By: Youheng Zhong

As the sweltering summer days unfold, many of us eagerly soak up the sun’s warm embrace or hide in fear of a horrible sunburn. But, while the sun is shining, a curious cosmic event took place overhead. This week, our planet Earth reached its greatest distance from the sun, a point known as aphelion.

“It’s a little counterintuitive, but the fact that we’re farthest from the sun during the summer months is all thanks to the tilt of the Earth’s axis,” explains Dr. Kirby Runyon, a geologist at the Planetary Science Institute. “While our distance from the sun does have a small impact, the primary driver of the seasons is that axial tilt, which causes the hemispheres to lean toward or away from our star.”

The reason that Earth’s distance from the sun does have impact on the seasons is because Earth’s orbit around the sun is not a perfect circle; it’s an elongated ellipse. This elliptical path results from a “chaotic tug of war” between the gravitational influences of the other planets, especially Jupiter.

“All the planets tend to jostle each other around, pulling their orbits from perfect circles,” says Dr. Runyon.

At aphelion, which occurred this year on July 4th at 1:06 AM Eastern Time, our planet reaches its farthest point from the sun, about 94.5 million miles away. In six months, Earth will be at the exact opposite point, its closest approach, called perihelion by astronauts, during the winter solstice. At this point, Earth will be only 91.5 million miles from our star.

“The difference in distance is only about 3 million miles, which is really not much on an astronomical scale,” notes Dr. Larry Wasserman, an astronomer at Lowell Observatory. “The size of the sun in the sky appears about 4% smaller at aphelion, an effect that’s too small to be noticed without precise instruments.” But don’t stare at the sun for too long, you don’t want to burn holes in your eyes while trying to notice the difference!

While this variation in distance does lead to a 7% decrease in the amount of sunlight Earth receives at aphelion compared to perihelion, the impact on temperatures and seasons is relatively minor. “It’s a common misconception that the seasons are driven by our distance from the sun,” says Dr. Runyon.
For planets with more eccentric orbits, however, the changing distance can have a much more dramatic effect. For example, on Mars, sunlight can vary by as much as 31% over the course of a year. And if Earth’s orbit were to become significantly more elliptical, the consequences could be catastrophic, leading to extreme seasonal variations that would threaten the viability of advanced civilizations.

Fortunately, our planet’s orbit is currently becoming more, not less, circular, with a decreasing eccentricity. So, as we bask in the glory of summer, we can take comfort in knowing that Earth’s position in the cosmic “sweet spot” will continue to support the conditions necessary for us to live.

Image Credit by Rachel Claire