Week of August 17-23, 2025

Have you ever wondered why the night sky is dark?

Doesn't seem like much of a brainteaser, does it? Everyone knows the sky darkens at night because the sun sets ... or more accurately, because our part of planet Earth rotates away from the sun, and without sunlight illuminating our atmosphere, the sky becomes dark. The end.

Well, not so fast! You don't think I'd begin my weekly column with a question that has such a quick and simple answer, do you? I wouldn't, and I haven't. It seems the question of the dark night sky has been on astronomers' minds for centuries.

Ancient skywatchers believed that the universe — and the number of stars in it — was infinite. If that's true, they argued, the sky should never become dark. It should always appear as bright as the sun, no matter where we look.

At first thought this may seem nutty, but early natural philosophers thought about the universe as being built with crystalline spheres surrounding us — much like layers of an onion — with stars being distributed evenly on each.

So it's not difficult to imagine that, on the sphere closest to us, stars appear nice and bright. The shell 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. From this, we might conclude that, because the most distant stars would be so faint, there's no way we could see them.

But remember, while stars on larger spheres are farther away, the shells also contain more stars. For example, stars on the shell twice as distant are four times fainter, but there are four times as many stars there. On the shell three times farther, stars are nine times fainter, but there are nine times as many.

In other words, each shell would contribute the same amount of starlight to our sky, no matter how far it is. So if there were an infinite number of shells surrounding us, there would also be an infinite number of stars. And this means that, no matter where we look in the sky, our gaze would intersect the surface of a star. And this would add up to a night sky as bright as the sun itself!

But this isn't true, is it? The nighttime sky is dark. This paradox, now known as Olbers' paradox, is named after Heinrich Olbers, who tried to explain it in 1826. Interestingly, the paradox hasn't been totally explained even today.

The solution could be as simple as: There is not an infinite number of stars, as many ancient philosophers believed. Or it could be as profound as the universe having an origin, so that the light of the most distant stars hasn't yet had time to reach us.

Whatever the explanation is, go outdoors tonight and look into the southern sky, toward the constellations of Scorpius and Sagittarius, and the beautiful Milky Way streaming between them. And then cast your gaze to the seemingly starless sky around them. After today, you may discover a whole new appreciation of the dark night sky!

Ancient stargazers believed the universe contained an infinite number of stars.

Visit Dennis Mammana at dennismammana.com. To read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at creators.com.

Photo courtesy of Dennis Mammana