At many locations around the world, the night sky shines hundreds of times brighter than it did before the introduction of artificial light. The introduction of light into the nighttime environment is one of the most striking changes humans have made to the Earth’s physical environment — and it is associated with several unintended negative consequences, as researchers are discovering.
Phil Langill, director of the Rothney Astrophysical Observatory (RAO) at the University of Calgary, along with researchers from Europe, North America and Asia, found remarkably large variations in artificial night sky brightness at the different observation sites. The international study was published Feb. 12 in Nature Publishing Group’s open-access journal, Scientific Reports.
“Skyglow, the artificial brightening of the night sky, is among the easiest type of sky brightening to observe,” Langill says. “We measure the sky brightness continuously throughout the night, and take images simultaneously with a sensitive all-sky camera. The images allow us to see exactly what the Sky Quality Meter (SQM) is responding to.”
Until now, all published skyglow research had been local or regional in scale. This study greatly expands on this earlier work, examining light patterns at 50 locations worldwide, including Calgary. The findings reveal that 30 of the sites had considerable skyglow and the sky was more than twice as bright as a natural star-filled sky more than 95 per cent of the time.
Langill says the RAO has very sensitive SQMs that measure variations due to all sorts of artificial and natural light sources. An SQM is a little black box that has a lens on top, pointed straight up. Every 15 seconds, the SQM carefully measures how much light goes into the lens and the higher the reading means the darker the sky.
Effect of clouds on sky brightness varies considerably
The international study examined the effect that clouds have on the night sky brightness, and found that it varies remarkably, depending on the location. For millions of years, this made overcast nights the darkest, with starlight reflected back into space. However, this occurred at only two of the 22 sites where nearby meteorological observations were available.
At most sites, the overcast nights were many times brighter than clear nights. The researchers were surprised to discover that the ratio between overcast (bright) and clear (dark) sky brightness grows most rapidly as cities are approached. Once the "city limit" is crossed, the rate of this increase appears to slow.
“Despite being close to Calgary, the RAO still experiences dark skies when there are no clouds and the moon is below the horizon, so we haven’t crossed the city limit yet,” says Langill. “But with the impending development of the Southwest Calgary Ring Road, this could change.”
In the study, the brightest individual observation came from a site near the town of Schipluiden in the Netherlands. There, the sky was 10,000 times brighter than the darkest observation reported from Kitt Peak in the U.S.
“This difference is much larger than what is observed in the daytime” says Christopher Kyba, lead author, from the Freie Universitat Berlin in Germany. “It is roughly comparable to the difference between a surface illuminated by direct sunlight and one in the dim area between two street lamps.”
Unforeseen consequences of light on animal behaviour and navigation
The impact of brighter nights on the natural environment is still largely unknown. Researchers hypothesize that this change affects the behaviour of nocturnal animals, affects navigation and migration for some species, and unbalances traditional predator-prey relationships. Even social interactions such as reproduction are believed to be affected.
“When one thinks of threatened natural resources, the first to come to mind are clean water and fresh air. But I would argue that a star-filled nighttime sky is a natural resource worth protecting too,” Langill says.
Although the present study is the most widespread to date, it considered only a small fraction of the Earth’s nightly lit area. The researchers call for an international network of similar monitoring stations. The data gathered by such a network would allow researchers to calibrate and test models that predict skyglow in areas for which monitoring doesn’t exist.
The RAO is located southwest of Calgary, near Priddis, and is the Southern Alberta chapter of The International Dark Sky Association.