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Why Is the Nighttime Sky Dark?

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Week of Aug. 30 - Sept. 5, 2015

On the last weekend of August, we'll all enjoy the beautiful full moon as it rises in the east at dusk. Some early North American tribes knew the August full moon as the Sturgeon moon, since this is the time when this large fish is readily caught.

By early September, however, the moon will have again departed our early evening sky, and we'll have a chance to gaze skyward and ponder a rather simple question: Why is the nighttime sky dark?

Now before you exclaim "Because the sun isn't up, you dolt!", and accuse me of having way too much free time to ask such nonsense, let's think about this for a moment.

Yes, it's true that, during the early evening, our part of planet Earth turns away from the sun and, without sunlight to illuminate our atmosphere, the sky appears dark. And that, you might think, would be the end of the discussion. But if it were, this would be a very short article.

For ages, sky watchers and philosophers believed that the universe is infinite and, therefore, must contain an infinite number of stars. If true, they reasoned, then our sky should never become dark; it should always appear brilliant, no matter where we cast our gaze.

Think about it this way. Imagine the universe to be composed of stars that are evenly distributed on crystalline spheres surrounding us, much like layers of an onion. On the sphere nearest to us, stars would appear bright. The layer twice as far would also contain stars, but each would appear four times fainter; those on the shell three times farther would appear nine times fainter, and so on out to infinity.

From this we might easily conclude that, because the most distant stars would appear so terribly faint, we could never see them. But remember that, with increasing shell sizes come more stars. So, while those on the sphere twice as distant appear four times fainter, there would also be four times as many of them. On the shell three times farther, there would be nine times as many, and so on.

In other words, each shell would contribute an equal amount of starlight to our sky, no matter how far away it lies. Wherever in the heavens you look, your gaze would intersect the light of a star, and the entire nighttime sky should appear as brilliant as the sun itself!

But it doesn't. The night sky is dark.

This apparent contradiction between what people saw and what they wanted to believe — now known as Olber's Paradox — is named after Heinrich Olber who tried to explain it in 1826. The explanation could be as simple as the universe not actually being infinite — and, therefore, not containing an infinite number of stars — or as profound as an infinite universe having an origin, and the light of the most distant stars not yet having time to reach us.

A mind-bender, to be sure and, believe it or not, this paradox is still being pondered today.

Whatever the answer, go outside and gaze skyward. You may just see the dark nighttime sky in an entirely new light!

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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