Astronomers rate the darkness of our skies on a scale (the Bortle Scale) of 9 (brightest) to 1 (darkest). Most of us spend our lives in the radiance of levels 5 through 8, only rarely venturing into areas ranked 3 or darker. Because of the rapid growth of light pollution over recent decades, most Americans under 40 have never known real darkness. All over the globe our nights are growing brighter, and almost nowhere are they growing darker.

We are just beginning to learn the true cost of all this light. Studies increasingly link our overuse of light at night with health concerns such as sleep disorders, diabetes, obesity, and cancer. Other studies report the damaging ecological consequences, the tremendous