LOS ANGELES — The flood of artificial light that washes the stars from the sky has left one in five human beings unable to see the Milky Way at night, according to a new study of the global effects of light pollution.
The study is the first to document the extent to which humans have wrapped the inhabited world in a luminous fog, shutting out much of the heavens – including the very galaxy we call home – from view.
Once the sun goes down and the lights come on, as many as one in four people around the globe basks in a perpetual twilight, under skies brighter than on nights when the moon is full.
“The thing that struck me is there are large numbers of people who really have lost the panorama of the night sky – that’s no longer available to them because there is so much of this sky brightness,” said Chris Elvidge, a National Oceanic and Atmospheric Administration physical scientist and one of the study’s three co-authors.
The study matched global population density information with Department of Defense satellite images, captured over 28 nights in 1996 and 1997, of the upward flux of light scattered from artificial sources around the globe.
The Air Force Defense Meteorological Satellite Program satellites were designed to observe clouds illuminated by moonlight. On moonless nights, however, they also pick out the distinct pinpricks, blobs and smears of light cast off by the world’s cities and towns.
Light pollution has long been recognized as a problem in the United States, Europe and Japan. Elvidge said the broader look shows that no country in the world is untouched. About two-thirds of the global population lives under skies polluted by artificial light.
“That’s not surprising, it’s just very frustrating,” said Elizabeth Alvarez, associate director of the International Dark-Sky Association, a Tucson, Ariz., group that works to keep the night sky dark.
Far from the city lights at night, about 2,000 stars are typically visible. In major cities, that number shrinks to a few dozen at most.
“For a large percentage of people ... they’re no longer able to see what their ancestors saw on a nice, clear night,” said Elvidge, who works in NOAA’s National Geophysical Data Center in Boulder, Colo. While astronomers can retreat to remote mountain tops to stargaze, most city dwellers do not have that luxury, said John Mosley, an astronomer at the Griffith Observatory in Los Angeles. The impact is incalculable, he said.
“The sky has always been a source of mystery and wonder. To lose that connection with something that is so much older and bigger than we are is a tremendous loss,” Mosley said.
Elvidge said future studies by the team would chart the growth rate of artificial sky brightness over time.
Alvarez said any potential increase could be stemmed with increased education. Already six states have passed laws that focus on limiting outdoor lighting levels, she said.
“We’re working hard to make people realize it’s not something you have to give up,” Alvarez said.
On the Net: The Night Sky in the World
International Dark-Sky Association