The Milky Way over Mitten Park and the Green River in Dinosaur National Monument, one of the darkest places in the United States. The galaxy we call home is a sight one-third of humanity can no longer see. Image credit: Dan Duriscoe

The view of an ink-black, star-studded night sky is becoming a rarity—something only a small fraction of humanity can hope to experience, according to the most detailed study of light pollution compiled so far. “Light pollution” refers to the stray light from vehicles, homes, and industry—a form of pollution that’s often overlooked, but which has been on the rise ever since the invention of electric lighting. It now hampers the view of the night sky for the majority of people around the world. According to an international team of scientists, more than 80 percent of the world’s population now live under light-polluted skies. A smaller fraction—about one-third—live under skies that are murky enough to blot out the Milky Way. A summary of their findings was published today in the journal Science Advances.

Although long decried by both professional and amateur astronomers, the effects of unchecked light pollution also threaten to affect our lives and our environment in ways that impact more than just astronomy, according to the project’s lead scientist.

“Life on earth evolved over millions of years, and normally, it’s been light for half the time, during the day, and dark for half the time, at night,” lead author Fabio Falchi, of the Light Pollution Science and Technology Institute in Italy, tells mental_floss. “But in the last few decades, things have changed. Now, over large parts of our planet, we have light all day and also all night.”

Europe, Africa, the Middle East, and India in New World Atlas of Artificial Sky Brightness, as seen in Google Earth. Image credit: Falchi et al. in Science Advances

The extra light can have adverse health effects on humans and other animals, Falchi says, by affecting the production of melatonin, a hormone that helps regulate the circadian rhythms that control the sleep-wake cycle.

Falchi and his colleagues have released an updated and expanded edition of a light pollution atlas they first published more than a decade ago. The New World Atlas of Artificial Sky Brightness was compiled by correlating data from NASA’s Suomi NPP satellite (National Polar-orbiting Partnership) together with some 30,000 ground-based light-level measurements. The result is the most accurate assessment yet of the worldwide effects of light pollution. 

While other kinds of pollution, such as air and water pollution, often take the heaviest tolls in the developing world, light pollution is most pronounced in well-off regions, such as the United States, Europe, and parts of Asia. Some 99 percent of Americans and Europeans live under light polluted skies, the study found. In contrast, in the African nations of Chad, the Central African Republic, and Madagascar, some three-quarters of residents still live under dark skies. In the developed world, some of the darkest skies are found in Canada and Australia. 

Light pollution over Joshua Tree National Park. And yet—as the National Park Service says—the park has some of the darkest skies in southern California. Image credit: Dan Duriscoe

The good news, says Falchi, is that fairly simple measures can be taken to mitigate light pollution. For example, street lights can be equipped with shields that minimize the amount of light that escapes upward. And modern LED lighting can be dimmed more easily than older kinds of lights, allowing them to shine at reduced brightness levels when that’s all that’s needed.

Alan Dyer, a Canadian photographer known for his stunning portraits of the night sky, compares a dark sky to an endangered species of animal—a rare treat which, for some people, is worth traveling a great distance to see. “When you lose contact with the night sky, you really lose contact with your place in the universe,” Dyer tells mental_floss. “There’s nothing in nature that inspires more curiosity, wonder, and awe than looking up at the stars, and particularly the Milky Way.” Based in rural Alberta, Dyer has easier access to dark skies than most North Americans, but over the last 25 years, he’s seen the lights of Calgary, as well as smaller towns, steadily getting brighter.

For Falchi, who lives near Milan in northern Italy, a dark night sky is virtually impossible to find. “A really good night sky is no longer available in Italy,” he says. “I can drive two hours to a fairly good mountain site, but even there, there’s some light pollution.” A few more hours gets him close to the Austrian border, which is darker still, but even there, he notes, the sky is only really dark directly overhead. When he looks southward, toward Italy’s industrial region, an orange glow looms above the horizon.