​A visit to an astronomical observatory at night is a magical experience. On a far mountaintop, the starry vault overhead, they are like temples to the study of the cosmos. Telescopes silently scan the heavens all night, shutting their eyes before the first light of dawn. But with increasing frequency, these distant sites are under threat from artificial light at night. What were once thought to be the most defensible locations are less so now. Some suggest that a new management paradigm is the only way to save them.

Astronomical discovery depends on access to faint cosmic light. It is the one physical attribute of the universe directly accessible to science over very large distances. Information about physical conditions in faraway stars and galaxies is encoded in the light we collect with telescopes. Absent this fact, we would know very little about outer space. Because the intensity of light decreases with the distance to its source, these signals are very faint when they reach Earth. Telescopes with massive mirrors collect this light in enough quantity to make sense of the information it brings.

Yet it is now often the case that cosmic light must compete with light from artificial sources on the ground. This began as early as the 18th century, when astronomical observatories were still situated in cities. The installation of gas lighting systems, later replaced by electric ones, began brightening the night sky. Astronomers moved out of cities and took their telescopes with them to the countryside. Many such locations, especially in Europe were unfavorable for observations. Bad weather got in the way, and turbulence in the atmosphere made for poor-quality images. They then moved telescopes to the summits of mountains far from cities. In the 20th century, they started launching them into space. All this increased the difficulty and cost of collecting astronomical data.

Well into the last century, many important observatories in and near cities continued their work. Some sought to hold back the rising tide of light pollution with public policy interventions. Lowell Observatory, in Arizona, U.S., pioneered this approach in the late 1950s. It convinced the council of the city of Flagstaff, its home since the 1890s, to enact what became known as the "Searchlight Law". [1] It aimed to reduce the impact of the use of searchlights for advertising purposes on the observatory's work. But this could only go so far. Flagstaff's population now over four times that in 1960, and the observatory has moved its operations out of town.

Rising night sky brightness over observatories means that scientists need more time and money to achieve the same science outcomes. Consider a case in which the brightness of the night sky at an otherwise unpolluted observatory site doubles. The exposure time for a given telescope and camera needed to reach some science goal then doubles. [2] Given the dollar cost associated with operating a modern observatory, the cost to achieve that goal also doubles. Light pollution thus not only threatens to slow the pace of discovery, but it also makes that discovery more expensive.

There are some recent successes. In 1998, the Chilean National Congress enacted the "Norma de Emisión para la Regulación de la Contaminación Lumínica" ("Emission Standard for the Regulation of Light Pollution”). It required a few basic restrictions on the operation of outdoor lighting in three northern provinces of the country. The goal was to protect observatories in the mountains above the Atacama Desert. Successive updates to the 'Norma Lumínica' have proven rather successful. Yet even the most remote observatory sites there are now threatened. Light can travel hundreds of kilometers through the atmosphere, fouling the night sky far from where it is emitted. Now, no observatory is entirely safe from light pollution. [3] 

Astronomers began confronting this problem in a systematic way over a half-century ago. In 1973, the International Astronomical Union (IAU) established a 'commission' tasked with identification and protection of existing and potential observatory sites. At its 1976 General Assembly, the IAU adopted this statement of concern:

The IAU notes with alarm the increasing levels of interference with astronomical observations resulting from artificial illumination of the night sky, radio emission, atmospheric pollution, and the operation of aircraft above observatory sites. The IAU therefore urgently requests that the responsible civic authorities take action to preserve existing and planned observatories from such interference. To this end, the IAU undertakes to provide through Commission 50 information on acceptable levels of interference and possible means of control.

​It issued guidelines for what it considered 'acceptable' levels of light-pollution interference in 1977. By the following year, the issue attracted the attention of the International Commission on Illumination (CIE). As the international authority on light and illumination, it took note of the increasing problem faced by observatories. It adopted a statement acknowledging the problems caused by uncontrolled outdoor lighting near the best observatory sites.  Further, it urged authorities to take all possible action to protect these sites.

The IAU and CIE joined forces in 1980, releasing the joint publication "Guidelines for minimizing urban sky glow near astronomical observatories". [4] Among its main findings, the document recommended that:

The increase in sky brightness at 45° elevation due to artificial light scattered from clear sky should not exceed 10 per cent of the lowest natural level in any part of the spectrum between wavelengths 300 and 1000 nm.

A lot has changed in 45 years. For one thing, we know more about by how much the natural night sky varies in brightness. [5] That's true on timescales ranging from minutes to years. For example, the 1980 CIE-IAU report presumed a typical night sky brightness at unpolluted sites that is only found sometimes near the minimum of the 11-year solar activity cycle. At the cycle's maximum the brightness can be over 50% brighter, even in the absence of light pollution. Natural sources of light in the night sky varying from one night to the next can yield even bigger changes.

​The bottom line in all this is that the natural night sky itself is a dynamic system "alive with its own light," as the American astronomer Al Grauer says. The night sky, apart from stars and other sources, is not a pure black due to the absence of light. It varies in brightness and color like a noisy audio signal varies in intensity and tone. To continue the analogy, light pollution is like a loud sound atop this noise, with a pure timbre that stays mostly constant from night to night. In the midst of this racket, astronomers are trying to sense a mere whisper.

In 2019, astronomers reacted to a new threat from above: the light of thousands of new satellites launched into orbit around the Earth. They held an international conference in 2021 on the emerging idea of preserving "dark and quiet skies" (D&QS). The report of this conference make a series of recommendations for protecting observatory sites from encroaching light pollution. Some of these are technical in nature, while others leverage the power of public policy to regulate outdoor lighting installations.

The current IAU view, echoing the D&QS report, is:

Present-day professional observatories are located in remote, high-altitude locations; a key selection criterion is the actual sky darkness being as close to the natural background as possible. These sites have an artificial light contamination significantly below the 10% limit recommended by the IAU in 1979. Hence, this limit is not appropriate for the protection of modern professional astronomical sites.
Its new guidance is to "keep the total contribution to skyglow from ALAN substantially below the 10% dark site limit defined by the IAU" (emphasis in the original). At the same time, it recognizes that there is no "one size fits all" approach:

The core of the recommendations is that each major site has a unique limit that should not be exceeded by growing ALAN. It requires each observatory to know what its ALAN contribution is and the rate at which it is currently growing, quantities that can be easily measured.

We can monitor the light situation near observatories from the ground and space, but this involves making long-term observations. Changing the trajectory of that situation requires the commitment not only of governments, but also of the public they serve. There are various proposals for how to do this. We previously wrote about one such idea: the active management of so-called "lightsheds" near observatories.

Another approach is to incentivize reducing light pollution through means such as cap-and-trade schemes. But this still requires treating observatory sites as a special kind of reservation. "For example," the IAU writes,

if an observatory has a current ALAN growth rate of 0.04% per year, which is to be brought to zero within five to eight years, then the ALAN contribution will be less than 0.5% for the foreseeable future. The condition that the ALAN growth rate must be brought to zero and reversed at a site that now has an extremely low artificial contribution sets strong constraints; there will be no way to accommodate a new major artificial light source within these rules, as there is no offset for any sources that could be reduced.

There is now movement in the astronomical community to revise the existing 1980 standard with new information gleaned during the intervening decades. It is also informed by radical changes in the way the way the outdoor world is lit at night since the introduction of LED technology. The high directionality of LED and the ease by which active controls manipulate its light is a game-changer. And non-white light sources, such as amber LED, are more available than ever before. These sources emit light in a limited range of colors, leaving much of the rest of the spectrum dark.

​Astronomers are optimistic that these factors enable a tightening of the guidance that governs outdoor lighting. Over time, it could make a real difference for the wellbeing of the world's observatories. It may also extend their useful lifetimes. Controlling light pollution in this way could lead to a renaissance of science's "cathedrals of the night". 

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.
​
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.