It’s summer in the Northern Hemisphere. But as you enjoy long, balmy days at the beach or out in nature, you may be surprised to learn that our planet is slowly but surely getting closer to the sun, a point known as aphelion.
Here’s what you need to know about this celestial event that takes place every year as summer begins.
What causes aphelion and when does it happen?
Earth reaches aphelion every year in July, and this year it will occur on Friday at 1:06 a.m. Eastern Time.
The reason Earth has an aphelion is because its orbit is elliptical rather than circular. According to Kirby Runyon, a geologist at the Planetary Science Institute, all the planets in the solar system orbit the sun in elongated circles, rather than perfect ones. And that’s probably true of worlds around other stars, too.
The culprit for all these elliptical orbits is gravity.
“All the planets tend to push around each other,” taking their orbits away from perfect circles, Dr. Runyon said. “It’s literally a chaotic tug-of-war between the tiny amounts of gravitational influence the planets have on each other.”
Jupiter has 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, is between 129 and 155 million miles from the sun. Pluto, whose distance from the sun ranges 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 just 0.017. “Earth’s orbit is fairly circular,” says Larry Wasserman, an astronomer at Lowell Observatory in Flagstaff, Arizona. “If you were to draw it to scale on a piece of paper, you probably wouldn’t notice that it’s flattened at all.”
How far are we from the Sun at aphelion?
At aphelion, the Earth is about 94.5 million miles away from the Sun. Six months later, in early January in the winter, the Earth is at its closest point to the Sun at 91.5 million miles. This location is known as perihelion.
Three million miles may seem like a lot from the ground, but on an astronomical scale, it’s not much. The size of the sun in the sky appears about 4 percent smaller at aphelion than at perihelion, an effect too small to notice without precise instruments, Dr. Wasserman said.
Does aphelion affect Earth’s temperature?
A common misconception is that the Earth’s varying distance from the sun is what causes the seasons. It does have a small impact: we get 7 percent less sunlight at aphelion compared to the amount we’re exposed to at perihelion, leading to slightly milder summers and winters in the Northern Hemisphere.
But that effect is counteracted by the tilt of the Earth’s axis, which means that the hemispheres tilt toward or away from the sun at different points along the Earth’s orbit.
During aphelion, which occurs just a few weeks after the solstice, the northern half of the planet leans toward the sun, making summer days longer and warmer, even though Earth is farther away.
And during perihelion in January, the Northern Hemisphere tilts away from the sun, causing days to become shorter and temperatures to become colder.
In the Southern Hemisphere, this effect is reversed. Because the hemisphere is tilted away from the sun when the Earth is at aphelion, southern winters are slightly cooler than they would be if our orbit were perfectly circular. And as the planet approaches perihelion in January, the hemisphere is tilted toward the sun, making southern summers slightly warmer.
For planets with more exaggerated eccentricities, the changing distance can have a greater impact. For example, sunlight on Mars can vary as much as 31 percent along its orbit.
It is a coincidence that the Earth reaches aphelion just before its tilt toward the sun is greatest. And this will eventually change, as other planets in the solar system will gravitationally pull and compress Earth’s orbit in the future. The eccentricity is currently decreasing, meaning that the path around the sun is becoming increasingly circular.
What would happen if there was no aphelion?
If our planetary orbit were a perfect circle, the seasons would last 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 we could somehow snap our magic fingers and make Earth’s orbit more circular, it would probably be fine,” Dr. Runyon said.
But if something were to happen that caused the Earth’s orbit to become more eccentric, the consequences could be catastrophic. The seasons in the southern hemisphere would become too extreme: summers would be unbearably hot and winters unbearably cold. This could lead to crop failures and frost.
“If things got really bad,” Dr. Runyon said, “an advanced civilization wouldn’t be possible.”
For now, be grateful that our planet is in a favorable position.