Sky watch: The enigma of darkness at night

With the bright moon absent from our evening sky on the week beginning Nov. 1, it might be a great time to ponder this question: Why is the sky dark at night?

If you're like most people, your response might be that our part of planet Earth has turned 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 the universe is infinite and contains an infinite number of stars, as early sky watchers believed, then our sky should never become dark.

Imagine the universe is 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 appear bright. The layer 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 we might conclude that the most distant stars would be too faint to see. But remember that with increasing shell sizes come more stars. So, while stars on the shell twice as distant are 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.

Each shell would contribute an equal amount of starlight to our sky, no matter how distant it is. And an infinite number of shells (and stars) would produce a brilliant nighttime sky. No matter where you look, you'd see starlight, and the "nighttime" sky would appear as bright as the sun!

Obviously, this is not the case. The fact is that the night sky is dark. This apparent contradiction - now known as Olbers' Paradox - is named after Heinrich Olbers, who tried to explain it in 1826. It remains a bit of an enigma even today. The explanation could be as simple as there not being an infinite number of stars, or as profound as an infinite universe having an origin and the light of the farthest stars not yet having time to reach us.

Whatever the answer, go outside tonight and look up. You may find that a simple dark nighttime sky now takes on a completely new meaning.