About two-thirds of the world population - and 99 percent of people in the continental United States and Western Europe - never see a truly dark starry sky from where they live.
Most of them cannot see the Milky Way, and for many, the sky never gets darker than it would during natural twilight, because so much artificial light brightens the atmosphere.
These are just some of the statistics in the First World Atlas of Artificial Night Sky Brightness by Pierantonio Cinzano and Fabio Falchi (both of the University of Padua, Italy) and Chris Elvidge (NOAA National Geophysical Data Center, Boulder, Colo.).
Although there has been growing awareness of the problem of light pollution, this is the first time that artificial illumination of the night sky around the world has been quantified and related to where people live.
Dr. Cinzano and his colleagues used data acquired in 1996-97 by the US Air Force Defense Meteorological Satellite Program. The scientists then calculated how artificial light is propagated through the atmosphere, to arrive at a set of maps showing the extent and severity of light pollution around the world.
In areas where about half the world's population live, the sky is as bright as it is on days close to a full moon at a good astronomical site. "Night" never really comes to such places, and the sky is always as bright as nautical twilight (the period of time when the sun is between 6 and 12 degrees below the horizon).
For 40 percent of the US population, one-sixth of the European Union, and one-tenth of world population, it is never dark enough at night for human eyes to become adapted to night vision.