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Earth at Aphelion

Skywatch

By Dennis Mammana- While you’re enjoying the pool and soaking up some rays this summer, try this fun experiment.

Ask some friends which season of the year they think the Earth is closest to the sun. Unless your acquaintances are atypical, I’ll bet most will guess that we’re nearer the sun during our summertime.

Many people believe that our warm temperatures are somehow caused by a closer proximity to the sun, but this just isn’t true. In fact, we’re closest to the sun in early January!

Like most celestial bodies, the Earth orbits its star not in a circle, but along an ellipse. An ellipse is simply a circle that’s been squashed, and because we orbit along an elliptical path, our distance from the sun varies slightly throughout the year. This was discovered some four centuries ago by the German mathematician Johannes Kepler.

After struggling for many years to calculate a circular orbit for the planet Mars, Kepler threw up his hands in frustration. All he had to show for his work was 900 pages of calculations and 70 worthless orbits.

And then, around Easter of 1605, he decided he had seen enough circles for one lifetime. He concluded that all he had left to try was an ellipse or, as he so eloquently described it: “a single cartful of dung.”

As Kepler drew an ellipse over his data, his eyes lit up. It fit beautifully. In this single moment of unrivaled genius, Kepler solved a problem that had confounded astronomers for centuries. With unbridled joy, he sketched on his work the goddess of victory, riding her chariot above the clouds. “The truth of Nature, which I had rejected and chased away,” he later wrote, “returned by stealth through the back door, disguising itself to be accepted. … ah, what a foolish bird I have been!”

We now understand that the Earth, too, orbits the sun along an elliptical path, and our distance from the sun varies by about 3% — hardly enough to contribute to seasonal temperature differences. Those are caused, instead, by the tilt of the Earth’s axis.

Our planet’s equator is tipped about 23.4 degrees to the plane of its orbit around the sun. This means that during the months of June, July and August, the Earth’s Northern Hemisphere is tilted toward the sun and allows sunlight to beat directly down upon us. Six months and half an orbit later, our planet’s tilt aims the Northern Hemisphere away from the sun; solar rays then shine down on us at a much shallower angle and cause our temperatures to be lower.

It may surprise your friends to learn that the Earth reaches its farthest point from the sun during our summertime. This year, “aphelion” occurs on July 4 at 6:11 p.m. EDT (3:11 p.m. PDT). Our nearest point to the sun — “perihelion” — won’t occur until Jan. 5, 2020, at 2:48 a.m. EST (11:48 p.m. PST the previous night). Of course, if you happen to live in the Southern Hemisphere where seasons are reversed from those north of the equator, our planet’s nearest point to the sun does occur during summertime. But that’s a story for another time. The Earth reaches its farthest point from the sun during our summertime.

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