A dark sky — a truly dark sky — is so rare these days that it's now a tourist attraction. The Wall Street Journal reports that two-thirds of the world's population (including nearly everyone in the US and Europe) "no longer see a starry sky where they live." Most people live in places where it never gets dark enough for the human eye to adjust to night vision. "Our children grow without seeing what is possibly the most extraordinary natural wonder," says Italian astronomer Fabio Falchi.

There is a movement, led by the International Dark Sky Association, to promote lighting that doesn't ruin the night sky. But the amount of artificial light worldwide has tripled since 1970. Even Death Valley is lit up by the glow from Las Vegas.

Women who work night jobs or who live in brightly lit cities report higher rates of some kinds of cancer, the WSJ reports. Bright lights disorient turtles and birds. For most of us, it's just hard to see the stars.