Learning Series: How Artificial Lighting Limits Modern Astronomy

From March 2026

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Transcript

Traci: Thanks for coming to our learning series this evening, everyone. I’m happy to introduce Dr. Wes Ryle. He’s the astronomer for the Cincinnati Observatory, a historic observatory that focuses on public outreach and science education. He was formerly a professor of physics and astronomy at Thomas More University and director of their campus observatory. During this time, he led many undergraduate observational astronomy research projects, focusing on topics such as eclipsing binaries, transiting exoplanets, and cataclysmic variables. I have no idea what those are, but I’m excited to learn more. So, take it away.

Dr. Ryle: Thank you so much for having me, Traci. This presentation is based a little bit off of a class that I gave at the Cincinnati Observatory on light pollution a few months ago. Some of the folks here were actually in attendance for that one. It’s also in partnership with and based off of a presentation that my friend from grad school, Dr. Steven Williams, gave to the Friends of the Observatory group about a year ago. He’s based in Flagstaff, Arizona with the U.S. Naval Observatory, and he’s done a lot of work maintaining the dark skies out there. If you’re familiar with the dark sky initiative, Flagstaff is kind of world-renowned for being a dark sky-friendly city. There are some slides in here that I borrowed from him. He’s on the call; I asked him to join just to keep me honest. I told him that if anybody has any really tough questions, I’m just going to send them to him. I’ve also recommended to Traci that maybe at some point in the future, if DarkSky Ohio wanted to have him give a little more detail on how Flagstaff tackles this, that would be a great example to go off of.

So we’re going to talk about how artificial light, how light pollution, affects professional astronomers. I know you’ve probably had a lot of presentations on the wide range of effects that light pollution can have, but we’re going to try to focus in on those specific issues related to doing professional astronomy. The talk will have a quick recap of light pollution and how those effects are worsening over time, a look at how light has encroached on the majority of professional observatories in the world, the actual direct effects on astronomical observations, a bit on how Flagstaff tries to deal with this problem in terms of interactions between the community and the observatories, and then we’ll talk a little bit about the other type of light pollution problem.

Satellites

Any guesses on what the other light pollution problem is for astronomers?

Satellites, right? It’s still really a form of light pollution, but it very much impacts astronomical observations, whether you’re talking about professional or amateur astronomers taking astrophotography. It’s a huge problem and gets lumped into this discussion quite a bit, so I thought it was worthwhile mentioning.

Light Pollution: Is It Worsening?

A couple of slides here at the beginning have some more recent publications related to the effects of light pollution and how they’ve been changing over time. This is available via Globe at Night, a group that actually does measurements of sky brightness worldwide. It mentions that from 2011 to 2022, about 51,000 citizen science observations of night sky brightness showed that the number of visible stars was decreasing by an amount that can be explained by an increase in sky brightness of 7 to 10% per year. This was actually higher than what was expected based on estimates of the growth of lighting worldwide, and it gives you an idea of how quickly the problem is increasing.

This other article talks about how widespread the problem is. A group from Colorado shows that 80% of the world, or more than 99% of U.S .and European populations, live under light-polluted skies, and the Milky Way is hidden from one-third of humanity. It’s worse for North Americans than for Europeans, but still bad for the majority of people out there.

Of course, you’ve all seen light pollution maps like this. These are updated pretty regularly and based on satellite data. In general, the eastern half of the U.S. is worse off than the western half. Zooming in on our region, you can see that at least for me in Cincinnati, it’s hard to go in any direction to escape the light pollution. Whenever people ask me at the observatory where they can go to see dark skies, we mention places like Stonelick State Park, Caesar Creek, or Hocking Hills as examples. But really, you need to travel pretty far to the east of Cincinnati. That’s the only direction where you have any chance of seeing a dark sky. This is also a simulated view showing how things have progressed from the 1950s up until about 2025, illustrating how rapidly this problem has grown.

These are all things you’ve probably heard of, along with the widespread effects of light pollution. We’re not going to focus on all these different effects. I’m going to focus in on just the astronomical aspects. A lot of astronomers would probably say we’ve been trying to address this problem for a very long time, because after the development of electricity, we’ve gradually had to deal with it more and more as time has gone by. It’s especially a problem for a place like the Cincinnati Observatory, which is one of the first observatories in the U.S. The original observatory was built literally on a hillside overlooking downtown Cincinnati. It was on Mount Adams, and this was before the widespread use of electricity. Interestingly, it was built in 1843, the telescope was first used in 1845, and then within a decade, actual physical pollution, coal smoke, was being produced so thickly in downtown Cincinnati that astronomical observations became impossible. They moved the observatory a few miles east to a very rural location to get away from the smog. But now, even though we moved away from the city, we’re still very light-polluted. The observatory’s research arm in terms of making astronomical observations essentially ended as early as the early 1900s because of light pollution, and that made us switch to become an outreach and education facility. You actually see that happening at a lot of other institutions across the world too.

This is an extreme case: Mount Wilson Observatory. If you’ve ever been, it’s a beautiful site up in the mountains above the LA basin. This is showing the view from Mount Wilson looking back towards LA: 1910 versus 1925 versus 2002, how that basin basically filled in and is now a very light-polluted area. There are still research facilities on Mount Wilson. Georgia State University, where I went to grad school along with Dr. Steven Williams, has a facility out there. So it is still possible to do research there, but the light is definitely encroaching.

This is a graphic from a 2023 study showing sky brightness above natural background levels along the x-axis, and the different professional observatories around the world. Some of the classic ones that made huge contributions to astronomy early on, like Lick Observatory or Palomar, are now in a situation where they are incredibly light-polluted. In general, the majority of continental U.S. sites have become quite light-polluted. Kitt Peak as the National Observatory location was once a very nice dark sky site, but Tucson has been expanding, so you see it in the middle of the list now. At the very top, the top three are sites in Chile. There’s been a huge expansion of observatories built in Chile, partly to access the southern sky, but also to exploit very dark locations in the foothills of the Andes or in the Atacama Desert.

Very much related to that, you were just mentioning the datacenter project that got cancelled in the area. There was a lot of worry and many articles about the best site on this list down in the Atacama Desert. An industrial mega-project was being planned to be built basically within sightline of this observatory, and there was an outcry in the astronomy community to try to preserve that dark sky location. A lot of money has been invested in these telescopes specifically at this location for their dark skies. The industrial project was actually a green project, a solar farm and wind farm, and if you can imagine, the same thing that makes it good for an observatory, clear skies and dry conditions, also makes it good for solar and wind energy. Fortunately, just within the past month, there’s been word that they have abandoned that project in Chile. It’s not completely official yet, but they have indicated their intention to stop it. So this is still definitely an active battle and something that’s taken place for observatories around the world.

How Light Pollution Impacts Astronomy

Now, in terms of how light pollution actually affects astronomical observations, and this is true whether you’re doing amateur astrophotography or professional astronomy, the biggest contributor is skyglow. The same thing that makes it impossible to view the majority of stars in the night sky is what affects our observations. On the left are some images from Celestron’s website. To be fair, I think those images are probably just adjusting contrast and brightness on a single picture to show the effect. On the right is a more realistic representation; this is the Pleiades star cluster. One was taken at a very light-polluted location and the other at a dark sky site, and you can see the relative difference in background sky glow. That decreases your contrast in terms of what you can actually see.

Obviously it affects the visual appearance of a picture, but what’s important for astronomers is signal to noise. When you’re trying to detect a source, whether it’s a star, a galaxy, whatever it might be, you want the signal, the light from that source, to be high compared to the background noise. That background noise comes from a few sources: some of it is related to the camera itself, but the majority is related to the light in the sky, the sky noise. This graph shows, for a single image, how the different types of noise increase as you increase the exposure time, and how the signal of the actual object changes. The solid black line is the signal, which increases roughly linearly with exposure time, double the exposure time and you get twice the signal from your object. But the biggest contributor is the blue dashed line corresponding to sky noise, which is also increasing with exposure time. If you’re at a site where sky noise is high, you’re going to be in trouble, especially if you’re relying on single images. Amateur and professional astronomers know you can sometimes stack images, but that requires more telescope time and still has its limits in terms of improving signal to noise.

With astronomers, the thing you always have to keep in mind is that time is literally money. We have lots of big professional observatories, but they’re usually over-prescribed in terms of the missions and projects they’re carrying out. Astronomers can apply for time at different facilities, their proposal gets judged against others, and you have to make the case that your use of the telescope is worthwhile. When you have to combat light pollution with increased exposure times or a greater number of images, you’re in a situation where the time simply isn’t available for everyone.

Color Temperature

The other thing we know, and this is true for how it affects both the human eye and imaging, is that the color of the light matters. The reason our sky is blue is because of Rayleigh scattering, where blue light is more susceptible to scattering in our atmosphere than red light. That blue light from the sun has a tendency to spread out over the atmosphere, causing the blue appearance of the sky. But this is also true for any light source on the ground. If you have a light source with a higher concentration of blue light, that light is going to get scattered through the atmosphere more than red. This has been a big problem because LEDs, while they are very energy-efficient and a good replacement for incandescent bulbs, also have a tendency to produce bluer light. The widespread use of LEDs for outdoor lighting has led to an even worse skyglow problem, because that blue light spreads more through the atmosphere.

Just quickly, for folks who may not be as versed in what we mean by a spectrum: when we talk about different colors of light, it’s useful to refer to a spectrum: what you get if you take light and pass it through a prism or a grating so it spreads out into a rainbow of colors. It’s the same effect that creates a rainbow during a rainstorm. In astronomy, we collect light through a telescope, pass it through a slit, then through a prism or grating, and get that spectrum. The spectrum tends to show features, sometimes dark lines on a rainbow background, other times bright lines on a dark background. Those lines contain a huge wealth of information. Even if an object is halfway across the observable universe, a detailed spectrum can tell you what it’s made of, something about its motion, its rotation, and even pressure, density, or temperature conditions. There’s a huge wealth of information in spectra.

When scientists are investigating spectra, they almost always look at it as a graph, brightness or intensity on the y-axis and wavelength or color along the x-axis. Wherever there are dark lines in the picture, that corresponds to dips in the graph. Most astronomers would consider spectroscopy to be the most valuable form of observation you can have, but it’s also costly in terms of resources. You need bigger telescopes and longer exposure times, basically because you’re spreading out the light. For all the reasons we’ve talked about, that also makes spectroscopy highly susceptible to the effects of light pollution.

The other thing we do in astronomy, particularly when measuring the brightness of an object, is use standardized filters so we can share data between observatories and telescopes on an apples-to-apples basis. If you just put a camera on any given telescope, that camera, telescope, and site are all going to have slightly different responses. By using standardized filters that are commonly used worldwide, we can calibrate the data and share it all on the same scale. One of the filter sets that has been around for a very long time is the Johnson-Cousins UBVRI system. On this graph, visible light goes from about 400 nm to about 700 nm, and you can see the B, V, and R filters are typical ones used for photometry, the measurement of light. With the LED explosion that’s happened, observations in B and V have tended to be more affected by light pollution, and this can cause problems because the nature of the night sky has changed over time.

This slide shows the spectra of different types of light bulbs. LPS stands for low-pressure sodium and HPS stands for high-pressure sodium; some older traditional outdoor lighting options, especially for dark sky-friendly places.

NBA LED stands for narrowband amber LED; PCA is phosphor converted amber. The F in front of LED indicates it’s a filtered LED, and then warm white or cool white are terms you might see on packaging. The color correlated temperature is often listed on the bulb itself.

What I want you to notice is that in general for astronomy, a narrower spectrum is better, because then you’re only interfering with a very narrow portion of the entire spectrum of whatever object you’re observing. In general, redder is also better, because of the Rayleigh scattering effect and skyglow we discussed. Low-pressure sodium are still excellent lights for outdoor lighting, and one of the newer alternatives, narrowband amber, functions pretty similarly and has become a popular option. High-pressure sodium can be replaced with phosphor-converted amber and produce the same type of light with less leakage into the blue. In general, all LEDs have a higher blue contribution than the other options and therefore tend to be worse for astronomical observations.

You can also look at how the different types of light affect skyglow as you move farther and farther away from your light source. These graphs are all scaled to high-pressure sodium, which is why high-pressure sodium is at 1 on both graphs. In the bluer portion of the spectrum, 350 to 500 nm, LEDs are very bad even at large distances in terms of skyglow. In the redder portion of the spectrum, LEDs perform a little better, but low-pressure sodium and other options we discussed still perform very well in either situation.

Real-Life Example of Lighting Upgrades

Now, briefly, and Dr. Williams is on the call if you have questions about what Flagstaff has done, sometimes referred to as the “Flagstaff Solution.” It is a great collaboration between the community and the observatories there. Flagstaff is known as the world’s first International Dark Sky City, and it’s because of the connection between Flagstaff and Lowell Observatory that this really stuck and has worked over time.

This image shows two similarly-sized cities: Cheyenne, Wyoming at the top and Flagstaff, Arizona at the bottom, showing the skyglow produced by those two towns. They’re both roughly the same population, yet you can see the huge difference in sky glow. You can actually stand in downtown Flagstaff on a moonless night and see the Milky Way, which is pretty impressive for a city of that size. You can also see in this image that Flagstaff’s sky glow and Phoenix’s sky glow are almost identical, despite Phoenix being about five times farther away, and of course a much, much larger city.

These light sources we talked about before are ones that Dr. Williams has dealt with out in Flagstaff. High-pressure sodium and low-pressure sodium were the go-tos for a very long time, but we’re starting to see a transition over to the phosphor-converted amber and the narrowband amber as well. Phosphor-converted amber has a layer of phosphor around the blue LED chip, which filters the light and produces that orange or amber glow. Dr. Williams is often out on the streets of Flagstaff making sure that lighting enforcement code is being followed. This is a photo he took at an intersection, and you can see that even there, there’s a wide variety of lighting fixtures using different types of bulbs. He uses a little handheld spectroscope to go around and measure the spectrum. In this case, you can see the bulb with the orangest glow is one of those narrowband amber lights, and this one here is an LED, you can see the bright blue bump in its spectrum. You can also visibly see in the photo that this one is much brighter and has much more glare. In terms of skyglow impact, the LED has a value of 5.6 and the narrowband amber has a value of 1.29, many times worse for the LED.

This is another intersection with no LEDs, which is good, comparing high-pressure sodium in the distance to phosphor-converted amber on the near side. Their spectra are very similar, but the high-pressure sodium does have a small blue bump, which makes the phosphor-converted amber just a little bit better for astronomical studies.

Flipping between the two: high-pressure sodium and phosphor-converted amber are often seen as interchangeable, and you can see they’re pretty well-matched in terms of overall color. But the PCA eliminates that blue bump, making it slightly more accommodating. And then this is low-pressure sodium compared to narrowband amber, narrowband amber is a little wider than low-pressure sodium, but still very comparable.

In terms of the big picture, this shows the relative visual skyglow impact of these different light sources. Any color correlated temperature LED is going to be much, much worse than the other options. HPS can be replaced by PCA with a slight improvement. Low-pressure sodium is technically still the winner, but narrowband amber performs just as well. These types of lights, phosphor-converted amber or narrowband amber, are not necessarily easily accessible to individuals yet. That’s hopefully coming in the future. But if you were talking to a municipality or someone putting in a big project, these are all terms that would be known by the outdoor lighting companies. They’re available at a corporate or commercial level but not yet for the individual consumer.

More About Satellites

And then just quickly, the other light pollution problem. If you’re an astrophotographer or professional astronomer, this happens very regularly now. I’ve been doing astronomy for about 20 years. When I was doing this as a college student, you might get a satellite streak across your image maybe five times a night. Nowadays, when you sit down to take a night’s worth of data, it’s every other image that has a satellite streak through it. This is an example of an image taken by one of the larger scopes in Cerro Tololo, Chile, shortly after one of those Starlink launches, where there are 24 or more satellites going in a line across the sky.

The scary thing is that it’s already bad in terms of Starlink satellites. The total satellite count right now is around 14,000, and Starlink is just the beginning. Lots of other corporations want to put their own communication satellite constellations into orbit. There’s even a proposal from Starlink to do a constellation of a million satellites, which is currently under review. The lower right of this slide shows that there are 1.23 million proposed satellite projects in various stages of development. This is a problem for astronomers, and also a security concern. Having too many satellites up there creates risk of cascade failure if satellite debris disrupts other satellites. That’s called the Kessler effect.

We should never leave out our radio astronomer friends, either. When talking about Earth-based observations, it’s really visible light telescopes and radio telescopes that can do the majority of the work. Radio astronomy deals with its own form of interference or “light pollution” as well. For example, if you get a chance to go to the middle of nowhere in West Virginia, there’s a beautiful, gigantic telescope called the Green Bank Telescope; that dish is a football field wide. It’s intentionally placed inside a national radio quiet zone with strong restrictions on radio sources. That could be a malfunctioning microwave, or even someone’s cell phone. For a long time, you weren’t allowed to use cell phones anywhere near the Green Bank Observatory. The Starlink satellites even affect radio telescopes, because they are leaking radio signals at some frequencies that are supposed to be restricted for radio astronomy use. So it’s a problem not just for visual astronomy, but for radio astronomy as well.

Just a few resources, many of these you’ve probably come across before, but if you want more detail on what Flagstaff is doing, there are some related sites. And I’m putting my friend Dr. Williams on the hook. I’ve got his email address listed there. He’s getting more and more into dark sky advocacy and battling light pollution in his role at the U.S. Naval Observatory, and he’s helped get me into this as well. His information is up there in case you’d like to contact him.

Thank you all so much.

Q&A’s

Traci: Thank you so much, Dr. Ryle. That was a great presentation. So many good, helpful facts, and I think a lot of us have been fighting light pollution for a really long time, but those graphs really cemented it in my mind. Very helpful, thank you. My first question: can we borrow any of your slides if we wanted to?

Dr. Ryle: Yeah, I checked with Dr. Williams because I was really worried at first; some of his slides obviously come from a government and military organization. At least last time, I got his approval to share these slides, so as long as he says it’s still okay…Steve?

Dr. Williams: Hi. Yes. The process for any talk I’m required to give is that it has to pass several checks: content, technical, and then a security review. Everything that Dr. Ryle has shown here has gone through all of those and passed through our public affairs office. Our public affairs officer even uses a lot of these slides. Feel free, just as you noticed how Dr. Ryle cited individuals like Dr. Mark Mirrison for that original photograph, if you could just do that, that’s all we could really ask.

Traci: Okay. I’m going to open it up to the rest of the group. Do we have any individual questions right now?

I had a couple more. I’m assuming everybody understands the Bortle scal. Ideally a one, terrible is nine. As far as Ohio or the Midwest, what do you think is a maximum Bortle we could aspire to in more metropolitan areas that would make it easier to see stars and use telescopes and observatories? If Cincinnati could get down to four or five, what would be reasonable?

Dr. Ryle: It’s hard to give a blanket answer because it depends on which type of study you might be doing. For example, I spent 16 years at Thomas More University, which is an incredibly light-polluted site, as bad as if not worse than the Cincinnati Observatory. If you’re doing studies that rely on measuring brightness changes, called differential photometry, where you’re measuring the brightness of a star, planet, or galaxy and comparing it to other objects in the same picture, then you can still do science and research. It’s only when you go beyond that, into more calibrated forms of photometry or spectroscopy, that you really become limited. You can do differential photometry from light-polluted skies. For more intensive observations, it’s hard to give a blanket number because it depends on the brightness of the objects you’re studying. Obviously the more light-polluted the skies, the more you limit yourself in terms of what any given telescope can actually achieve.

Traci: Next question: the narrowband amber. I don’t see that a lot when I’m recommending different light fixtures to folks. Do you know offhand if there’s a cost difference between that and warmer LEDs?

Dr. Ryle: Steve, can I send that to you?

Dr. Williams: Absolutely. Dr. Ryle touched on it a little bit. This is part of the cutting-edge of the lighting industry right now. Developing the LEDs is one thing, but coating them with some sort of material, typically phosphor, to give a more broadband emission so you get more of a white light, is easier and cheaper to do. When you want to really cut down the blue contribution, it does cost quite a bit more. Those narrowband amber fixtures are typically quite expensive compared to white light color correlated temperature LEDs, at least twice as expensive.

Even if you find bulbs on lighting websites or Amazon labeled “turtle-friendly,” for turtle hatching on Florida beaches, for example, those are typically the equivalent or close to narrowband amber or phosphor-converted amber. And even those can be as much as 10 times more expensive than equivalent LED bulbs. I’ve seen them at retailers like Target, and unfortunately right now, getting people with limited budgets to make the switch is just very difficult.

Traci: Well, maybe as time goes on the cost will come down. We can only hope.

Dr. Williams: Yes, and that is one of the things worth noting. The metrics that the lighting industry and municipalities use when installing lights include things like lumens per watt. The amber lights, whether phosphor-converted or narrowband, are getting better all the time, but so many municipalities have already switched to white light LEDs that they’re not going to spend five to ten times as much to replace them. You can only hope to change their minds going forward when they start to have to replace those lights for whatever reason.

Traci: I’m going to ask you another question, Dr. Williams. What have you found to be successful in Flagstaff for maintaining your dark skies as far as messaging goes, so that we can take what you’ve learned and transition it into Ohio? Specifically, one of the issues we face is around crime. People say if they don’t leave their front porch lights on, that’s how criminals will get them. Can you say anything about that?

Dr. Williams: Sure, I can give a short version. As far as I know, criminals still need to see to open a door or pick a lock. One of the best methods we put forward is motion sensor lights, though those can get tricky. In Flagstaff, when a deer runs by, your light turns on, your heart rate goes up, and then it’s just a deer staring back at you in the dark.

As of now, and I am very open to suggestions for research studies and in-depth analyses, there has been no correlation found indicating that lighting something to a certain level, whether with white light versus amber light, makes a difference in crime. One study I saw was done in New York City where they used diesel-powered white LED lights, the kind you see at road construction sites, placed them in certain areas, and crime was lower there. But it might have been something other than the light itself. I haven’t seen anything definitive on this.

That usually stops the conversation, for better or worse. As far as successes in Flagstaff, I could give an entire talk on it, but one of the primary things is that just like with new construction requiring plumbing codes and electrical codes, we have a lighting code. You won’t be able to open your business if you don’t meet the lighting code, and that gets enforced at the municipality level. Grandfathered-in fixtures eventually have to be replaced and must follow the code. Flagstaff has had a long history of collaboration between the Naval Observatory, Lowell Observatory, and the community, and the general attitude of the whole place is very much in favor of dark skies. It was kind of heralded as an astrotourism destination: come see the dark skies. Even the Chamber of Commerce, the city council, and business people embrace it. Hotel chains want people to come because of the dark skies, so they’re willing to put in amber lights. It’s a serendipitous confluence of a lot of different things.

Traci: I love that slide showing the difference in skyglow between Cheyenne and Flagstaff. It’s such a great example of what protecting your night skies can look like. And I would be interested to see if there is any difference in crime. That would be very interesting to look at. Does anybody else have any questions?

Mark sent me a question: Have you seen any studies about how the newer, brighter white car headlights are affecting light pollution?

Dr. Ryle: I haven’t seen any studies directly related to car headlights. I know they sure blind me when I’m driving. They’re a horizontal source of light, but there’s still plenty leaking up into the sky, and you’d imagine they’d have to be a contributor in some way. But at least I haven’t seen any particular studies yet.

Dr. Williams: I haven’t seen anything either.

Dr. Ryle: And really the main focus is going to be those lights that are left on all night long and what we can do to improve those, whether it’s something as basic as shielding, changing the spectrum, adding a motion sensor so it’s only on when needed, things like that.

Traci: Any other questions?

Audience Member: What was used to take those photographs of Flagstaff and Cheyenne from similar distances?

Dr. Ryle: If you look down at the bottom of that graphic, you’ll see it’s credited to the U.S. National Park Service Night Skies Program. You’ll also notice the dates in the upper right; these were taken in 2016. If you go to the National Park Service website, I think it’s nps.gov, though, don’t quote me, just do a search, they actually have a section on dark skies. This was a very special setup. If you look closely at those images, you’ll notice they’re almost pixelated; they took images of the entire night sky in chunks and then processed and stitched them together so you could measure sky glow relative to other locations. They do this at national parks, so you can see the progress of light pollution in places like Yosemite or Yellowstone. I’d strongly encourage you to explore that. Essentially it was a CCD device, a digital camera with a lens, programmed to scroll through the night sky over a couple of hours. They tried to make it as portable as possible so they could go from park to park. The details will be there on the website.

Audience Member: I was just wondering if we could do a comparison shot doing something similar at Caesar Creek, John Bryan State Park, Hocking Hills, Lake Hope, or any of the parks around here, because as Traci pointed out, that is an awesome graphic for conveying where somebody is paying attention to the issue versus where they’re not. Thank you.