Light pollution and starlight

City skies (right) are often significantly dimmed by sky glow. Credit: Jeremy Stanley

Step out into the darkness a few hours after sunset. What do you see overhead? If you live in a relatively unpopulated part of the world, you might see the broad stripe of the Milky Way splashed against a backdrop of black sky punctuated by countless stars. If, on the other hand, you live in a teeming metropolis, what’s visible might have much more to do with where you find yourself on the planet than where we find ourselves in the galaxy.

Artificial illumination has dramatically changed the night sky across the globe. And considering that more than half of people worldwide live in urban areas—and more than one fifth live in large cities of one million or more inhabitants—light pollution obscures the stars above billions of people.

Since 2006 a project called GLOBE at Night has been quantifying light pollution using the very people it affects as measuring instruments. The project enlists citizen scientists to make naked-eye observations of a given constellation, then compare what they see with a series of star charts calibrated for different levels of light pollution. Participants submit their observations via an online form. The GLOBE at Night’s most recent campaign, for 2012, gathered nearly 17,000 observations from participants in 92 countries. (The 2013 campaign is still accepting data for a few more weeks.)

Now a new study analyzing several years of GLOBE at Night data reveals some of the promise of the project—as well as its limitations. Aggregated data from the citizen scientists are indeed strongly correlated with levels of sky glow—artificial light scattered by the atmosphere—as estimated from satellite data. The amateur observations could thus find use in tracking regional or global changes in light pollution over time, note Christopher Kyba of the Free University of Berlin and his colleagues in a study published May 16 in the online, open-access journal Scientific Reports. (Scientific American and Scientific Reports are both parts of Nature Publishing Group.) But the data points are too imprecise to say much about local changes.

Atmospheric scattering of artificial light

Sky glow over a populated area. Credit: Ralf Steikert

The researchers note that although the human eye has its foibles, it does have some advantages over satellites and other electronic sensors. For one thing, humans are everywhere—and, increasingly, so are smartphones and other portable devices with Internet access. Then there is the fact that the human eye is relatively stable instrument. “Satellite sensors are replaced on a timescale of a few years, whereas the evolution of the human eye proceeds much more slowly,” the researchers note. As such, gathering naked-eye observations over many years or decades may allow for apples-to-apples comparisons of sky glow now with that in the future.

Light pollution is more than just an aesthetic and astronomical concern, Kyba and his colleagues note. Some species, such as dung beetles, have been shown to navigate by the light of the Milky Way, and those creatures could struggle in places where the stars are veiled by light pollution.