The average brightness of the night sky is increasing 10% every year, making the stars less visible.
How many stars can you see at night from where you live? According to results from a new study appearing this week in Science, the number is likely less than it was just a few years ago.
To arrive at that conclusion, Christopher Kyba (Ruhr University Bochum, Germany) and his collaborators analyzed data from the Globe At Night citizen-science project. The researchers found that the brightness of the night sky increased on average by about 10% every year between 2011 and 2022.
Skyglow is familiar to most amateur astronomers as the wash of light across the night sky, mostly from artificial light sources on the ground. It reduces the contrast between astronomical objects and the sky, making some observations difficult, if not impossible. The effect is greatest in and near cities.
Researchers have found that the nighttime light that yields skyglow also has important effects on the ground. Artificial light at night can negatively impact wildlife, energy security, and public safety. As a result, light pollution is increasingly considered an important environmental problem.
Until now, our best global view of light pollution came from satellite measurements of the Earth at night. They indicated that the amount of light emitted on the ground grew at an average rate of 2% per year between 2012 and 2016. But the new result from citizen scientists on the ground hint that satellites have underestimated the increase in a big way.
The new estimate differs from earlier results in part because the instruments aboard the current generation of satellites are effectively blind to blue light. In the past decade, governments in many parts of the world rushed to adopt white light–emitting diode (LED) technology. LEDs emit much more in the blue part of the spectrum than earlier light sources did, and that light scatters more efficiently in the atmosphere than other colors. Also, because LEDs use much less energy than earlier light sources did, regions tend to install more (and brighter) fixtures, which may negate the cost savings. But even as observers on the ground generally saw their night skies brighten from the transition to brighter, bluer LEDs, satellites missed much of the change.
Furthermore, satellite sensors don't see light emitted toward the horizon from sources like windows and illuminated signs very well. That light is an important contributor to what observers perceive as skyglow. And particularly in the early evening hours, these sources may be the dominant source of skyglow near cities.
Visual observations like those collected by Globe at Night can account for at least some of the “missing” light. By counting the number of visible stars in the night sky, citizen scientists include the influences of both increasing light emissions and the shift toward generally bluer light sources.
Salvador Bará, a light pollution researcher formerly of the University Santiago de Compostela in Galicia, Spain, who was not involved in the study, says this approach was crucial to the outcome of the investigation.
“This work reports on the real vision of the night sky with the naked eye, without relying on technological instruments,” Bará said. He added that it “underscores the power of citizen science to assess changes in our environment on a global scale.”
Globe at Night participants contributed over 50,000 individual observations during the study period. Many came from parts of the world where deploying ground-based light sensors was impractical or even impossible.
Yet even that mountain of data could sample night-sky conditions only on large scales. “We were only able to look at continental trends, because we don't have sufficient data to look at smaller regions,” Kyba explains.
More program participants would improve the results, Kyba suggests. It's also important to recruit volunteer observers in parts of the world underrepresented in past Globe at Night campaigns. Satellite data suggest that’s where light pollution and skyglow may actually be growing the fastest.
Despite high confidence in certain approaches known to reduce light pollution, the researchers concluded that current efforts don't seem to be working. And failure to cut back on wasted outdoor light at night can have real impacts on communities. The main messages of light pollution reduction might be better conveyed to governments if the study results were more targeted to specific geographies.
Raising the number of Globe at Night observations each year by a factor of 10 would make it possible for researchers to look at trends on much smaller scales than they can now. “If we could do that, we would surely find places that are doing better than average,” Kyba says, “and we could try to figure out what they are doing right.”
Kyba argues that this is why the conclusions of the paper affect everyone — not just astronomers. “Even if you don't care about the stars, this result should matter to you,” he said. “It's a visual representation of your community's waste of money and energy as a result of poor lighting design or installation.”
You can join Globe at Night — find instructions here.