Stars Disappear Before Our Eyes: Startling Analysis From Globe at Night
Stars Disappear Before Our Eyes: Startling Analysis From Globe at NightNOIRLab’s Globe at Night educational program reveals how increasing light pollution is robbing us of the night sky.A startling analysis from Globe at Night — a citizen science program run by NSF’s NOIRLab — concludes that stars are disappearing from human sight at an astonishing rate. The study finds that, to human eyes, artificial lighting has dulled the night sky more rapidly than indicated by satellite measurements. The study published in the journal Science showcases the unique contributions that citizen scientists can make in essential fields of research.
“At this rate of change, a child born in a location where 250 stars were visible would be able to see only abound100 by the time they turned 18,” said Christopher Kyba, a researcher at the German Research Centre for Geosciences and lead author of the paper detailing these results.
Light pollution is a familiar problem that has many detrimental effects, not only on the practice of astronomy. It also has an impact on human health and wildlife, since it disrupts the cyclical transition from sunlight to starlight that biological systems have evolved alongside. Furthermore, the loss of visible stars is a poignant loss of human cultural heritage. Until relatively recently, humans throughout history had an impressive view of the starry night sky, and the effect of this nightly spectacle is evident in ancient cultures, from the myths it inspired to the structures that were built in alignment with celestial bodies.
Despite being a well-recognized issue, however, the changes in sky brightness over time are not well documented, particularly on a global scale.
This gives an estimate of what is called the naked eye limiting magnitude, which is a measure of how bright an object must be in order to be seen. This can be used to estimate the brightness of skyglow, because as the sky brightens, the fainter objects disappear from sight.
The authors of the paper analyzed more than 50,000 observations submitted to Globe at Night between 2011 and 2022, ensuring consistency by omitting entries that were affected by factors including cloud cover and moonlight. They focused on data from Europe and North America, since these regions had a sufficient distribution of observations across the land area as well as throughout the decade studied. 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.
After devising a new method to convert these observations into estimates of the change in skyglow, the authors found that the loss of visible stars reported by Globe at Night indicates an increase in sky brightness of 9.6% per year over the past decade. This is much greater than the roughly 2% per year global increase in surface brightness measured by satellites.
Existing satellites are not well suited to measuring skyglow as it appears to humans, because there are no current instruments monitoring the whole Earth that can detect wavelengths shorter than 500 nanometers, which corresponds to the color cyan, or greenish blue. Shorter wavelengths, however, contribute disproportionately to skyglow, because they scatter more effectively in the atmosphere. White LEDs, now increasingly commonly used in high-efficiency outdoor lighting, have a peak in emission between 400 and 500 nanometers.
“Since human eyes are more sensitive to these shorter wavelengths at nighttime, LED lights have a strong effect on our perception of sky brightness,” said Kyba. “This could be one of the reasons behind the discrepancy between satellite measurements and the sky conditions reported by Globe at Night participants.”
Beyond wavelength differences, space-based instruments do not measure light emitted horizontally very well, such as from illuminated signs or windows, but these sources are significant contributors to skyglow as seen from the ground. Crowd-sourced observations will therefore always be invaluable for investigating the direct human effects of sky brightness.
“The increase in skyglow over the past decade underscores the importance of redoubling our efforts and developing new strategies to protect dark skies,” said Walker. “The Globe at Night dataset is indispensable in our ongoing evaluation of changes in skyglow, and we encourage everyone who can to get involved to help protect the starry night sky.”