Light pollution threatens amour and astronomy.
Night never truly falls on a quarter of the world's population. More than two-thirds of Americans and half of all Europeans have lost sight of their cosmic home, the Milky Way galaxy, according to the first comprehensive map of light pollution. Unchecked, artificial light could soon leave astronomers in the dark.
"The night sky is more seriously endangered than commonly believed," say Pierantonio Cinzano of the University of Padua, Italy, and colleagues. Their global atlas of night-time brightness shows that "mankind is proceeding to envelope itself in a luminous fog"1.
City dwellers have long been accustomed to their night sky containing little more than the Moon and a handful of the brightest stars, because of the obscuring effects of street and building lighting. But the problem is not confined to industrialized countries, says Cinzano.
"More than 99 per cent of the US and European Union population, and about two-thirds of the world population, live in areas where the night sky is above the threshold considered polluted," they now report. Many people, including 80 per cent of the US population, have no real night at all - defined as darker than twilight in the middle of the ocean.
Like many other forms of pollution, light shows no regard for national boundaries. The fact that night-time light from one nation can obscure the stars from its surrounding countries may warrant new systems of international legislation, Cinzano's team suggests.
Many astronomical observatories, located in remote places where light pollution is currently low, risk having their vision degraded by encroaching artificial light in less than 20 years, the researchers warn. Urban development up to 250 kilometres away from such sites can have a severe impact on astronomical viewing conditions unless lighting levels are controlled.
The new atlas has been prepared from global satellite measurements of low-level visible and infrared light emanating from the top of the Earth's atmosphere, made by the US Air Force Defense Meteorological Satellite Program (DMSP). The researchers excluded transient light sources, such as fires, by omitting light issued from any one place for less than three consecutive nights.
Several previous studies of light pollution have attempted to derive it from population distributions. But the night-sky glow is not always directly related to population.
Not every city dweller is nocturnally impaired, the maps show. There is just one Italian city with more than 250,000 inhabitants, for example, from which one has a chance of seeing the Milky Way on a clear night: Venice.
This, explain Cinzano and colleagues, is due mainly to the unusually low intensity of the city's lighting, which is preserved to retain its romance. As long as safety is not compromised, other cities might do well to emulate Venice's example. After all, how much more romantic is a night sky aglow with stars?
Cinzano, P., Falchi, F. & Elvidge, C. D. The first world atlas of the artificial night sky brightness. Monthly notices of the Royal Astronomical Society, (in press)
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Ball, P. Night in bright light plight. Nature (2001). https://doi.org/10.1038/news010809-6