Gazing up at the night sky in awe and wonder can be a comforting and almost pristine joy, but stargazers are seeing fewer and fewer stars. A study published in the journal Jan. 19 Science notes that the night sky is getting seven to 10 times brighter each year, faster than measurements of artificial light emissions from Earth first suggested.
A research group from institutions in Germany and the United States analyzed over 50,000 observations made by citizen scientists around the world from 2011 to 2022. The telescope shows the important role citizen science data can play in complementing satellite measurements.
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An artificial twilight glows in many places around the world long after sunset, and this skyglow is a form of light pollution. Not only is it detrimental to stargazing and astronomy, but the extra light can affect the animals’ sense of daily and seasonal cycles.
“Skyglow affects both diurnal and nocturnal animals and is also destroying an important part of our cultural heritage,” says Constance Walker, co-author and leader of the Globe at Night project from the National Science Foundation’s NOIRLab, in a statement.
So far, the change in skyglow over time has not been measured globally. Satellites can measure it, but existing sensors don’t have enough sensitivity or accuracy.
Enter Citizen Scientists. Harnessing the power of humans to collect observational data through citizen science experiments is a promising approach to get more measurements. globe at night Participants report which of eight star maps best matches what they observe in the sky, and each map shows the sky under different levels of light pollution.
“The contributions of individual people work together like a global sensor network and make new science possible,” said Christopher Kyba from the German Research Center for Geosciences GFZ in Potsdam and the Ruhr University Bochum in a statement.
This study analyzed data from 19,262 locations worldwide for cloudless and moonless night recordings over the past 11 years. To calculate the rate of change in sky brightness, they used a global sky brightness model based on 2014 satellite data.
In Europe, the team found a 6.5 percent increase in brightness per year and a 10.4 percent increase in North America. “The speed at which stars are becoming invisible to humans in urban environments is dramatic,” Kyba said. “If things continue at this rate, a child born where 250 stars can be seen will only be able to see 100 stars by the time they are 18.”
The satellite data showed a slower growth in the skyglow and the team was surprised at the rate at which the skyglow was increasing. One of the reasons for different measurements from the satellites in space compared to those here on Earth has to do with changes in lighting practices.
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“Satellites are most sensitive to light directed upward toward the sky. But it’s the light emitted horizontally that accounts for most of the skyglow,” Kyba said. “So if advertisements and facade lighting become more frequent, bigger, or brighter, they could have a big impact on how the sky glows without making a big difference in the satellite imagery.”
Another factor is the switch from orange sodium vapor lamps to white LEDs, which emit significantly more blue light. “Our eyes are more sensitive to blue light at night, and blue light is more likely to be scattered in the atmosphere, so it contributes more to the skyglow,” Kyba said. “But the only satellites that can image the entire earth at night are not sensitive to the blue light wavelength range.”
While the citizen science approach is helpful, it has some limitations. One of them are the places where the participants observe and report on the stars. So far, mostly people from Europe and North America have participated, and half of the entries from Asia come only from Japan. Rapid changes in artificial skyglow are also suspected in developing countries, but not as many measurements have been made. “Most of the data comes from the regions of the world where Skyglow is currently most prevalent. That’s useful, but it means we can’t tell much about the change in skylight in regions with few observations,” Kyba said.
The results show that current lighting policies (such as switching to LEDs) have not yet brought improvement, despite growing awareness of light pollution on a continental scale. They also demonstrate the role that citizen science data can play as a supplemental research tool.
“If we had broader participation, we could identify trends for other continents and possibly even for individual states and cities. The project is still ongoing so please take a look around tonight and let us know what you see!” said Walker.