Citizen scientists show light pollution is erasing stars from the sky

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Many years ago Christopher Kiba was skeptical about the astronomical data collected by civilian scientists – after all, they rely on people assessing the night sky with the naked eye. But when a student wrote to him with a question about measuring the brightness of the sky, he thought about Globe at night a citizen science project that was launched in 2006 to allow students to track the stars they could see. He downloaded and studied the data. “I became completely converted,” says Kiba, a scientist at the German Research Center for Geosciences in Berlin. He has since devoted his career to studying light pollution and has now analyzed Globe at Night data from around the world to quantify its astounding growth in recent years.

The project is administered by the NOIRLab of the National Science Foundation, an astronomical research center in Tucson, Arizona. Volunteers – most of them in North America and Europe – are given eight possible maps of the local night sky, which show stars visible at various levels of sky brightness. Volunteers look outside and choose the map that best matches what they’re actually seeing, based on the brightness of the faintest star they can spot at the moment. They then do a short report on the Globe at Night website using their phone, tablet or computer. (A few citizen scientists, mostly amateur astronomers, also own a light-intensity-measuring instrument called a sky quality meter, and the website also has a place to mark its readings.)

In January, Kiba and his team published their analysis in the journal The science data collected between 2011 and 2022 showing a dramatic increase in light pollution, with the night sky brightening by nearly 10 percent annually during that decade. A startling trend means that in at least some areas the brightness of the sky is doubling every eight years. “It was a big shock. I like to be optimistic, but I’m constantly disappointed by the data,” says Kiba.

For millennia, people have gazed into the sky, and people all over the world have seen night skies that are virtually identical to what their ancestors saw, except for the occasional star that dies in a supernova explosion. But in recent years this has clearly changed. Artificial light from glowing urban areas continues to seep into surrounding regions, while areas of dark skies, such as sparsely populated national and state parks in the United States, are receding. (This phenomenon is different from the problem posed by reflective satellites in orbit, which add artificial illumination to the night sky.) Amateur and professional astronomers have known about light pollution for decades, if not centuries. However, the problem continues to get worse.

Kiba and his team fitted the model to the NOIRLab data, finding that each year the sky’s brightness increases by about 6.5% in Europe, 10.4% in North America, and 7.7% in the rest of the world, with a global average of 9 .6 percent. Globe at Night volunteers also reported that fainter stars are becoming less visible, and some are even disappearing from the shrinking sky. While Kiba was doing his research, people contacted him to say that they could no longer distinguish between the Pleiades or the Milky Way’s iconic band of stars.

A poorly lit staircase in Potsdam, Germany.

Contributed by Christopher Kiba

Outdoor staircase with dim light illuminating it

A better and dimmer hospital, also in Potsdam.

Contributed by Christopher Kiba

“At first I was amazed,” says Connie Walker, a scientist at NOIRLab, director of Globe at Night and co-author of the study. These results dwarf the mere 2 percent increase previously estimated by meteorological satellites. But these spacecraft are completely blue-blind, so they missed much of the trend, Walker says. Over the last decade or so, many cities have switched from yellow high-pressure sodium streetlights to energy-efficient but bluer LEDs, and human eyes (and the eyes of some wildlife) are especially sensitive to blue light at night. The satellites also let in light sources pointing sideways, like on billboards. In general, brightness is accumulated from sources such as lights on the side of houses or businesses, as well as on streets, stairs, and signs.

It’s possible that air pollution explains some of the trends in certain areas, but Kiba says it can’t increase to that extent. And while the estimates of individual citizen scientists may vary or contain some inaccuracies, they are canceled out when the reports of hundreds of thousands of volunteers are averaged, he says. He concludes that light pollution is the main cause of star extinction.

So what can be done about it? “Light pollution activists like to say, ‘The easiest way to solve the problem is to just turn off the lights.’ This is both true and useless,” Kiba says. Light pollution comes from many sources and lighting decisions are made by numerous people, businesses and city governments. But, unlike climate change, changing the situation is not so difficult, and the benefits are felt immediately. For example, it is neither difficult nor expensive to use only the minimum amount of light required, to turn on the light on a timer if it is not needed at night, or to shield it or direct it downward. It is simply a matter of convincing the many thousands of people involved in large scale lighting solutions to make the best choice.

An infographic illustrating the effect of light pollution on our ability to see stars and other objects in the night sky.

An illustration of how more light pollution results in fewer visible stars. The digital scale is similar to that used by Globe at Night members.

Courtesy of NOIRLab/P. Marenfeld

Walker and other astronomers worry that an entire generation will lose access to the starry night sky. “As an astronomer, I am terrified that we will lose the inspiration that draws people to our field. There are millions of people in major cities who will be lucky if they see Venus and Saturn. The moon is all they have,” says Tezni Pugh, head of the University of Texas at McDonald Observatory in Austin and co-chair of the American Astronomical Society Committee on Light Pollution, Radio Interference and Space Debris. National policies or international regulations on light pollution may be difficult to implement in the near future, so she and her colleagues are focused on putting more emphasis on supporting local campaigns.

There are good reasons for lighting at night, including public safety. But that shouldn’t be at odds with protecting the night sky, says John Barentine, a Tucson-based astronomer and chief executive of Dark Sky Consulting, which advises companies and cities on outdoor lighting design. He points to Tucson as a success story. The population of the city and its outskirts is about 1 million people, but many residents understand how lighting affects the night sky, and it helps that there are large observatories nearby. About five years ago, the city converted 20,000 street lights to LED, but opted for a lower light output than many other cities. The city has been “over-lit” for years, Barentin said, and after street lighting was reduced by about 60 percent, city officials received little to no complaints that it was too dim. “I don’t understand why the rules here can’t be exported to other places,” he says. “We know they work.”

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