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.
Earth reaches aphelion every July, and this year it occurs 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.
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, Runyon said. “It’s literally this chaotic tug of war between small amounts of gravitational influence that the planets have on each other.”
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.
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% 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.)
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.
