Sky Watch: Why is the nighttime sky dark?

During the final days of October, we will all enjoy the beautiful full moon as it rises in the east at dusk. Some early North American tribes knew October's full moon as the Hunter's moon, since this was a time for hunting and stocking up on supplies for the cold winter months ahead.

By early November, however, the moon will depart our early evening sky, offering us an opportunity to ponder a rather simple question: why is the sky dark at night?

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

Yes, it's true that, in 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 column.

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 looked.

Think of it like this. Imagine a universe composed of stars evenly distributed on crystalline spheres surrounding us, much like layers of an onion. Let's say, for example, on the sphere nearest to us, stars appear nice and bright. The sphere twice as distant 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 model, 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.

The result: each shell would contribute an equal amount of starlight to our sky, no matter how far away it lies. In other words, no matter where in the heavens you look, your gaze would intersect the light of a star. 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, nor as profound as an infinite universe having an origin and the light of the most distant stars hasn't yet had time to reach us.

Interestingly, this paradox is still being pondered today.

Whatever the answer, go outside and gaze skyward. You may find that your perception of the dark nighttime sky might just take on a whole new meaning.