In 1999 Chad Moore was working as a park ranger near California's Salinas Valley, monitoring falcons' nests and hiking back to the station after dark. "But it wasn't really dark," says Moore. "The glow from nearby towns was drowning out the stars." 

Since then Moore and his team have used a specialized camera to take photos at more than 86 national parks and found that in most, vibrantly starry skies—like the stunner that inspired Van Gogh—are fading, thanks to suburbs and illuminated highways. 

But there's more at stake than constellations: Light pollution can cause depression in humans and disrupt animal migration. In that sense, dark skies are a natural resource that needs protecting, just like the oceans. Moore's research inspired the National Park Service to create the Night Sky Program, which covers park lamps so that less light escapes and educates nearby homeowners, since light can affect areas 200 miles away. "When you realize the consequences of leaving your porch light on," says Moore, "you might turn it off."

1. Give your eyes 20 minutes to adjust; if you have a flashlight, cover it with red cellophane to reduce glare.