How Could we Light our Cities and Still See the Night Sky?

The night sky is a part of humanity’s natural heritage. Gazing up at the heavens is a unifying act, performed by almost every human that’s ever lived. Haven’t you looked up at the night sky and felt it ignite your sense of wonder?

But you can’t see much night sky in a modern city. And the majority of humans live in cities now. How can we regain our heritage? Can quiet contemplation make a comeback?

It may be possible, and a team of researchers from Spain, Portugal, and Italy have tackled the problem. In their new paper, titled “Can we illuminate our cities and (still) see the stars?” the team outlines how we could not only keep cities well-lit at night but also make the night sky open to contemplation. The paper is available on the pre-print site arxiv.org.

The Dark Sky Movement is a worldwide effort to reduce light pollution and change the way we light our cities. Its proponents argue that our cities are over-lit and that it’s not only bad for humans and our circadian rhythms but bad for nocturnal animals, too. They also say that we waste too much energy lighting our cities, and much of the light is directed up into the sky haphazardly for no good reason. That creates a phenomenon called sky glow, which is an impediment not only to quiet contemplation but also to scientific astronomical observations.

The authors claim that their paper shows how we could create reasonably dark skies, even in the center of large metropolitan areas, by controlling both the light emission levels and the direct glare. “These results may support the adoption of science-informed, democratic public decisions on the use of light in our municipalities, with the goal of recovering the possibility of contemplating the night sky everywhere in our planet,” they write.

A glittering night-time map of Europe. Most of the lights in this image are streetlights, which make up the majority of light pollution. The atmosphere scatters the light, creating sky-glow. NASA Earth Observatory images by Joshua Stevens, using Suomi NPP VIIRS data from Miguel Román, NASA’s Goddard Space Flight Center

The Dark Sky Movement has been fighting a defensive battle for the most part. They’ve been trying to preserve the dark sky areas that already exist, especially in our urban areas. They’ve worked to limit light pollution in places like urban parks and popular star-observing spots in light-polluted areas. And those efforts have helped.

But it’s not enough. Not for the future, and not for the authors of this paper. “Notwithstanding that, a purely defensive, reactive stance seems to be nowadays insufficient for ensuring the future of the dark nights on the planet,” they write. “The persistent increase of radiance and illuminated surface progressively encroaches the dark areas, reducing their size and natural values.”

The authors worry that there won’t be enough dark skies to defend in the future. “The dark territory to defend becomes progressively smaller and fragmentary, breaking in many cases the continuity of the nocturnal ecological
corridors,” they write.

For some, the battle is already lost. Some think that current levels of light emission in our urban areas are unavoidable. Streetlights create the majority of the light, and some argue that roads must be as well-lit as they are now, either for safety, to ward off crime, or even to encourage our compulsive consumption. But the authors say these are “old truths” and that there are no good reasons to keep doing things the way we do them now. “… modern research fails once and again to find convincing reasons to support the actual recommendations for road lighting, be it in the name of purported traffic safety, fostering of compulsive consumption, or the even less proven effect of increased photons densities when it comes to avoiding some behaviours…”

This much we can all relate to. But this is a scientific paper, and the authors dig deeper.

“The results strongly suggest that the complete loss of the starry nights is not an unavoidable fate, even in our large metropolitan areas.”

From “Can we illuminate our cities and (still) see the stars?”

Two questions arise naturally, according to the researchers: What is the maximum level of light emissions that are compatible with dark urban skies? What are the compromises and balances?

The ability to see stars in the night sky depends on several factors. The type and quantity of light in the background and coming from the star, the difference in eyesight among humans, and even the skill and experience of an individual observer. The researchers say that all of that can be contained in a single number called the luminance contrast threshold. It basically states that whenever background luminance increases, the luminance of the object being observed must also increase, according to several factors.

The night sky is never completely dark. Nature itself can sometimes provide plenty of background light, even on moonless nights, and that light can help lower the contrast and make stars more difficult to see. But nature’s background light, coming from unresolved stars in the Milky Way and other sources, is part of what we want to see. It’s artificial light that’s the problem.

The authors came up with a simplified situation that captures the physics of the problem and then went about analyzing it.

This night-time image of Mexico City shows the power of sky glow. Imagine trying to see stars in this setting. By Fernando Tomás from Zaragoza, Spain – Flickr, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=348732

Imagine yourself in a small urban park with a radius of about 200 meters (656 ft.) The little park is in the center of a large urban area with light that is spatially homogenous, ie no particularly dominant directional light sources. The park itself is free from annoying light sources, but the paths are lit. The pathway lights are not directed up into the sky or into the observer’s eyes because of the tree canopy. Got it? The authors go into much more detail around the accepted formulas for computing these variables, which interested readers can delve into. But for the rest of us, just imagine the park. Imagine that most of a city could be like this.

There is an enormous number of variables in the authors’ calculations, even though they consider them to be simplified for this work. The atmosphere changes a lot from season to season, and from location to location on Earth. Buildings and obstacles of different heights affect the night sky. Affluent cities produce more light, etc. But the researchers come to some conclusions.

“How our city nights should look like is a social and political decision.”

From “Can we illuminate our cities and (still) see the stars?”

They say that we can improve the darkness of our urban night-time skies by making more judicious decisions around what we light up and how much. That probably sounds obvious, but now there’s data to help support and explain it.

“The darkness of our urban skies can be improved by a sensible choice of the area to be illuminated and the
average spatial density of the urban light emissions,” they write in the paper’s conclusion. “The simplified equations derived in this paper can be used for getting quantitative insights on the trade-offs between emissions and starry skies. The results strongly suggest that the complete loss of the starry nights is not an unavoidable fate, even in our large metropolitan areas.”

Obviously, we need to light up our cities at night. But how much? We seem to want more and more light and to use more and more energy. What are the benefits of that? Is it well thought out? Or reflexive?

“How our city nights should look like is a social and political decision,” they write. “The use of artificial light sources creates by definition a new reality different from natural light, an artificial nightscape whose main features should be collectively decided before choosing the technical solutions that should allow us to fulfill these goals.”

The authors say it’s time to take control over how we light our cities. There are no good reasons, according to the researchers, to keep doing things the way we are. It’s poorly thought out, and we’re losing our heritage. The preservation of the night sky is an important issue, they emphasize. Millions and millions of the world’s children don’t get an opportunity to gaze up into the full force of nature and wonder about themselves and humanity.

If we don’t understand that we’re part of the natural world, if we can’t even see the heavens, what will happen? Are we courting disaster in our well-lit cities?

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Evan Gough

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