Johannes Kepler: Charting the planets' ellipticity


The next time you gather with friends and family, ask a question that's sure to generate a fun discussion: "During which season does our planet lie closest to the sun?"

Unless your guests are atypical, most will insist we're farthest during December, January or February.

This isn't surprising. Many people believe that our cold wintertime temperatures are somehow caused by our greater distance from the sun. This just isn't true, however. In fact, our planet's farthest point to the sun actually occurs in July!

It was little more than four centuries ago that German mathematician Johannes Kepler found that the planets, including the Earth, orbit the sun not in uniform circles but in ellipses.

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, a circle is also an ellipse ... with zero ellipticity.

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

But determining the shapes of planetary orbits wasn't easy. After struggling for years to fit circular orbits to the measured motions of Mars, all Kepler 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 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!"

As a result of this amazing discovery, 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 three percent and is hardly enough to contribute to a significant difference in terrestrial temperatures.

And now, having turned the page on yet another year, I'd like to extend my hope that 2016 will offer us all an exciting time of cosmic discovery.

Keep looking up!

Visit Dennis Mammana at


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