Illuminate our cities without obscuring the night sky? It's possible!
A team of researchers from Spain, Portugal and Italy have tackled the problem. In their new article, titled "Can we light up 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 for contemplation. The study is available on the press site arxiv.org.
The Dark Sky Movement is a worldwide effort to reduce light pollution and change the way we light our cities. Its supporters argue that our cities are over-lit and that it's not only bad for humans and our circadian rhythms, but also bad for nocturnal animals. They also say that we waste too much energy lighting our cities, and much of the light is randomly directed skyward for no good reason, creating a phenomenon called sky glow, which prevents not only contemplation but also scientific astronomical observations. br>
Photo credit - depositphotos .com The authors claim their article shows how we could create reasonably dark skies, even in the center of large metropolitan areas, by controlling both light output levels and direct glare. The study starts with two questions: what is the maximum level of light emissions compatible with dark urban skies and what are the trade-offs and balances.
The ability to see stars in the night sky depends on several factors. The type and amount of light in the background and coming from the star, the difference in sight between humans and even the skill and experience of a single observer. Researchers say all of this can be contained in a single number called the luminance contrast threshold. Basically it states that every time the background luminance increases, the luminance of the observed object must also increase, based on several factors.
The night sky is never completely dark. Nature itself can sometimes provide a lot of background light, even on moonless nights, and that light can help reduce contrast and make stars harder to see. But nature's background light, coming from stars in the Milky Way and other sources, is part of what we want to see. It is the artificial light that is the problem.
According to the authors, we could improve the darkness of our urban night skies by making more judicious decisions about what we illuminate and how much. It probably seems obvious, but there is now data to help support and explain it.
“How our nights in the city should be 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 nocturnal landscape whose main characteristics should be decided collectively before choosing the technical solutions that should allow us to achieve these goals."
The authors say it's time to take control of how we light our cities. There are no good reasons, according to the researchers, to continue doing things as they do now.
