Astronomical observatories enjoy some of the world’s darkest night skies. But even there light pollution is spreading, a new study suggests.

If you want to escape light pollution and experience a truly dark sky, go where the pros go. The World Atlas of Artificial Sky Brightness, first compiled in 2000 and updated in 2016, shows that most major professional observatories in the world lie in black spots, meaning their sky is almost as dark as it was when humans first started lighting up the night.

Light pollution map for North America
The World Atlas view of North America shows light pollution is strongest in the eastern half. But light pollution is also creeping up on observatories in the western half. (Explore light pollution all over the globe.)
Esri / HERE / Garmin / FAO / NOAA; Source: Airbus / USGS / NGA / NASA / CGIAR / NLS / OS / NMA / Geodatastyrelsen / GSA / GSI / GIS User Community

To put it into numbers: The measured sky brightness over these telescopes is less than 1% brighter than the assumed natural sky brightness. In some of those regions, laws have even been enacted to stop the spread of light pollution and secure an unhindered view into the cosmos.

But this strategy isn’t working particularly well, according to a new study published in the Monthly Notices of the Royal Astronomical Society. Light pollution is spreading even where the night was truly dark. As of 2021, only a handful of all large observatories resulted to remain below the 1%-line, while almost two-thirds have already seen their night skies brighten 10% over the assumed natural levels, the researchers report. Which means their locations in the World Atlas are no longer “black” or even “dark-grey.”

Eyes from Orbit

Fabio Falchi (Light Pollution Science and Technology Institute, Italy) and colleagues analyzed data gathered in 2021 by the Suomi NPP satellite’s Visible Infrared Imaging Radiometer Suite (VIIRS), collecting all the sources of light in a radius of 500km of major observatories. The sites investigated include all active or planned professional observatories with telescope apertures of 3 meters or larger, as well as potential and historic sites. They also included a selection of spots used by amateur astronomers, such as observatories offering rental telescopes.

Falchi’s team then employed a light propagation model to compute the sky brightness created by the visible light sources that VIIRS found, taking into account the difference between light sources’ and observatories’ elevation and the general topography between them. From this model, they then calculated five indicators of sky brightness, including the radiance at different altitudes above the horizon as well as an average all-sky radiance.     

Light pollution map for South America
Light pollution isn't as strong in South America, but even here observatories are not immune. (Explore light pollution all over the globe.)
Esri / HERE / Garmin / FAO / NOAA; Source: Airbus / USGS / NGA / NASA / CGIAR / NLS / OS / NMA / Geodatastyrelsen / GSA / GSI / GIS User Community

Volcanoes, "Friendly Fire," and Streetlights

The sharp resolution VIIRS affords enables scientists to disentangle individual light sources contributing to a site’s sky brightness. Some of them were quite unexpected: For example, the active Kilauea volcano on Hawai‘i, 50 km from the Mauna Kea observatory, emits mostly at infrared wavelengths, but some of its light spills into visible wavelengths.

Also surprising was strong light pollution affecting the Tokyo Atacama Observatory (TAO) in Chile, which turned out to be “friendly fire” from the Atacama Large Millimeter/submillimeter Array. The radio astronomers who operate ALMA might not appreciate the proble visible light presents. Without their pollution however, TAO would have won the contest for darkest observatory, as measured by the amount of radiance direct overhead (at zenith).

As it stands, though, the Paranal Observatory, also in the Atacama Desert in Chile, is the winner. This is despite the temporary nuisance of light emitted by workers’ lodging near the European Southern Observatory’s under-construction Extremely Large Telescope. Once completed, the telescope’s 39-meter mirror will be the largest in the world.

Other findings are more troublesome. A single partially illuminated highway 40 km away heavily affects the Las Campanas Observatory, home of the twin 6.5-meter Magellan telescopes and future site of the Giant Magellan Telescope (GMT). The highway  contributes more than 50% of overhead radiance in the otherwise pitch-dark Chilean desert.

Losing the Night

There’s some hope that such sources of light pollution could be removed or at least reduced relatively easy, the researchers say. However, the general trend seems more complicated. The lowest light pollution level in the World Atlas, which is a maximum of 1% over the assumed natural level of 22.0 magnitudes per square arcsecond at zenith, only occurs over seven major observatories. The only one of these on U.S. soil, on Mauna Kea in Hawai‘i, is already at the critical 1% mark.

“All other major astronomical observatories in the continental U.S. have already crossed the 10% limit,” Falchi and colleagues write, “while most Chilean ones are still below it, even if some are relatively close.” Potential future sites, like the GMT’s, may cross critical limits even before they become operational, the researchers worry.

The sky brightness at zenith can be relatively forgiving, since most of the light directed overhead escapes to space rather than scattering around the sky. But sky radiance at lower angles matters, too. While professional telescopes rarely observe at angles lower than 30° above the horizon, the radiance in the first 10° above the horizon matters to night-sky observers of any stripe. Thanks to the effect of forward scattering, even very far away sources can ruin the nocturnal landscape: The city of Antofagasta can be seen from Paranal, Honolulu from Mauna Kea and Las Vegas from the Grand Canyon National Park. The impact on the near-horizon therefore serves as an early warning, Falchi and his team suggest.

The results are “the last call for a serious, collective, unambiguous, no-compromise action to lower light pollution now,” Falchi’s team concludes. “Failing to take action implies a progressive decline of the ability to explore our universe.”

Astronomical observatory sites, due to their remote locations, are the least affected by light pollution, which makes them the proverbial canary in the coal mine: “If we are not able to keep the canary alive, then we can forget being able to solve the problem of light pollution as a global environmental issue.”


Image of Doug


January 6, 2023 at 4:50 pm

It is sad. I’ve almost given up night observations from my backyard.

You must be logged in to post a comment.

Image of Tom-Gwilym


January 7, 2023 at 1:57 pm

I know the feeling Doug. I'm starting to wonder sometimes if it's time to give up on on the hobby and sell my backyard Nexdome and contents. I'm fighting it here in our small Wisconsin city, but it's a losing battle I feel. I moved here 4 years ago and loved that I could see the Milky Way over my house, but a billboard installed to my SE pretty much wiped out the core of the Milky Way.

You must be logged in to post a comment.

Image of Martian-Bachelor


January 7, 2023 at 4:31 pm

I live hardly ten miles from a middle-of-nowhere small national park that was given international dark sky status maybe 8-10 years ago. There seems to be zero awareness around here that light pollution is even an issue, that responsible use of outdoor night lighting is something of value people need to do as a matter of course.

With each new housing development it gets worse. And, after several tries on the ballot, they finally got a tax increase to build a new public atheletic club (indoor pool, weight room, etc.) The parking lot is immense, and of course they leave the lights on all night -- for the usual reasons, like "safety" -- and so that RV-ers passing through won't spend the night in the parking lot there, rather than at the WalMart or one of the grocery stores.

It's a losing battle because there's so little cost in destroying the darkness, and it's being funded by taxes and the real estate industry.

You must be logged in to post a comment.

Image of Anthony Barreiro

Anthony Barreiro

January 6, 2023 at 6:28 pm

Thank you for this detailed report.

The light pollution from the ALMA observatory reminds me of an experience some years ago at a star party on Mount Diablo, California. A ham radio club also set up their antennas around a trailer in the same parking lot, with floodlights blazing all night long. I wished I had a powerful radio transmitter so I could give them a taste of their own medicine.

I live in badly light polluted San Francisco. I often walk to a nearby hilltop to at least get above the streetlights. In recent years more and more people walking in the park at night have the flashlights on their cell phones lit up all the time, even when there's enough moonlight to cast shadows. Many people now seem to live their entire lives under bright artificial illumination. It's troubling.

On the other hand, light pollution is probably the easiest kind of pollution to solve. Once you turn off a light, or replace a light fixture with a night-sky-friendly alternative, the problem is immediately solved, there is no residual light pollution.

You must be logged in to post a comment.

Image of PGT


January 6, 2023 at 7:35 pm

My situation, as well. Over the years, Orion has grown dimmer and the Pleiades are now almost lost to sight. The new-tech style street lights do not help, as they seem brighter than the old style ones.

The irony is, of course, as the neighborhood at night grows brighter, "second-hand" observations via the Webb telescope, etc., are growing by leaps and bounds.

Sad not to see things firsthand anymore, but exciting to see things I'd never have seen via the S&T website.

You must be logged in to post a comment.

Image of Peter Jacobs

Peter Jacobs

January 7, 2023 at 3:42 pm

Like many "improvements" we humans seem to take pride in, the implementation of LED street lights seems to be a step backwards with regard to efforts to improve our night skies. Because it involves several technologies a discussion about dark skies has become more difficult.

High pressure sodium lamps with good shielding provide both good light control and the color is neither obnoxious, nor penetrating. But, new LED fixtures tend to be broad spectrum and particularly cool or blue. Even with good light shielding, the ground and surrounds scatter white LED light much more than the old "yellow" HPS fixture. And due to long life, as well as higher energy efficiency, utilities and governments are adapting them rapidly.

You must be logged in to post a comment.

Image of Martian-Bachelor


January 7, 2023 at 4:17 pm

Not to mention that for the same wattage you get a ton more light out of an LED due to their higher efficiency, so the new improved versions tend to be brighter, which of course is going in the wrong direction.

In my grocery store they use motion sensors to turn on the lights inside the display cabinets in the frozen foods section just a step or two ahead of someone walking down the aisle. (I think they're overhead, hanging from the ceiling) Why can't they put streetlights on motion sensors, too? They'd save a ton of money on routes that aren't heavily travelled during much of the night.

You must be logged in to post a comment.

Image of Jay-Eads


January 9, 2023 at 6:44 pm

I have said this for a long time, and most in the hobby don't want to accept it. As long as the human population is expanding, as long as there is money and a lot of it to be made in development and construction and there is, light pollution will continue to increase. In my 15 years of going to a favorite dark site which is only about an hour and fifteen minutes from my home, the SQM-L readings I have taken have moved from the upper 21.7 to lower 21.8 range depending on the season to the upper 21.6 range, sometimes on a bad night I can only get a mid to upper 21.5 range. That means I have to drive an additional hour to get the readings I use to get. Still that is good enough for what I do and enjoy but LP from the Salt Lake to Provo metro dome is ever increasing. I can see it even on my annual trip to a very, very dark spot in western Utah or eastern Nevada, something 20 years I never saw.

I don't think the LP battle is going to be won or remedied in my life or even in the life of my children or grandchildren. It is why the visual arm of the hobby is dying and will continue to die, as the imaging side continues to grow. However, I think with the loss of visual observing will come a reduction of those interested in even exploring the hobby and trying it out. The hobby isn't dying, just changing and that is a reality. Dealing with LP is perhaps best done with being an amateur imaging hobby enthusiast.

You must be logged in to post a comment.

Image of Bob


January 9, 2023 at 10:06 pm

I was lucky enough to retire and have some funds to build an observatory in an astronomy community in SW New Mexico. My current residence in northern VA is bloody awful.

However, we too are now being faced with "improvements" along US180 that runs from Deming to Silver City.

And don't get me started on Starlink.

You must be logged in to post a comment.

Image of Alan


January 10, 2023 at 6:25 pm

I got involved with the IDA 30 years ago and told them that you can't bring about change by trying to teach stubborn people about light pollution. You need laws to bring about change. The IDA wouldn't listen. 🙁

You must be logged in to post a comment.

You must be logged in to post a comment.