Week of June 28-July 4, 2009 by Dennis Mammana on Creators.com

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Week of June 28-July 4, 2009

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.

But this just isn't true.

Like most celestial bodies, the Earth orbits the sun 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 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 Mars, Kepler threw up his hands in disgust. All he had to show for his work was 900 pages of calculations and 70 worthless orbits.

And then, around Easter 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 his ellipse over the data, his eyes lit up. It fit beautifully. In a single moment of unrivalled genius, Kepler solved a problem that had confounded sky watchers for centuries.

We now understand that the Earth, too, orbits the sun along an elliptical path.

But our distance from the sun varies by only about 3 percent — hardly enough to contribute to seasonal temperature differences. That is 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, when solar rays shine down on us at a much shallower angle. This is what provides our seasonal temperature changes.

It might surprise your friends to learn that the Earth actually reaches its farthest point from the sun during our summertime. This year, "aphelion" occurs around 7 p.m. PDT on July 3. We won't reach our nearest point to the sun — perihelion — until early January.

To find out more about Dennis Mammana and read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at www.creators.com.