Two astronomers in Italy and an American environmental scientist have produced the first world atlas that shows the effects of artificial light on the night sky. The atlas shows the dramatic impact of what astronomers call "light pollution."
The study that produced the first atlas showing the effect of artificial light on the night sky found that one-fifth of mankind - including more than two-thirds of Americans - cannot see the Milky Way, even on a crystal-clear night.
The global map of nighttime light sources is based on a composite of photographs taken in 1996 and 1997 by a U.S. Air Force weather satellite. Italian astronomers Pierantonio Cinzano and Fabio Falchi of the University of Padua used the satellite views, all snapped on clear nightsto produce a computer model.
The astronomers' American colleague, Chris Elvidge of the National Geophysical Data Center in Boulder, Colorado, says the model estimates the amount of light that escapes the earth's surface on a cloudless night, then bounces back to earth, spoiling starlight views. "The light is being scattered by gas molecules in the atmosphere," he said. "And then dust. And aerosols that are in the atmosphere."
Dr. Elvidge says it is not for him, as a scientist, to make value judgments as to whether the loss of night sky views is good or bad. He says he leaves terms like "light pollution" to others. He prefers to call the escape of light into the night sky "artificial sky brightness."
"Certainly," he said, "our ancestors spent a lot of time looking at the stars, perhaps because they didn't have as many other things to do after dark as we do today - reading a book or watching television or listening to the radio. How can you say that those new activities are more or less valuable than the old stargazing activities?"
Chris Elvidge says he and his family can see only a few stars on a clear metropolitan Denver night. "I consider it one more good reason to head for the hills," he said.
The new world atlas of the night sky has buoyed the work of an organization in Tucson, Arizona, called the "International Dark-Sky Association." The nonprofit membership group supports efforts to restore what it calls the heritage of the nighttime environment through improved lighting.
Elizabeth Alvarez, the group's associate director, says it's important to preserve a clear night sky for science, and also for more abstract reasons. "Inspiration," she said. "Over the centuries you have seen it inspire religion, culture, poetry, art. It's so integral to our society. We have evolved on this planet with a day-night cycle - the Circadian rhythm. It affects humans as well as plants and animals. We need to have those periods of darkness as well as periods of light to stay healthy."
Elizabeth Alvarez says the new nighttime atlas will give cities ammunition to reduce the intensity and scattershot trajectory of street and security lighting. She points out that in a city with soft and romantic lighting like Venice, Italy, an observer can still see the Milky Way, right downtown, on a clear night. She says other cities like Rome have committed millions of dollars to moderate street lighting for asethetic reasons and to save energy.
Evironmental scientist Chris Elvidge in Colorado says satellite images are now being collected for a follow-up, comparative study of the night sky. He says he expects it will show that the middle of the night in even more of the world will look more like twilight than nighttime. "The world population is growing," she said. "Our economies are still growing. As people become more wealthy, they use more light."
In its story about the new atlas and the effects of escaping light on the night sky, the Christian Science Monitor altered the words of the old lullaby. The story was headlined, "Twinkle, Twinkle Little Street Light."