An international citizen science project has discovered that people around the world are seeing fewer and fewer stars in the night sky. The change in star visibility can be explained by an increase in the sky brightness of 7-10% per year.
“The rate at which stars are becoming invisible to people in urban environments is dramatic,” says GFZ German Research Centre for Geosciences, Dr Christopher Kyba.
“At this rate of change, a child born in a location where 250 stars were visible would be able to see only around 100 by the time they turned 18.”
The analysis is from Globe at Night – a citizen science program run by the National Science Foundation’s NOIRLab – and concludes that stars are disappearing from human sight at an astonishing rate.
Satellite observations had been taken and researchers thought they had a good understanding of the issue, however the new study has shown that human eyes seem to be more sensitive than satellites.
“We investigated the change in global sky brightness from 2011 to 2022 using 51,351 citizen scientist observations of naked-eye stellar visibility,” the researchers explain in their new paper.
“The number of visible stars decreased by an amount that can be explained by an increase in sky brightness of 7 to 10% per year in the human visible band.”
This increase is faster than the change in light emissions from the ground indicated by satellite observations.
The researchers think that the difference between human observation and satellite measurements is probably due to changes in lighting practices.
“Satellites are most sensitive to light that is directed upwards towards the sky. But it is horizontally emitted light that accounts for most of the skyglow,” Kyba explains.
“So, if advertisements and facade lighting become more frequent, bigger or brighter, they could have a big impact on skyglow without making much of a difference on satellite imagery.”
Another factor the authors cite is the widespread switch 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, so contributes more to skyglow,” Kyba says. “But the only satellites that can image the whole Earth at night are not sensitive in the wavelength range of blue light.”
Although Globe at Night is international, the research team focused on data from Europe and North America since these regions had the most data across the timescale.
However, the paper notes that the sky is likely brightening more quickly in developing countries, where satellite observations indicate the prevalence of artificial lighting is growing at a higher rate.