Is the night sky ever really dark?

In a word, no. Anyone who's ever walked their dog at night knows that when you step out the door, it can seem awfully dark out there. By and by, our eyes get used to the darkness. After 10 minutes, we can see the stars, make out the shapes of trees and safely navigate the road without artificial illumination. Granted, everything is in black-and-white, but that's because we're using our eyes' rod cells, which are sensitive to low light but don't perceive color.

Recently, I drove to a rural area north of Duluth to observe the sky with my telescope. It was an exceptionally dark night. The Milky Way crossed high overhead, so I set up tripod and camera and made several time-exposures. When I reviewed the images on the camera's back screen, I noticed soft r

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