People around the world see fewer and fewer stars in the night sky. The change in the visibility of the stars can be explained by an increase in the brightness of the sky of 7-10% per year.
The rate of change is faster than would be suggested at first glance by satellite measurements of artificial light emissions on Earth.
This is the conclusion of a study published in the journal Science, conducted by a research group led by Christopher Kyba of the GFZ German Research Center for Geosciences and Ruhr-Universität Bochum, with colleagues from the GFZ and NSF’s NOIRLab. (National Science Foundation) of the United States.
They analyzed more than 50,000 naked-eye observations made by citizen scientists around the world between 2011 and 2022 as part of the “Globe at Night” citizen science project. The results show that citizen science data is an important complement to previous measurement methods.
Over much of the Earth’s surface, the sky continues to glow with artificial twilight long after sunset. This “sky glare” is a form of light pollution that has serious effects on the environment and therefore needs to be investigated, says Constance Walker, co-author of the study and leader of the Globe at Night project at NSF’s NOIRLab. since its inception.
Participants in ‘Globe at Night’ look at their night sky and, through an online form, indicate which of eight star maps best corresponds to what they see. Each graph shows the sky under different levels of light pollution.
“Individual people’s contributions work together as if they were a global sensor network, making a new science possible,” says Christopher Kyba of the German Research Center for Geosciences GFZ Potsdam and the Ruhr University Bochum. Together with her GFZ colleague Yigit Öner Altintas and Constance E. Walker and Mark Newhouse from NOIRLab, she has analyzed data from 51,351 participants from around the world taken on cloudless and moonless nights between 2011 and 2022. They were collected at 19,262 locations in worldwide, including 3,699 locations in Europe and 9,488 locations in North America.
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To calculate a rate of change in sky brightness from this data, and to take into account that the observers were also in different locations over the years, they made use of a global model for sky brightness based on data satellites from 2014.
“The rate at which stars are becoming invisible to people in urban environments is dramatic,” summarizes Christopher Kyba, lead author of the study. The researchers found that the change in the number of visible stars can be explained by the increase in the brightness of the night sky. In Europe, the data coincides in a 6.5% increase in brightness per year; in North America, by 10.4%.
To put these figures in a more understandable context, Kyba explains the consequences of seeing the stars in a place with an annual increase of 9.6%, which was the average for all places in the world. “If development continued at this rate, a child born in a place where 250 stars can be seen will only be able to see 100 stars there by the time he turns 18.”
Based on the slower growth of upstream emissions observed in satellite data, the researchers were surprised by the speed of this skybrightness development. In fact, for the observer locations, the artificial brightness measured by satellite had decreased slightly (by 0.3% per year in Europe, 0.8% in North America).
Christopher Kyba believes that the difference between human observation and satellite measurements is likely due to changes in lighting practices. “Satellites are most sensitive to light that is directed up into the sky. But it is the light emitted horizontally that accounts for most of the sky’s glare,” Kyba explains. “So if billboards and streetlights are made more frequent, larger, or brighter, they could have a big impact on skyglow without showing up much on satellite imagery.”
Another factor the authors cite is the widespread shift from orange sodium vapor lamps to white LEDs, which emit much 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, thus contributing more to the glow in the sky,” Kyba says. “But the only satellites that can image the entire Earth at night are not sensitive in the blue light range of wavelengths.”