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Monday, March 25 , 2019, 2:15 pm | Fair 62º


Dennis Mammana: Why Is the Night Sky Dark?

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Learn why the night sky is dark this week. (Dennis Mammana illustration / creators.com)

If you’re out looking for the moon early this week, you’ll have a tough time finding it. On Tuesday, it will lie nearly in the direction of the sun (its new-moon phase), but by Thursday it will appear not far from the brilliant planet Venus in our western sky at dusk.

Before the moon begins illuminating our night sky again this month, we stargazers 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 fool!” and accuse me of experiencing yet another senior moment, 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 (sunset, we call it), and that without sunlight to illuminate our atmosphere, the sky appears dark. You might think that is 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 similar 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 brightest. 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. The shell three times farther would contain 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 see and what they want to believe, now known as Olbers’ paradox, is named after Heinrich Olbers, 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, with the light of the most distant stars not yet having time to reach us.

It’s a puzzler, to be sure. And believe it or not, this paradox is still being pondered today.

Whatever the answer, go outdoors and gaze skyward this week. You may just view the dark nighttime sky in an entirely new light!

Dennis Mammana is an astronomy writer, author, lecturer and photographer working from under the clear dark skies of the Anza-Borrego Desert in the San Diego County backcountry. Contact him at [email protected] and follow him on Twitter: @dennismammana. Click here to read previous columns. The opinions expressed are his own.

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