"DON'T ACT LIKE BAIT," warns Chad Moore, only half-joking. As we hurry down a dark, quiet path toward the edge of Bryce Canyon, we're living on mountain lion time, and hoping the cats will grant us an uneventful evening. 

Moore, a pale, wiry 37-year-old, is a leader of the National Park Service Night Sky Team, and he's used to unusual hours. He and his colleagues travel to parks around the country, scaling peaks and rooftops to measure the depth of darkness. 

When Moore and I reach the edge of the canyon - using only small red flashlights for better night vision - the sky is powdered with stars, and darkness pools around us. The sculpted rim of the canyon, in south-central Utah, forms a high, cold ridge between the sparsely populated Colorado Plateau and the Great Basin, but the darkness here isn't complete. The small cities of Kanab and Cedar City throw pale domes of light on the horizon, and Page, Ariz., glares yellow in the distance. The sophisticated digital camera Moore carries, whose high-resolution photos form a detailed mosaic image of the sky, will confirm that this view is about 5 percent brighter than a perfectly dark sky. 

"When I got into this project, I had no idea of the extent to which light could be seen," says Moore. "I thought there would be lots of parks that were absolutely pristine, that our mission would be to take pictures of them before the darkness was degraded. But it's been really, really hard to find a park that doesn't have a trace of light pollution." 

MOORE GREW UP in Honolulu, where city lights obscured the night sky. In high school, when his parents gave him a telescope, he and his friends started to take stargazing trips outside the city. "By high school, we were having to go further and further out to find areas that were darker," he remembers. "We literally got to the edge of the island, and had no place left to go." 

The world has continued, in the words of Italian astronomer Pierantonio Cinzano, to "envelop itself in a luminous fog." Cinzano's 2001 atlas of artificial night sky brightness estimated that two-thirds of the U.S. population, and one-fifth of the world population, can no longer see the Milky Way with the naked eye. 

Years after leaving Hawaii, Moore was working as a physical scientist at Pinnacles National Monument in California, and he once again noticed the lights of nearby cities on the horizon. Darkness, he realized, was another park resource, like soils or water or air, that needed to be measured and protected for both wildlife and human visitors. Yet there was no formal Park Service program for darkness protection, and no one inside or outside the agency offered a practical way to monitor light pollution in the backcountry. So Moore made up some letterhead for the "Park Service Night Sky Team," and, with a team of one, launched a new park program.

A few months after this unofficial founding in 1999, Park Service scientist Dan Duriscoe made the team a reality, and seasonal staffers and volunteers later rounded out the crew. By 2004, thanks to a series of small internal grants, Moore and Duriscoe were working full-time on the Night Sky Team. With advice from professional astronomers, the pair experimented with methods of measuring darkness, spending several years developing a camera system that was precise, affordable, and light enough to lug into the backcountry. 

"We spent a lot of time with flashlights stuck between our teeth, trying to get things to work in the dark," says Moore. One setup included both an unwieldy telescope and a 35-pound set of lead-acid batteries, and Angie Richman, a Bryce Canyon ranger who works closely with the team, remembers that she once threatened to throw her heavy load over the edge of the canyon. "I thought she was serious," says Moore. "I was like, ‚ÄòPlease don't.' "