circles, Dr. 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, this is quite pronounced: Mars, with an eccentricity of 0.094, ranges from 129 to 155 million miles away from the sun. Pluto, whose distance from the sun varies from 2.8 to 4.5 billion miles, is even more eccentric at 0.244.

On the other hand, our home planet has an eccentricity of only 0.017. “Earth’s orbit is fairly circular,” said Larry Wasserman, an astronomer at Lowell Observatory in Flagstaff, Ariz. “If you drew it on a piece of paper to scale, you probably wouldn’t notice it was slightly flattened.”

At aphelion, Earth’s distance from the sun is about 94.5 million miles. Six months later, at the start of January in the winter, Earth is at its closest point to the sun at 91.5 million miles. This location is known as perihelion.

From the ground, three million miles may seem like a lot, but it doesn’t amount to much on astronomical scales. 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 be noticed without precise instruments, Dr. Wasserman said.

A common misconception is that Earth’s varying distance from the sun is what gives rise to the seasons. It does have a small impact: We get 7 percent less sunlight at aphelion compared with the amount we are exposed to at perihelion, leading to slightly milder summers and winters in the Northern Hemisphere.

But that effect is offset by Earth’s tilt on its axis, meaning that at different points along its orbit the hemispheres slant either toward or away from the sun.

At aphelion, which occurs just weeks after a solstice, the northern half of the planet is leaning toward the sun, resulting in the longer, hotter days of summer even though Earth is farther away.

And at perihelion in January, the Northern Hemisphere tilts away from the sun, making the days shorter and the temperatures colder.

In the Southern Hemisphere, this impact is reversed. Because the hemisphere leans away from the sun when Earth is at aphelion, southern winters are a little cooler than they would be if our orbit were perfectly circular. Then as the planet approaches perihelion in January, the hemisphere’s lean toward the sun, making southern summers slightly warmer.

For planets with more exaggerated eccentricities, the changing distance can have a bigger impact. Sunlight on Mars, for example, can vary as much as 31 percent along its orbit.

It is a coincidence that Earth reaches aphelion close to when its tilt toward the sun is greatest. And this will eventually change, as other planets in the solar system gravitationally yank and squeeze Earth’s orbit in the future. Its eccentricity is currently decreasing, meaning its path around the sun is becoming more circular.

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 shift. “If, somehow, we snapped our magic fingers and Earth’s orbit became more circular, it’d probably be fine,” Dr. Runyon said.

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 civilization would not be possible.”

For now, be thankful our planet is in a sweet spot.

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