The news reports are at first ominous. Purple glows appear over towns. Some headlines are even comical: "Tomato factory lights mistaken for 'lovely aurora'". But they share a common origin. To extend growing seasons and further the agriculture industry, more food crops are grown in greenhouses. Artificial lighting used at night aims to increase yields. And that light is having an unwanted effect on neighbors and the night sky. Yet some are betting the farm that new technologies and better regulations can solve this problem. 

Greenhouse farming is on the rise around the world. In the U.S. alone, its economic output was almost $6 billion in 2024. Rising at an annual rate of almost 9% per year, analysts expect it to top $11 billion by 2030. Industry experts make a case for investing further into this sector. Last year Agritechture wrote: "The agricultural landscape in the United States is undergoing a transformative shift, driven by supply chain weaknesses identified during the pandemic, the changes in our climate making it harder for field farmers to grow consistently, and the increasing demand from consumers for more sustainable and locally sourced produce. Greenhouses offer a viable solution to meet this shift in consciousness by providing a controlled environment for year-round cultivation."

With year-round cultivation comes an effort to grow plants faster by lengthening the growing day. Illuminating plants during overnight hours can make them bigger and more productive. "Horticultural lighting" can also reduce the time from seed to crop by accelerating growth. Rapid improvements in lighting technology better support year-round growing seasons.

In particular, the arrival of LED lighting was as much of a game-changer for agriculture as in other sectors. Earlier lamps consumed a lot of electricity and radiated much of their power in the form of heat. Besides being much more energy-efficient, LED has remarkable color characteristics. Its properties are selectable to give plants only the light they need for photosynthesis, explaining the pink or purple colors reported in news stories.

Given the challenge of providing adequate food for the global population, no one doubts the value of greenhouse farming. Food insecurity is a serious threat to economic development. According to the World Bank, "food security continues to be at alarming levels in most low-income countries." That is particularly true in Africa, where famines and rising food prices put millions at risk.
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At the same time — and ironically in part due to light pollution — populations of pollinating insects are in decline. Thus far, machines cannot replace the "ecosystem services" pollinators provide for free. These factors combine to create distinct threats to humans through potential disruptions in the global food supply. 

Their clear glass walls and roofs that make greenhouse ideal during the day is the source of a problem at night if owners use light to extend the growing day. Even with good lighting design and best-in-class lighting products, greenhouses are the source of "obtrusive" light. Light scatters and reflects from glass, interior surfaces, and plants themselves. Some of that light emerges from the greenhouses sideways and can cause light trespass. Light leaving from transparent roof panels travels unimpeded into the night sky. This causes the strange glows reported in news stories. And it can have ecological consequences of its own. For example, one recent study found that greenhouse lighting is harmful to songbirds.

What can be done about this? The fundamental solution involves keeping light contained within the building at night. That's an obvious challenge for structures whose very nature is to let outside (sun)light in. To address that particular aspect, some have experimented with so-called 'smart glass'. This method uses exotic materials that are alternately transparent or opaque to visible light. The state of transparency changes when, for example, an electric current is applied to a glass panel. But these materials are rather expensive, and their opacity is usually not enough to keep all interior light contained.

​A lower-tech solution involves a much lower-tech approach: close the blinds. Roof-mounted machinery deploys physical window coverings at dusk and retracts them at dawn. As shown in the example below, these can be an effective mitigation for much of the light that would otherwise escape the greenhouse. Yet even this trick isn't inexpensive, and it involves moving parts subject to wear and tear.

These mitigations can be combined with local regulations to enforce changes. The simplest laws impose a lighting "curfew" time, after which greenhouse owners must switch lights off each night. That may be most fair to people who live near greenhouses to reduce the impact of lighting. Of course, growers may take exception to such policies as unfairly limiting their operations. Zoning restrictions can help put some distance between greenhouses and their neighbors. And some jurisdictions may choose to prohibit greenhouses entirely within their territories.

​But regulation is challenging to put in place correctly and consistently. It often fails to completely address the problem. Like other examples of conflicting land uses, satisfying all those involved may be impossible. The benefits to society that greenhouse farming represents must be weighed against its social costs. In the ideal case, workable solutions respect the rights and wishes of all stakeholders.

Modern greenhouse lighting is here to stay, and by all accounts will only be more important to the global economy in the future. Yet there are clear challenges to enabling its future development while reducing its effect on the nighttime environment.

The controllability of LED could be the key to solving this problem, alongside other tech. But good old-fashioned "shutting the blinds" is the best approach. It presents an added expense to operators, which could as easily be counted alongside other cost of doing business. In that sense it's not unlike complying with building codes or workplace safety laws. And it's up to each jurisdiction to decide how much (and how best) to regulate.

Recent experience suggests that jurisdictions should get out ahead of this issue before it lands on their doorstep. Often the first sign of a new greenhouse locals notice is the nighttime glow. Reactionary efforts at regulation have a habit of proceeding rather poorly. Proactive attention to the problem can reduce that tendency.

There may be also some lessons learned here about things like land-use zoning. They may point the way to how to deal with other outdoor lighting challenges from 'speciality' lighting applications. Because of its associated effects, greenhouse farming might be subject to geographic restrictions. Regulators should, however, be mindful of the fact that light pollution can drift far from its sources. Other interventions may still be necessary to avoid conflicts.

​Can the future of greenhouse farming remain bright without compromising the dark? Any serious discussion of that question must consider both lighting design and regulation. It must further weigh its economic benefits against potential social and environmental harms. As in many such instances, the ideal solutions find the right balance between the needs of both people and planet.

It sounds like something out of a bad dream. You're outside in the early evening, enjoying the night air and contemplating the stars. All of a sudden, a glowing message drift across the sky: EAT AT JOE'S. It has all the appeal of a commercial message in a nature reserve. Yet such a scenario may only be a few years away.

​In a previous post here, we asked whether we should worry about satellite light pollution. At that time we argued there was little reason for concern. Yet it's easy to conceive of a future in which it is a problem. We made that argument based on certain realities about shortcomings in how we govern human activities in outer space. "Space advertising" is in even earlier stages of development, but the same anxieties apply. The difference is that for space advertising, there is still time to get out ahead of the situation.

​The uses and occupation of orbital space near the Earth have transformed since 2019. In the time since, the number and pace of launches of satellites into orbit around our planet more than doubled compared to their Cold War peak. An important difference compared to decades ago is that private actors conduct most of these operations. This brings activities motivated by profit rather than, say, geopolitical superiority.

Yet long before the arrival of this new use regime, some eyed outer space as the next frontier of product marketing.  For some, it was enough to engage in product placement on rockets or spacecraft. Others saw space as the medium of advertisement itself.

One notable episode in 1993 involved a proposed orbiting billboard made of Mylar that would appear about half the size of the full moon in the sky. Outcry from astronomers and environmental groups led to an act of the U.S. Congress that prohibited such activity if launched from U.S. territory.

​The 1993 law defined obtrusive space advertising as "advertising in outer space that is capable of being recognized by a human being on the surface of the Earth without the aid of a telescope or other technological device." Congress worded it so to draw a distinction with a more passive form of advertising such as corporate logos on launch vehicles. But in enacting the prohibition, Congress made clear that the U.S. would not abide "obtrusive" commercial messages in space. 

Besides the potential to annoy billions of people, obtrusive space advertising poses a threat to astronomy. To be bright enough for people on the ground to see it, ads in space must be bright. Any such object wandering through the field of view of a telescope would obliterate faint cosmic light beyond. These object would also be bright at infrared wavelengths.

Even if the lights switched off while flying over observatories, dark objects would block starlight for several seconds. There are already concerns that much smaller conventional satellites will have similar effects on astronomical observations.

At the same time, it seems inevitable that someone will try this. The lure of passive income from space advertising is great. It was once the case that the high cost of launching objects into orbit was a disincentive to potential space advertisers. But the recent mass-commercialization of launch services caused per-kilogram launch costs to plummet.

​Another factor is a shift in government attitudes toward commercial activities in space. From its founding, the U.S. National Aeronautics and Space Administration (NASA) faced restrictions on even appearing to promote commercial products or services. That stance is shifting. For example, in 2018 the NASA administrator formed a committee to investigate changing the policy.

Nothing in international law now governs this variety of space advertising. The founding document of international space law, the 1967 Outer Space Treaty (OST), is silent on this topic. It does not prohibit commercial activity in space. In fact, it refers to the "use of outer space", which many construe to include both commercialism and exploitation.

But it doesn't render such activities limitless. Its Article IX calls for "due regard" among spacefaring nations toward "the corresponding interests of all other States Parties to the Treaty". And it considers the possibility that some of those activities could drive conflict. Any activity that "would cause potentially harmful interference with activities of other States Parties in the peaceful exploration and use of outer space" requires consultations between countries causing and receiving the interference.

The U.S. prohibition on obtrusive space advertising also faces civil liberties challenges. Some assert that the First Amendment to the U.S. Constitution precludes such laws. In their 1995 law review article "People Do Read Large Ads: The Law of Advertising from Outer Space", Don Tomlinson and Robert Wiley came to different conclusions on this point. Tomlinson argued "Ban Without Reservation", while Wiley promoted "Regulate with Reservations".

Still, the world often looks to the United States to lead on international policy matters. The Outer Space Treaty led to the establishment of the United Nations Committee on the Peaceful Uses of Outer Space (COPUOS). It serves as an international forum for diplomatic discussions of matters flowing from implementation of the OST. At its annual meetings (and those of its subcommittees), national delegations debate various concerns.

​Because COPUOS operates on a consensus basis, the pace of legal conventions over the decades has been very slow. The U.S. and its allies exert an outsized influence in the international community. To that extent they have led efforts to recognize norms of behavior outside the realm of binding, 'black-letter' law.

The American Astronomical Society (U.S.) and Royal Astronomical Society (U.K.) recently asked for exactly that. Both are associations of astronomers that advocate for their members on both national and international matters. In the past year, both issued statements calling for their national delegations to promote discussing obtrusive space advertising at COPUOS. That led to media coverage (e.g., here, here and here) that raised more public attention to the issue.

A particular episode spurred them on. As the AAS statement notes, "Avant-Space Systems LLC, a private entity incorporated in the Russian Federation, recently launched a prototype cubesat intended to demonstrate the feasibility of this technology." Avant-Space launched a 3U cubesat in April 2024 to test objects that would make glowing message formations in space. The successful effort made the prospect of near-term advertising missions palpable.

The AAS statement calls out two specific aspects of the OST. First, it declares that astronomy is a kind of Article I "use" of outer space. And second, as a result Article IX dictates that it is entitled to "due regard" from launching States to limit harmful interference.

But it goes further. Because "no known mitigation of such harmful interference enables the peaceful co-existence of obtrusive space advertising alongside astronomy consistent with Article IX", it argues that States should simply prohibit launch of such payloads from their territories. That's exactly what U.S. law already demands. AAS thus asks the U.S. COPUOS delegation to promote the same policy worldwide.

The RAS takes a similar stance, noting that the Society "opposes all space-based advertising, recognising it as detrimental to the science of astronomy and to our shared heritage of the night sky". It references the 2007 Declaration in Defence of the Night Sky and the Right to Starlight, a nonbinding statement supported in part by the United Nations Educational, Scientific and Cultural Organization (UNESCO). That document called an unpolluted night sky "an inalienable right of humankind equivalent to all other environmental, social, and cultural rights."

It also builds on moves the international astronomical community made almost a quarter-century ago. In its 2001 COPUOS statement, the International Astronomical Union (IAU) referenced the 1993 U.S. obtrusive space advertising prohibition. It called on COPUOS to encourage member States to "adopt similar legislation on obtrusive space advertising, so that this activity is regulated by all space-faring nations."

​This month, the COPUOS Science and Technology Subcommittee will hold its annual meeting in Vienna, Austria. At that meeting, it will hear again from the IAU: "[T]here is no means of mitigating the potential harm to astronomy of obtrusive space advertising.  It represents the ultimate light trespass because its purpose is to be visible as widely as possible on the surface of the Earth.  The IAU urges COPUOS national delegations to consider a prohibition on this technology that creates the risk of unwanted messaging and disruption of the dark night sky."

No one yet knows whether COPUOS will take this advice to heart. Reading the diplomatic tea leaves is often difficult. But this effort may well have the majority of public sentiment on its side. As effective as advertising is, many people tire of being targets of a nonstop, 24-hour-a-day marketing regime. Especially given the proliferation of electronic media and mobile devices, their waking hours are often bombarded with commercial messages. And for some, the notion of advertisements brightening the night sky beyond the pale.

The world faces an imminent choice. It can allow this kind of activity in space, or it can prevent it from ever taking root. If it chooses the former, experience shows that it will be difficult to put the proverbial genie back in the bottle. Of course, a rogue actor could (and may well) launch an space advertisement in violation of domestic law. Some argue against the very notion of law in space to begin with; others note that the OST enforcement regime has no effect at the international level. It remains an evolving geopolitical matter.

The night sky is one of the few remaining aspects of the natural world that all humans share. Our increasingly connected world has blurred other boundaries, but the sky remains a kind of public commons. Whether the world will assert proper control over that commons remains to be seen. But if we lose that shared sense of ownership by allowing narrow commercial interests to take it over, we will have lost something much more significant: a rare medium that draws us together more than it pushes us apart.

We've all been there at some point. You're walking in a dark space at night, alone, maybe to your car in a parking lot. And a kind of fear seizes you. You can't see very well around you, so you don't know if there is someone waiting in the shadows. You get to where you're going as fast as you can, unlock your car door, get in and lock the door behind you. For a moment, you feel safe again.

My friend and colleague Nancy Clanton, a lighting designer based in Colorado, explains how the situation is even more acute for women. She told me about an incident once, some years ago, when she was in an unfamiliar city to attend a lighting conference. She had to walk alone at night along a dark stretch of road between the convention center and her hotel. Even with all her knowledge about lighting and darkness, she still felt uneasy being out there by herself.

She saw two people, both silhouettes, approaching. Like many people in the same situation, Nancy had to make a quick decision. Were they two threatening men? An innocuous couple? Something else? In her mind, at that moment, was born the idea of "lighting for reassurance". With a small amount of well placed light, she could have made a quick friend-or-foe determination. And if circumstances required, she could have made a quick exit. It would have changed the situation in a fundamental way.

There are many reasons that people use artificial light at night. For example, we light sidewalks and pathways to help people orient and find their way from Point A to Point B. We light roadways and "conflict zones" where different kinds of traffic come together, because we know that doing so saves lives. We like to add nighttime amenity to outdoor spaces, making them more inviting for activities like commerce. But we also liked it because many people believe that light at night deters or even prevents the incidence of crime.

Does outdoor lighting yield any real, positive influence on nighttime safety and security? Does it reduce criminal behavior? "The influence of outdoor light at night on crime is mixed," writes DarkSky International in its most recent annual report Artificial Light at Night: State of the Science 2024. "Some of the same studies that looked at lighting and traffic/pedestrian safety also considered nighttime crime incidence. Certain studies reported crime reduction when lighting is added to outdoor spaces. Others find either a negative effect, no effect, or mixed results."

​Whatever the ways in which lighting and crime interact, it's not like the effect of administering a drug to a person. That is, there is no sense that a dose X of the medicine always produces effect Y in the individual.

Crime and lighting seem to have a very context-dependent relationship. There are very many of what researchers call "confounding variables". These are influences unrelated to the study subject that might produce some kind of effect in the system. Without careful control of these variables, the researcher can draw an incorrect conclusion on the basis of observing the intended effect.

To continue the medical analogy, drug researchers try to remove differences among people in patient study samples. These include age, gender, geographic, location, and other factors that might produce a result that mimics what the drug should do. Failure to deal with these "confounders" means that one can be fooled into believing there is an effect from a drug that is in fact, unrelated to its mechanism of action.

It is also very difficult to carry out high-quality, well-controlled studies. Designing reliable (and replicable) lighting studies is challenging. There is not much research funding for this. Sometimes the sources of research funding are questionable. Researchers worry they may want to see some sort of pre-defined result from studies they support.

And there are instances where experimental design is just bad. For example, a 2019 study conducted in New York City housing projects claimed a strong relationship between high levels of light and decreased crime. The study looked at the effect of placing intense light sources on portable towers outside housing buildings and tracking changes in the incidence of crime in the immediate environment around them.

​To the surprise of few, crime dropped. But that's what happens when you treat people like criminals and light up their homes like they were prison yards: you get unreliable results.

What underlies concerns about outdoor lighting, public safety and crime is the unspoken element of fear. It is a powerful and underestimated motivator of human behavior. Our evolutionary heritage has given us limited abilities to see at night. As diurnal animals are active during the day, it made night a dangerous place for our early human ancestors.

Although we tend to think of fear of the dark as something that particularly afflicts children, many people as adults feel a deep-seated sense of terror toward dark places at night. How much we know about a place determines our sense of ease (or unease) about it. Because we are so dependent on our sense of sight, it's easy to feel that we lose control when we cannot see what is happening around us.

Psychologists have long understood that these influences have tremendous power in shaping our perception of the world. In recent years, social scientists have begun investigating what they call "feelings of safety". That is, they try to measure the degree to which people feel safe or unsafe in different situations that may involve danger.

It is important to know that feelings of safety are independent of whether people are actually safe. There are many instances in which people feel unsafe about a situation but the evidence shows that there was no actual danger.

There are ways in which feelings of safety can increase in dark places at night that lead people to accept lower light levels. This relates to a sense in which people feel trapped by their surroundings. They are looking for the exits, so to speak, in case danger suddenly emerges. Research shows that lower light levels are less acceptable to most people in any situation where they feel trapped. If there are clear opportunities to escape if needed, the acceptability of lower light levels begins to rise. Other work shows that most people prefer somewhat higher levels of outdoor light at night. Most prefer light that is cooler than warmer in color appearance.

​But the value of this extra light diminishes very quickly as light intensities increase. The relationship between feelings of safety and light intensity appears to be logarithmic. What that means is that as light levels increase, the amount of an increase necessary to produce some certain amount of increase in feelings of safety becomes larger.

​The biggest increases in feelings of safety happen in moving from situations where there is no light to those in which there is a very small amount of light. To raise feelings of safety by again as much requires increasing the light intensity by much more, often more than a factor of ten.

​The increase in feelings of safety flattens out at highlight levels. We don't quite understand why. One theory is that high intensity lighting makes people feel insecure by promoting the sense that they are on display. Bright lighting can create deep, dark shadows between objects that obscure those spaces. A person moving through an outdoor space at night may not be able to tell whether there is a threat hiding in those shadows.

​The glare from very intense lighting sources also has a disabling effect on the viewer. Glare causes the pupil of the eye to contract, which reduces the depth of field of vision. It conveys a distinct disadvantage to the viewer. And it's the same "prison yard lighting" effect in the New York study mentioned earlier.

That's where "lighting for reassurance" comes in. It's an outdoor lighting ethic still new in the design community. While traditional design holds that more is better, lighting for reassurance asserts that better is better. Often that's less light chosen with a strategy in mind. The goal is to improve outdoor visibility at night and help users of spaces discern where threats might be.

​In particular, it makes use of the incredible properties of human vision, even at low light intensities, to see very small changes in contrast. It ensures that all potential safety hazards are clearly indicated. It also means not going overboard with lighting levels to the point that it becomes a disability for the viewer.

It also makes outdoor spaces at night more inviting to other users of those spaces. People generally tend to feel more unsafe when they are alone in such places at night. As more people fill outdoor squares, streets and other places, they begin to feel more safe in the presence of a crowd. This couples with the popularity of using outdoor light at night for aesthetic purposes. This can include lighting of building façades, statues and monuments, and other landscape features to enhance the sense of nighttime placemaking. And it avoids creating glare to the greatest practical extent.

The task is now to communicate this to more lighting designers. They don't usually learn it either in their formal education or as part of their praxis. We also have to change how lighting designers and engineers think about the concepts of minimum and maximum lighting levels. Right now, lighting standards generally tend to indicate only minimum lighting levels for various lighting applications. This is a belief, not always supported by evidence, that only certain minimum values are "safe". But it is often not clear why those values are "safe" while other values are not.

​It also does not take into account the idea that lighting can be too bright in some cases. That can create its own security problems. Organizations that make outdoor lighting standards are just beginning to embrace the notion that along with recommended minimum lighting values should come corresponding maximum values. The right lighting levels would make Goldilocks herself feel safe at night because they're neither too low or too high. They're just right.

What can we learn from all this? For one thing, many people experience fear of nighttime darkness, and that fear is visceral. We shouldn't tell them that they are wrong to feel fear because we have data that somehow show that their fears are unfounded.

It's also the case that feelings of safety are very powerful. We should recognize this and leverage that fact in outdoor lighting design. Furthermore, the human eye is an underrated detector of faint light at night. That said, lighting design often does not fully exploit its amazing properties. Instead, and as lighting has become much cheaper than ever to consume, design has pushed light levels very high.

The result is that the way we do lighting design now often does more harm than good, even though its practitioners clearly want to do good. What we can do for them is to help them put in place better design through lighting for reassurance. It combines the best of what scientists tell us about lighting with the power of psychology.

​Together, we can make outdoor spaces at night that are truly more safe while also helping people in them feel safer. If we overcome the fear that now leads to demand for brighter outdoor spaces, we'll increase support for measures that gradually draw down light pollution. To do so would be a win for all involved: for people, for the environment, and for the night sky.

On Wednesday, 6th November, the world awoke to the news that former U.S. President Donald Trump won election for the second time. The 2024 campaign was long and bruising, but the win was definitive. On 20th January 2025, Trump will take office as the 47th President of the United States. Changes are of course coming to many aspects of American governance. And some people are wondering what this means for efforts to advance the cause of dark skies in the United States.

As 2024 draws to a close, it's worth considering what the new year may bring. Here we must set aside politics and leave this year's campaign in the past. Instead, in this post we focus on the relevant policies of the first Trump Administration and what little we know about relevant plans for the second.

​The way the U.S. government manages federally owned lands has much to do with landscape-scale preservation of dark skies. Despite lacking a Congressional mandate, all the major land-management agencies take part. But they also oversee various activities on those lands that can yield light pollution. And there is a complex interplay between federal, state and local government in terms of activities next to public lands. The good news is that the land managers have largely embraced dark skies as an important conservation goal.

The world is in a precarious position at the moment. Our planet is experiencing a climate crisis. Scientists expect that its effects will become more plain in coming decades. In previous posts here, we explored how outdoor lighting and light pollution interact with environmental concerns. That includes topics such as climate change, corporate social responsibility, and the "Rights Of Nature".

The laws in many countries mandate the use of outdoor lighting because it serves the public interest. Lighting has demonstrated safety benefits and saves lives. But when lighting becomes excessive, light pollution can cause its own problems. As the world transitions to renewable energy, this can create new challenges. This post explores concerns on both sides of the issue and asks whether competing interests can be balanced in reasonable and effective ways.

Artificial light at night (ALAN) has several connections to environmental concerns. For one, lighting consumes electrical power. That has a significant climate impact via the burning of fossil fuels. Also, ALAN exerts pressure on species already experiencing environmental stress. That stress comes not only from climate change, but also from sources like habitat and biodiversity loss. Energy-efficient lighting like LED seems at first like a solution the first problem. But it can also contribute to making the second problem worse.

Its lower cost of operation has led to rapid adoption of LED technology in the past two decades. With this has come overconsumption of outdoor ALAN. Outdoor lighting is now perceived to be cheap and free of environmental consequences. There is evidence to suggest that had led to the installation of a lot of new and unnecessary lighting. It may have also eliminated any overall environmental benefit from the transition to LED. And it means more light in the nighttime environment, yielding further ecological stress.

In concept, the simplest solution to the climate crisis is to reduce humanity's energy consumption. But our recent history suggests that we're not going to do that. Instead, economists expect energy demand to grow coming years and decades. If so, the next-best approach is to 'decarbonize' the global energy generation portfolio. That requires finding sources of energy that aren't limited to an exhaustible fuel supply.

"Renewable" energy sources like solar, wind and geothermal harness the power of forces of nature. These forces are at work around us all the time, their energy is "free" and it yields no polluting byproducts.  We have used some, like the wind, to power our transportation for thousand of years. Yet until recently, we couldn't move that energy from where we captured it to where we wanted to use it.
Making the best use of renewables now involves constructing both facilities to collect the energy and the infrastructure needed to move it. For this reason, people who live in areas near renewable projects often oppose their construction. Often there are real concerns about the environmental impact of these facilities. When faced with the task of evaluating applications to allow them, governments have to make difficult choices. Sometimes they conclude that the benefits involved outweigh any adverse local impact. Those impacts can include light pollution.

There are certain "speciality" lighting applications that must evaluated in their own contexts. Lighting for specific public safety applications is among those applications. Wind farms, whether built on land or in bodies of water, are a particular concern.  Wind turbines are obstacles for both pilots and sailors in both daytime and nighttime. That's true whether they're in active operation or not. Countries have set rules for lighting windmills at night to reduce the chance of accidents. These rules often supersede other concerns in environmental impact assessments.

The principle underlying these rules is that certain safety concerns cannot be mitigated. As a necessary condition of permitting wind farms, they are seen as acceptable risks. These considerations have led governments to deny permission to build some projects. In other cases, the perceived benefits are too important not to permit renewable energy projects.

Neighbors often oppose wind projects on aesthetic grounds. Their concerns about property values are often labeled NIMBYism ("Not In My Backyard"). Yet there are also real concerns about the environmental impact of this lighting. Renewables projects are often built in remote locations. These are often ecologically sensitive places. Windmills can be hazardous to birds, for example, and in particular at night for migratory species. Very large wind projects involving hundreds or thousands of turbines could light up the night sky. This may happen even when the lighting employed is the smallest amount allowed by law.

So what is the real impact, and should we worry about it? Until recently there were essentially no scientific results to inform the debate. Salvador Bará (Universidade de Santiago de Compostela, Spain) and Raul Lima (Politécnico do Porto and the University of Coimbra, Portugal) just published the first significant work on this topic. They made a simple model of the visual impact of individual wind turbine lights. They used this model to compare their expected brightness to that of astronomical sources like bright stars and planets. It takes into account real-world influences like attenuation of light by the Earth's atmosphere .

Bará and Lima found that windmill lights can be very bright at short distances. Up to 4 kilometers away, medium-intensity turbine lights can exceed the apparent brightness of the planet Venus. After the Sun and Moon, it is often the brightest natural object in the night sky. At 10 km away, the lights can still be brighter than the brightest stars. They can remain visible to the unaided eye to distances as far as almost 40 km. The authors concluded that "the visual range of wind farms at nighttime may be significantly larger than at daytime, a factor that should be taken into account in environmental impact assessments".

​So far this does not address the environmental effects of these lights. The researchers also looked at the amount of illumination on the ground that the lights can cause. Except at very close range, the intensity is low. As a benchmark, the authors used the expected light intensity on the ground from a starry night sky in the absence of the Moon. They found that even the brightest beacon lights on turbines yield these conditions out to distances of only a few hundred meters. Within 5 km the intensity is almost completely negligible. That said, it could still be harmful to wildlife very close to the windmills, whether on land or at sea.

What about the night sky? "Skyglow, in absolute terms, should not be very intense far away from the poles," Bará tells us. "The luminous intensity of the standard lamps used here at nighttime is 2000 candela, and this would make less than about 24,000 lumens per beacon, something like a couple of streetlights." Of course, if designers packed turbines into wind farms, the collective effect might be detrimental to the night sky.

​In a paper published last year, Bará and colleagues considered the effect of skyglow immediately near streetlights. They found that the contribution to skyglow from an isolated, single street light is small and localized close to the light. Small wind farms may thus have little impact on the night sky in their surroundings. But that's less likely as installations grow larger.

What can be done about all this? Clearly there is a climate need to shift energy production away from fossil fuels and toward renewables. Almost one-third of global electricity production now comes from renewables. That percentage is steadily increasing. Certain flavors of renewable energy, like solar, don't need facility lighting at night. But they're not suitable for every condition or situation. For example, electricity generated from solar cells drops to zero at night. To reduce uncertainties, experts recommend diversifying investments in types of renewables. This ensures that no one technology dominates in an area.

To the extent that wind energy is a good option in many parts of the world, there remains the question of where to put it. The social unacceptability of wind farms near populated places is unlikely to change. That leaves locations that may be especially sensitive from an ecological perspective. At the same time, transportation safety concerns will persist and even grow.

Technology may be able to help. For example, some wind farms are now equipped with Aircraft Detection Lighting Systems (ADLS). These systems sense the approach of aircraft within a predefined volume of space around and above wind farms by using low-powered radar. When an aircraft enters a predefined detection zone, the ADLS switches on bright lighting to make windmills visible to the pilot. Once the aircraft exits the detection zone, ADLS turns off the lighting automatically. ADLS can improve situational awareness of obstructions like windmills even in challenging weather. It is sometimes deployed in conjunction with Enhanced Vision Systems (EVS) equipment. EVS combines views from infrared and visible-light cameras to help pilots see through fog, snow, and other low-visibility conditions.

At the end of the day, we can't have it all. So we have to make accommodations. Situations arise where even our best technology can't fully mitigate dangers associated with wind energy projects. But where to locate these facilities remains a choice. Decisions should consider ecological sensitivity in relation to the amount of traffic and the need for lighting. In other words, it's a problem of many variables that has more than one solution. Sometimes the answer is to not build the project at all.

Dark-sky advocates have long wrestled with such tradeoffs. It is often the case that the most "wildlife friendly light" is the one that is never installed to begin with. And yet, better lighting practices exist in the real world alongside complex social and political influences. Lighting for safety remains among the most complex of them all. As the climate crisis becomes more acute, we are again confronted with often difficult choices. The right balance between competing risks serves both the interests of nature and society. That balance leads in a direction toward a world that is more sustainable in all its aspects.