Earth at Perihelion

By Dennis Mammana

December 28, 2017 4 min read

Week of Dec. 31, 2017-Jan. 6, 2018

As you gather with family and friends over the New Year's holiday to share great food and watch all the holiday bowl games on TV, ask a question that's sure to generate a fun discussion: During which season does our planet lie farthest from the sun?

Many people believe that the cold wintertime temperatures are somehow caused by our planet's greater distance from the sun, but this just isn't the case. In fact, it's during early January when the Earth reaches its nearest point to the sun.

Seeing how this fascinating discovery came to be requires a bit of an imaginary journey back in time.

It was little more than four centuries ago that German mathematician Johannes Kepler discovered that our planet — and others — orbits the sun not along a circular path but an elliptical path. An ellipse is simply a circle that's been squashed, and the amount of squashing an orbit shows determines its "ellipticity." Technically, a mathematician would say that a circle is also an ellipse — with zero ellipticity.

What this means, Kepler said, is that all planets' distances must change over the course of their orbits.

He stumbled upon this fact after struggling for years to fit circular orbits to the measured motions of Mars. All he had to show for his work was 900 pages of calculations and 70 worthless orbits — quite a headache, I'm sure. And then, around Easter in 1605, he decided he had seen enough circles for one lifetime. The only figure he had not tried was an ellipse, or, as he so poetically 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 know that the Earth's distance from the sun changes over the course of its annular orbit. But this change is only about 3 percent and is hardly enough to contribute to a significant change in seasonal temperatures.

We also know that our Earth reaches its nearest point to the sun in 2018 — a point that astronomers call "perihelion" — at 9:35 p.m. PST on Jan. 2 (12:35 a.m. EST on Jan. 3), while our winter is just beginning. At that moment we will lie only 91,401,983 miles from the sun. Our farthest point — "aphelion" — won't arrive until July 6.

Of course, if you live in the Southern Hemisphere, where seasons are reversed from those north of the equator, our planet's closest point to the sun does occur during summertime. But that's a story for another time.

And now, as we prepare to turn the page on yet another year, I'd like to wish all my readers a happy, healthy and star-filled 2018.

Just remember to keep looking up!

View the Earth at perihelion this week.

Visit Dennis Mammana at www.dennismammana.com. To read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at www.creators.com.

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