How dark is the night sky?
How dark is dark?
When you go outside on a clear, moonless night, you can see not only lots of stars but also the black space between them—literally space, in this case. Given that you can see stars and that the sky is black, you might think the sky is transparent. But it’s not—at least, not really.
Various molecules, atoms and particulates float around in the air, and these reflect light. During the day, sunlight entering the atmosphere is scattered by molecules such as nitrogen and oxygen at more or less random angles, like pinballs caroming off a bumper, so anywhere you look in the sky, you’ll see sunlight directed toward you. Complicated physics makes these molecules scatter more blue light than red, giving the sky its azure hue.
During the day, the sky is so bright from this light that no stars (well, except the sun) can be seen. They’re too faint. At night, after the sun sets and its scattered light diminishes, the sky gets darker. But does it get completely dark—that is, with absolutely no light coming from between the stars?
Nope. Even at night, there’s some light in the sky. There’s the moon, of course, but also, depending on where you live, illumination can come from city lights, street lamps, car headlights and other artificial sources that create what we call light pollution. The farther removed you are from those, the darker the night sky will be.
But even then there’s a limit because the sky itself glows! [Continue reading…]