A startling analysis from Globe at Night — a citizen science program from NSF’s NOIRLab — concludes that stars are disappearing from human sight at an astonishing rate. The study shows that artificial lighting dulls the night sky more quickly to the human eye than indicated by satellite measurements. The study published in the journal Science demonstrates the unique contributions that citizen scientists can make in key areas of research.

From the glowing arc of the Milky Way to dozens of intricate constellations, the naked human eye should be able to spot several thousand stars on a clear, dark night. Unfortunately, increasing light pollution has robbed about 30% of people around the world and about 80% of people in the United States of the nighttime view of their own galaxy. A new paper published in the journal Science concludes that the problem is rapidly getting worse.

New citizen science-based research sheds alarming light on the problem of “skyglow” — the diffuse illumination of the night sky that is a form of light pollution. The data for this study came from observations collected from around the world as part of Globe at Night, a program of NSF’s NOIRLab and developed by NOIRLAb astronomer Connie Walker. The study shows that sky glow is increasing faster than satellite measurements of Earth’s surface brightness at night indicate.

“At this rate of change, a child born in a location where 250 stars were visible would only be able to see 100 by the time they turned 18.” said Christopher Kyba, a researcher at the German Geosciences Research Center and lead author of the paper describing these results.

Light pollution is a well-known problem that has many adverse consequences not only for the practice of astronomy. It also affects human health and wildlife because it disrupts the cyclic transition from sunlight to starlight by which biological systems have evolved. Moreover, the loss of visible stars is a poignant loss of human cultural heritage. Until relatively recently, people throughout history enjoyed impressive views of the starry sky, and the effect of this nighttime spectacle is evident in ancient cultures, from the myths it inspired to the structures built in alignment with celestial bodies.

Despite being a widely recognized problem, the changes in sky clarity over time are not well documented, especially on a global scale.

Globe at Night has been collecting star visibility data every year since 2006. Anyone can submit observations via the Globe at Night web application on a desktop or smartphone. After entering the relevant date, time and location, participants are presented with a number of star charts. Then they record which one best matches what they can see in the sky without telescopes or other instruments.

This gives an estimate of what’s called the limiting magnitude to the naked eye, which is a measure of how bright an object needs to be to be seen. This can be used to estimate the brightness of skyglow, because as the sky brightens, the fainter objects disappear from view.

The paper’s authors analyzed more than 50,000 observations submitted to Globe at Night between 2011 and 2022, ensuring consistency by omitting entries that were influenced by factors such as cloud cover and moonlight. They focused on data from Europe and North America, as these regions had a sufficient spread of observations across the land area and over the decade studied. The paper notes that the skies are likely to clear more quickly in developing countries, where satellite observations indicate that the prevalence of artificial light is growing faster.

After devising a new method to convert these observations into skyglow change estimates, the authors found that the loss of visible stars reported by Globe at Night indicates a 9.6% increase in sky brightness per year over the past decade. This is much greater than the approximately 2% per year global increase in surface brightness measured by satellites.

“This shows that existing satellites are not sufficient to study how night changes on Earth,” Kyba said. “We have developed a way to ‘translate’ Globe at Night star visibility observations made at different locations from year to year into trends in brightness changes across the continent. That shows that Globe at Night is not just an interesting outreach activity, it is an essential measurement of one of the Earth’s environmental variables.”

Existing satellites are not well suited to measuring skyglow as it appears to humans, because there are currently no Earth-monitoring instruments that can detect wavelengths shorter than 500 nanometers, which corresponds to the color cyan or greenish-blue. Shorter wavelengths, however, contribute disproportionately to skyglow, as they are more effectively diffused in the atmosphere. White LEDs, now increasingly used in high-efficiency outdoor lighting, have an emission peak between 400 and 500 nanometers.

“Since human eyes are more sensitive to these shorter wavelengths at night, LED lights have a strong effect on our perception of the brightness of the sky,” Kyba said. “This could be one of the reasons for the discrepancy between satellite measurements and the air conditions reported by Globe at Night participants.”

Aside from wavelength differences, space-based instruments don’t measure light emitted horizontally, such as from illuminated signs or windows, very well, but these sources are major 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 redoubled efforts and developing new strategies to protect dark skies,” Walker said. “The Globe at Night dataset is indispensable in our ongoing assessment of changes in skyglow, and we encourage anyone who can get involved to help protect the starry sky.”