By Will Sager
Light pollution from a mountaintop in Switzerland. Photo by Jan Huber.
Every astronomy enthusiast knows about light pollution, especially those of us under the Houston light dome. We watch our skies and get the feeling that the sky gets brighter as development grows apace. Using data collected by citizen scientists, a team of researchers from Germany and the US reports in the journal Science (Kyba et al., 2023) that light pollution is growing faster than previously thought. The researchers used more than 50,000 observations submitted by citizen scientists from around the world, in which observers estimated the faintest stars visible from their site (Naked Eye Limiting Magnitude or NELM). The connection, as we all know, is that as light pollution increases, light scattering increases and this limits the visibility of dimmer stars. The scientists found an unsurprising correlation between citizen observations and locations of increased light pollution measured by satellite. What was surprising was that to match observations of decreasing NELM, the researchers had to model an increase in light pollution (sky brightening factor) by 9.6% per year. With this rate of increase sky brightness doubles in eight years. This rate is significantly greater than satellite observations, which grew by 2.2% per year globally between 2012-2016 and about 1.6% from 1992-2017.
Light pollution in the continental United States. (from https://lightpollutionmap.info)
Why the mismatch? The authors note that the sensitivity of satellite sensors is with wavelengths shorter than 500 nm. As older gas-discharge streetlamps emitting longer wavelengths have been replaced by LED bulbs that emit shorter wavelengths. Thus, some of the light is not being detected by the satellites. Moreover, the satellites detect mainly direct light, but not light reflected off the ground or other objects, which contributes significantly to light pollution. The authors conclude that the visibility of the stars is decreasing rapidly and existing lighting policies are unable to prevent increases in skyglow. Moreover, the authors think that switching to LED lights is a double-edge problem. Not only are shorter wavelengths being emitted, but the fact that LED lights are cheaper is encouraging people to install more lights.
The study highlights a venue in which average astronomy enthusiasts can contribute. Data for the study were collected by the Globe at Night program (https://www.globeatnight.org), which is run by the National Optical-Infrared Astronomy Research Laboratory (NOIRLab). It collects data from anyone who wishes to contribute. No special equipment is needed (star maps are provided for the purpose) and are easy to make, but SQM (sky quality meter) data are also encouraged. Although the data set is heavily biased by observations in the US and Europe, a glance at maps of observation locations shows that data distribution is low, even in those highly sampled areas. It was surprising to find that yearly observations are declining (see figure) and there are only a few observations from Houston (and none from the Dark Site). This is clearly an area where astronomy clubs and their members can make a significant contribution to science.
Distribution of Globe at Night observations for 2011-2022. (from Kyba et al., 2023)
References
Falci, F., and S. Bará, 2023. (Perspectives) Light Pollution is Skyrocketing. Science, 379, 234-235, https://doi.org/10.1126/science.adf4952.
Kyba, C. C. M., Y. O. Altintas, C. E. Walker, and M. Newhouse, 2023. Citizen scientists report global rapid reductions in the visibility of stars from 2011 to 2022. Science, 379, 265-268, https://doi.org/10.1126/science.abq7781.