16 March 2023

White LEDs contributing to skyglow, satellites impacting astronomical observations, and stars vanishing from sight — the effects of light pollution are revealed thanks to alarming new evidence from Globe at Night, a global citizen science program of NSF’s NOIRLab. The study reveals that indeed the night sky is disappearing, but there is always something we can do. On 13 March 2023, Globe at Night will begin its next campaign, which will last until 22 March and coincides with the launch of a new Globe at Night website that features a revamped and more streamlined user interface.

Why is the dark night important, and what can we do to protect it?

If you happen to live in a large city or major metropolitan area, you certainly have the opportunity to enjoy a vibrant nightlife. What you won’t be able to enjoy, however, is a vibrant night sky. Travel to a more rural area, however, and a summertime evening stroll would likely showcase hundreds of stars and clearly delineated constellations. Aside from those lucky enough to visit National Parks, that is about as good as it gets for most of the people in the United States; a pleasant view of the stars and brightest planets, but nothing that stops you in your tracks. And, believe it or not, the night sky can indeed stop you in your tracks.

On a dark night at the right time of year, in just the right place on the planet, it seems as if the entire Milky Way, and even a few galaxies beyond, are visible to the naked eye. Our home galaxy is blazing overhead, and if conditions are just right, you may even see your shadow cast by light reflected by Venus. Surely that must already be the showstopper — but wait, there's more. If you were to stand on the summit of Cerro Tololo on a moonless night right next to the Víctor M. Blanco 4-meter Telescope, you could see the faint outline of your body against its outer white walls. Turn around and look at the sky: the only things producing that light are the stars of the Milky Way itself.

Why is the Night Sky So Important?

As incredible as this sounds, the fact is that it’s becoming increasingly difficult to experience these awe-inspiring sights. Ask your next-door neighbor, your coworker, maybe even your friend who goes on hikes, and you might be hard pressed to find someone who has actually seen our home Galaxy. In fact, an estimated 83% of people on Earth live under light-polluted skies that essentially wash out the Milky Way. Why is that?

An estimated 83% of people on Earth live under light-polluted skies that essentially wash out the Milky Way. Why is that?

The answer lies in global urbanization and the technological advancement of the last century. Urban life is synonymous with glimmering city lights at night, and, as attractive as that sounds, light pollution has dramatically increased because of it. Prevalent external sources such as streetlights, car headlights, and bright advertising billboards all contribute to the decreased visibility of the starry night sky, and the advent of new efficient illumination devices such as light-emitting diodes (or LEDs) has had notable repercussions as well. All of these add into the mix of why you might really only be able to see the Moon in the midst of a brightly-lit city.

The obvious impact of increased light pollution is the threat it poses to mountaintop sites where major world-class astronomical facilities, such as NOIRLab’s Kitt Peak National Observatory and Cerro Tololo Inter-American Observatory, operate. With the newest generation of ground-based observatories coming online and conducting cutting-edge research into dark matter, exoplanets around distant stars, and other thought-provoking astronomical and cosmological questions, preserving their ability to do this science is more important than ever.

In addition, light pollution has a detrimental effect on various ecologies and life on Earth. Compelling evidence shows how constant exposure to light may negatively affect nocturnal animals and insects, crucial ecological systems that concern agriculture and food supplies, and even human biological systems that are important to our health and wellbeing. Our circadian rhythms depend on dark skies in order to know when to take proper rest, as well as to keep our internal clocks in check.

Yet at the core of it all dark skies have an even deeper connection with human civilization and culture. Entire generations living in urban areas have never seen the Milky Way arching overhead at night, and understanding our place in the Universe starts with being able to see the night sky in the first place. The stars have ever been intertwined with human history, from celestial navigation to myths and legends spurred by apparent asterisms in the sky. And much of our cultural heritage is intimately tied to curiosity about how the world works, including what lies beyond our atmosphere. Thanks to light pollution, those dark skies will disappear along with our fascination with the stars above.

That is why monitoring and mitigating light pollution as much as possible are vital not just for our understanding of the Universe, but for humanity and the well being of our planet as a whole.

Globe at Night Data

To better understand the ever-evolving state of light pollution and ‘skyglow’ — the diffuse illumination of the night sky — a small army of citizen scientists have pooled their talents and resources to track and report on the impact of skyglow on the visibility of the night sky through a program called Globe at Night. Connie Walker and Steve Pompea, astronomers at NSF’s NOIRLab, developed this program in 2006 to give citizens the opportunity to contribute to the task of surveying night sky visibility.

Thanks to the collective efforts of over 250,000 people, Globe at Night has been gathering extensive data on stellar visibility for 17 years. Anyone can submit observations through the Globe at Night web application on a desktop or smartphone. By comparing what a participant sees in the night sky to a selection of star maps on their cell phone or tablet at that moment and location, a built-in algorithm produces a rough approximation of the ‘naked eye limiting magnitude’, or the measure of how bright an object must be in order to be seen. This in turn can be used to estimate the brightness of skyglow, because as the sky brightens, the fainter objects disappear from sight.

"At this rate of change, a child born in a location where 250 stars were visible would be able to see fewer than 100 by the time they turned 18,” said Christopher Kyba, a researcher at the German Research Centre for Geoscience.

What is particularly baffling from new research is the revelation that light pollution is diminishing our ability to see the stars more quickly than indicated by satellite studies. These data show that the unaided eye should be able to make out approximately 3000 stars on a clear, dark night; yet within the United States, these stars are disappearing from approximately 80% of the population’s evening view. And even more alarming, a new paper published in the journal Science implies that the situation is quickly snowballing as the problem becomes progressively worse.

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 confounding factors, such as cloud cover and moonlight. They focused on data from Europe and North America, since these regions had a sufficient distribution of observations both across the land area and 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.

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

By converting the results of these observations into estimates of the change in skyglow, the authors discovered that there is a 9.6% per year increase in sky brightness over the past decade, thanks to the reports of Globe at Night participants. When compared to the 2% per year global increase in surface brightness measured by satellites, the disparity becomes glaringly obvious.

Kyba notes, “[This] shows that Globe at Night isn't just an interesting outreach activity, it's an essential measurement of one of Earth's environmental variables.”

Evidently, the same satellites we rely on for much of our technology are not well suited to measuring skyglow as it appears to humans. This is due to the fact that satellites do not detect the blue emissions of white light as well as the sensitivity of the naked eye, especially at night. These bright sources, including LEDs, thus have a stronger effect on the perception of sky brightness, and why the information gained from Globe at Night’s crowd-sourced observations will remain invaluable for investigating the direct human effects of sky brightness.

NOIRLab and Protecting Dark Skies

Thanks to the eye-opening conclusions drawn from years of Globe at Night studies, NOIRLab is now at the forefront of protecting dark skies. This commitment starts in the local communities we share our facilities with.

For Arizona, an established Pima County/City of Tucson Outdoor Lighting Code helps protect the work of Kitt Peak National Observatory from surrounding light pollution. NOIRLab is also a member of the Arizona Astronomy Consortium, an advocacy organization that is currently gathering support for conventionally illuminated billboards over its digital LED counterparts.

In Chile, NOIRLab has been collaborating with other AURA staff, the Oficina de Protección de la Calidad del Cielo del Norte de Chile, University of La Serena, Fundación Cielos de Chile, and local authorities by monitoring the sky brightness resulting from the use of artificial lighting at night. These systemic measurements help determine areas that require correction or updating of their lighting fixtures in order to minimize light pollution, including locations near Cerro Tololo and Cerro Pachón. Combined with educational outreach events and campaigns, NOIRLab hopes to cultivate a broader awareness of a pristine night sky to the general public.

And in Hawai‘i, the University of Hawai‘i has taken the lead on action against light pollution on behalf of the Maunakea Observatories. While visible-wavelength light pollution on Maunakea is relatively small, steps are being taken to further mitigate any preexisting effects. NOIRLab has supported the university with requested data and other assistance, and plans to display advocacy materials in Hilo and Maunakea. By 2028, NOIRLab will also implement the same Site Quality Cameras used in Chile and Arizona at the summit facilities to replace its common site monitoring equipment.

Through these actions, NOIRLab endeavors to reduce light pollution in surrounding areas to a minimum. Yet, the vast usage of strong lighting doesn’t completely encompass the whole problem.

While increased skyglow has been an ongoing issue on the ground, the light from passing satellites far above has also startlingly become a serious issue as well. ‘Satellite light pollution’ has been around since the first launch of communication satellite constellations, many of them following a low orbit to provide broadband internet access. By the end of the 2020s, more than 5000 satellites will be just above the local horizon at any given time as seen from a dark-sky observatory. As useful as they are, the sheer number of them — not to mention the additional thousands of inactive satellites, space junk, and pieces of debris within Earth’s orbit — contributes to negative impacts on night sky visibility.

With that in mind, NOIRLab is actively working with the SKA Observatory (SKAO), an intergovernmental organization based in the UK, to mitigate these effects. NOIRLab and SKAO were named as co-hosts of the newly established International Astronomical Union Centre for the Protection of the Dark and Quiet Sky from Satellite Interference (IAU-CPS) in 2021, and this center will coordinate efforts to diminish the negative interference caused by large satellite constellations. By bringing together astronomers, satellite operators, regulators, and the wider community, the IAU will act as a bridge to moderate the night sky, and become the leading voice for astronomical matters that relate to its protection.

What Can You Do?

As grand as Globe At Night and the IAU-CPS sound, change doesn’t have to be complex for the individual and their communities; there is still much to do here on the ground that will create just as much of an impact.

By the end of the 2020s, more than 5000 satellites will be just above the local horizon at any given time as seen from a dark-sky observatory.

Each February, Globe at Night, in partnership with the International Dark-Sky Association (IDA), invites people to participate in the Love the Stars campaign. Starting the Sunday before Valentine’s Day, and running for ten nights, the Globe at Night campaign asks participants to look up at the night sky, find the bright constellation Orion in the northern hemisphere, or Gemini in the southern hemisphere, and then submit an observation identifying which of a set of star maps most closely matches what they see. Measurements can be submitted on the report page, accessible through a computer or smartphone. Scientists use these observations to measure and monitor how light pollution changes globally.

The Globe at Night program does not stop there. 13 March of this year marks the beginning of the next Globe at Night campaign, which will last until 22 March. The event is being kicked-off with the unveiling of a new website that features a revamped and more streamlined user interface. The interactive data map has also been redesigned to include all submitted data points across current and past years.

Despite the growing numbers of the population affected by this phenomenon, only a fraction truly understand why dark skies are so important. By participating in outreach events, talking to your friends and family, and spreading the word online, more and more people can reach an understanding that will enable them to better combat light pollution and protect the night sky. In doing so, we also protect our ecosystems, our cultural heritage, and our endeavors to understand the Universe.


Chiara Villanueva
Chiara Villanueva is a science communicator at NSF’s NOIRLab. They have a background in astrophysics and mathematics, and will be undertaking a master’s degree in science communication at University of California, Santa Cruz.

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