The Milky Way Is Disappearing
A new “dark sky atlas” suggests that light pollution is drowning out the stars faster than ever.
The Pawnee people took literally the idea that we are all star stuff. In their cosmology, which dates back at least 700 years, the first woman was born from the marriage of stars, and the first man from the union of the sun and moon. The stars themselves were sent by the creator god, Tirawa, who tasked them with holding up the sky.
The brightest stars were entrusted with Earth’s climate, which was thought to be the key to its fertility. But this arrangement made some lesser stars jealous, so they stole a sack of violent storms that belonged to the brighter stars and emptied them on the Earth, and this is how death came to the world.
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Today, the clouds, wind, and rain are still the principal ways that humans experience the sky, and that experience is changing. The Pawnee lived through thunderstorms and tornadoes, but ours are likely to become more violent as climate change worsens. And our night sky is changing too. As light pollution intensifies, it’s emptying out of stars, and life on Earth is paying a price.
One-third of humanity —and 80 percent of North Americans—can’t see the bright smear of the Milky Way, our home in the cosmos. For the first time in the history of our species, entire generations of people have never seen our galaxy.
A full 99 percent of the people in North America and Europe sleep under a bright haze at night, caused by light pollution. A new dark sky atlas describes just how widespread this problem is, and gives scientists a starting point for studying the impact artificial light is having on humans and the other creatures that share this planet.
“The light that we detected is not even seen by people, because they are asleep; it is only seen by astronomers,” says Fabio Falchi of the Light Pollution Science and Technology Institute in Thiene, Italy. “But I am convinced that light pollution is no longer a problem for astronomers. It is a global problem for everyone. All life on Earth evolved with the dark, with 12 hours of dark and 12 hours of sun. But now we are enveloping our planet in a perpetual glow. And life is affected by that.”
Falchi and Chris Elvidge, a scientist at the National Oceanic and Atmospheric Administration, have been studying satellite images of the Earth at night since the 1990s. Their first atlas, produced in 2001, used older satellite data that was taken around 8 p.m. local time, while the updated atlas comes courtesy of a new satellite that captured sky glow around 1 or 2 a.m. Because of these and other differences, the new atlas can’t be directly contrasted with the old one. But the scientists think light pollution is more widespread now, even as some communities are trying to bring back the night. This is partly because of LEDs.
“Awareness is rising, but not as much, I think, as the new lights,” Falchi says.
Many cities are replacing their older high-pressure sodium or metal halide street lamps with LEDs, which use less energy but shine more brightly, especially in the part of the visible-light spectrum that scatters the most. (This is the same effect that makes the sky blue.) This means cities are both getting brighter and spreading their light across greater distances. Standing in Death Valley National Park, for example, a visitor can see gumdrop-shaped domes of light hovering over Las Vegas to the east and Los Angeles to the west, both of which are hundreds of miles away.
It’s is a trend that troubles Dan Duriscoe, the night sky program manager for the National Park Service. The park service officially views the sky as a natural resource just as valuable as pristine meadows and streams. To safeguard it, Duriscoe has been tracking light pollution in the parks since 1996, using software he and a colleague developed. The new atlas incorporates their data, along with dark-sky observations from a worldwide crew of citizen scientists.
Duriscoe tries to encourage park-neighboring communities to waste less light, which could mean better-shielded, downward-directed lamps, or fewer lumens — a measurement of illuminance—per capita. He worries about the proliferation of bluish LED lamps, which spread light farther, and are more disruptive to nocturnal animals and human circadian rhythms.
“You hate to see all this blue light put in, which is supposed to have a 20-year life cycle. I’m thinking, ‘Oh, god,’” Duriscoe says.
Ironically, the spread of these lights will be harder to track in future atlas updates. The satellites that track light pollution don’t see this part of the light spectrum very well, so as LEDs proliferate, the robot’s-eye-view will actually seem better, while to our eyes it will look worse. Falchi and his colleagues estimated that if European cities continue adapting blue-light LEDs without blocking any of their glow, the sky will look two to three times brighter at night than it does now.
“This is not changing the lanterns. It’s simply changing the spectrum of light that we used,” Falchi says. “We are used to high-pressure sodium, which is yellow, and when we change to cool white LEDs, that is very bright to our eyes. The solution should be to use warm-light LEDs.”
Some cities are already worse than others, and this is not necessarily because of their populations. Munich and Milan each have roughly 1.5 million inhabitants, but Milan is much brighter than its German counterpart. The same is true of Rome and Berlin: The capital cities have about 3 million residents, but Rome shines far more brilliantly. Falchi, who is Italian, says he thinks his countrymen are just accustomed to brighter light levels.
Earth does have a few truly dark spots — which happen to include some of its poorest places — in Africa, southeast Asia, and South America. Just 1 percent of Mauritania does not have a pristine sky. A visitor can also see a virgin night in 99 percent of Greenland, and parts of New Zealand and the Australian Outback are pretty light-free.
But in the world’s most populous cities, like Singapore, New York, Tokyo, and others, a resident might never experience true darkness, let alone a pristine sky. Falchi and his colleagues estimate that in those places, the sky remains so bright at night that a person’s eyes don’t ever become truly dark-adapted. This is called scotopic vision, when the rods in the retina are the only cells processing light, and it is best experienced outside under starlight—sneaking down the hallway at night, guided by the glare of outdoor street lamps, doesn’t count.
Duriscoe hunted for a house in a community that has no such street lamps, and readily acknowledges that he is fortunate to have not only found one, but to be privileged enough to live there. You should not have to drive several hours into the nighttime wilderness merely to see the stars, he argues, and partly for political reasons.
“You are making it into an elite activity, where you have to be rich to enjoy it. You have to get a ticket to Mauritania or Namibia or the western United States to enjoy it. That’s really oppressive,” he says.
It’s easy to muse over what the loss of night means for civil rights or human culture. But much lowlier creatures than us are also sky watchers. Sea turtles are a famous example, and many species of birds are also thought to use moonlight to get their bearings. The humble dung beetle requires a view of the Milky Way to navigate and roll its precious balls of poop in the right direction. Other nocturnal creatures probably use the galactic plane in this way, too. But as the new atlas shows, the galaxy is disappearing from much of the Earth.
A rapidly growing community of scientists is studying what might happen as a result. The biological impacts of artificial light at night, from its effects on bats and insects to harms on human health, are just beginning to be appreciated. The atlas will serve as a crucial backdrop for these studies. Researchers will be able to quantify how bright a place is, and even study what happens if things change — if the lights get dimmer, and the natural nightscape returns.
“Just being able to go out and look at the constellations gives people a great sense of reassurance that something is still there. It was there when they were home, and it always will be there. Our world is so changeable that to have that view of the cosmos to ground yourself in is definitely good for your mental health,” says Duriscoe. “It has to be. If you are robbed of that, you can just go crazy. I know I would. I would think of it as an intolerable violation of my civil rights.”
Falchi and Elvidge say they have also sought out dark skies, but they have to travel to get them. A few years ago Falchi visited Chile’s Atacama desert, one of the driest and darkest places on the planet, where the view of the Milky Way presented him with what he called one of “the great natural wonders.”
Elvidge tries to escape the haze by visiting the mountains, an easy and obvious choice for someone in Boulder, Colorado. But sometimes he drives east instead, passing the exits for Denver and the smaller agricultural city of Greeley. After a two-hour ride, he arrives at the windswept Pawnee National Grassland.
Nobody lives out there now; the Pawnee themselves are long gone, and white farmers mostly abandoned the area after the Dust Bowl. Apart from the sandstone Pawnee Buttes, improbably rising from the plains like a pair of prairie ziggurats, the grassland’s most compelling feature is its sky. Here, you can see the same stars the Pawnee people did, centuries ago. Taking them in just as the Pawnee would have, you can wonder, as they did, where we came from.