Artificial light pollution from Earth is contaminating the skies over most major astronomical observatories, requiring immediate action to remedy, a team of international scientists has determined.
The team, including scientists from Italy, Chile and Spain, compared light pollution at almost 50 observatories, ranging from some of the largest professional observatories in the world to smaller amateur-focused facilities. The scientists then applied a model of how light travels through Earth's atmosphere to night views taken by satellites.
The researchers also accounted for light pollution across the entire night sky. Directly overhead, or the zenith, is less polluted by light and is therefore the darkest portion of the night sky, but the researchers also measured the average brightness at altitudes of 30 degrees — the lowest that a telescope at a ground-based observatory can point — as well as for the first 10 degrees above the horizon. The scientists also considered the overall average brightness across the sky and how artificial light coming from the night sky illuminated the ground.
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The scientists then compared these measurements with light originating from stars and the Milky Way, as well as the natural sky brightness provided by the faint emission of light by Earth's atmosphere called "airglow."
With all these factors combined, the researchers were able to model how artificial light affects the night sky and determined that the night sky over major observatories is more polluted by light than previously assumed.
The team found that only seven of 28 major observatories, defined as a facility with a telescope of a diameter of 10 feet (3 meters) or more, had a zenith brightness with light below the expected natural sky brightness threshold of 1%. The researchers considered anything above this threshold "contaminated" and anything below it "uncontaminated," so only seven major sites were considered contaminated by no more than the natural level of sky brightness.
Even worse, only one of the 28 major sites examined by the scientists had light pollution below the 1% threshold at 30 degrees above the horizon. The researchers also considered a higher maximum allowable artificial brightness threshold for major observatories of 10% set by the International Astronomical Union in the 1970s, but still two-thirds of major ground-based observatories exceeded this more lenient limit.
The least contaminated of all the sites featured in the study was a lodge in Namibia that houses several telescopes rented to amateur astronomers, according to Fabio Falchi, a physicist specializing in light pollution at the Universidade de Santiago de Compostela in Spain and lead author on the new research.
"I was recently there and I can confirm that it is the least light-polluted site I've ever seen," he said in a statement. "We must try to decrease the light pollution levels at other sites in order to protect the future of ground-based astronomy."
The team's research is described in a paper published Tuesday (Dec. 20) in the journal Monthly Notices of the Royal Astronomical Society.
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