When was the last time you stood under a truly dark sky? Today, the LED glare of streetlights, security lamps and car headlights overwhelms starlight for most of us. More than 80% of people worldwide now live under light-polluted skies, and in North America the vast majority can no longer see the Milky Way.
Away from urban centers, however, darkness thrives — even in North America. Astronomers measure sky darkness on the Bortle scale, a nine-point system where 1 is the darkest possible sky and 9 is an inner-city night. A Bortle 1 sky reveals the Milky Way so bright it can cast shadows; at Bortle 7 or higher, it all but disappears.
