If cities were to turn off all their lights — all their street lamps, billboards, neon signs, car headlights — a clear night sky would look something like this:

That shimmering river of stars is, of course, the Milky Way. Most of us living in urban areas can’t see it because of all the light pollution. In big cities, we’re lucky to even glimpse the Big Dipper. It’s becoming harder and harder to pick out our place in the universe.

How hard is it? In a new study for Science Advances, an international team of researchers created the most detailed atlas yet of light pollution around the world. They estimate that the Milky Way is no longer visible to fully one-third of humanity — including 60 percent of Europeans and 80 percent of Americans. Artificial light from cities has created a permanent "skyglow" at night, obscuring our view of the stars.

Here’s their map of artificial sky brightness in North America, represented as a ratio of "natural" nighttime sky brightness.

In the black areas, the natural night sky is still (mostly) visible. In the blue and green areas, stars start fading from the horizon and zenith. In the yellow areas, the natural sky is lost, drowned out by street lamps and lit-up buildings. In the red and white areas, it’s usually impossible to see the Milky Way with the naked eye — in many places, there are often fewer than 100 stars visible.

Here’s their map of Europe. Note that there are very few dark-sky regions anywhere on the continent:

And here’s the entire world:

The research team, led by Fabio Falchi of the Light Pollution Science and Technology Institute in Italy, used low-light imaging data from NOAA/NASA's Suomi polar-orbiting satellite to create the map, calibrating the data with thousands of ground readings.

"I hope that this atlas will finally open the eyes of people to light pollution," Falchi said in a statement.

It’s not just cities — dark-sky refuges are becoming rarer and rarer

Dan Duriscoe of the National Park Service, a co-author of the paper, told me he wasn’t surprised by the level of light pollution in cities themselves. We’ve known that for a long time. "What’s surprising," he said, "is how far the glow from these lights reaches out into outskirts and unpopulated areas."

That matters, because for those who do want to see the stars in all their glory (or for astronomers trying to use telescopes), it’s harder to find skies unblemished by artificial light. "If you lived in Switzerland, you’d have to travel more than 1,000 kilometers," Duriscoe says. The United States still has a few dark-sky areas — particularly around southeastern Oregon, western Utah, and northern Arizona. But even those are being encroached upon by light from nearby cities like Las Vegas.

"Most people are happy not to live in wilderness so long as there’s someplace they can go," Duriscoe says. "But the problem is that with the expansion of light pollution, these places are becoming more and more remote."

That said, there is an optimistic side to this story. As scientists have piled up evidence of the dangers of too much light pollution — from energy waste to sleep disruption — more cities are finding ways to cut down the glare. And many US parks are taking new measures to preserve what remaining dark skies we have. (More on this below.)

"We're definitely seeing a growing interest in night sky protection," Scott Kardel, the public affairs director for the International Dark-Sky Association, a nonprofit that works to reduce light pollution, told me last year. While it's implausible that cities will ever go totally dark, there are ways to reduce the harm from light pollution — and preserve what dark spots we have remaining.

Does it really matter if we can't see the stars?

At first blush, it's hard to imagine why light pollution is a problem. Sure, it's nice to gaze at the constellations. But artificial lighting is also incredibly valuable. We no longer have to squint to read by candlelight or hold up a lantern to walk down streets at night.

Put another way: No one looks at that famous satellite image of North Korea enshrouded in darkness at night and thinks they're better off than brightly lit South Korea next door.

But we’ve also arguably gone overboard with modern-day urban lighting. As Christopher Kyba of Deutsches GeoForschungsZentrum in Germany, another co-author of the Science Advances paper, explained to me, scientists have found that excessive artificial lighting at night can have a number of adverse consequences:

1) Light pollution can disrupt our sleep. Some research suggests that exposure to certain wavelengths of light at night can suppress our bodies' ability to produce melatonin — and disrupt our slumber. This is why it's harder to fall asleep right after staring at your computer screen all evening (blue light is particularly disruptive). But light pollution in cities coming through bedroom windows may also play a role here.

2) It messes with wildlife. In the last decade, scientists have begun to realize that artificial lighting at night affects animals and ecosystems in all sorts of unexpected ways. Sea turtles get confused by the lighting from coastal cities and lose their ability to navigate in the ocean. Migrating birds often get bewildered by tall buildings that are lit up and smash right into them. Some scientists even suspect that artificial lighting at night makes it harder for fireflies to find each other and mate.

These ecological impacts can trickle over to humans, too. There's some evidence that artificial lighting can, for example, make it harder for zooplankton to eat away at harmful algae in lakes, which lowers drinking water quality. (Kyba is currently involved in a follow-up study in Germany on this topic.)

3) It's a big waste of energy. Lighting at night is useful. But do we need so much of it? Many street lamps are built so that they shine light in every direction (including up at the sky) rather than focusing purely on the ground. And many streetlights shine even in the wee hours when no one actually needs them. By some estimates, 40 percent of a city's electric bill goes toward street lighting — and about half of that is simply wasted. That would imply the US squanders about $3.5 billion on excessive lighting each year.

4) It can diminish our sense of the universe. Okay, this one's more abstract. But many astronomers really do think it's a tragedy that we're cut off from the stars. "Increasingly, urban youth only see constellations on computer screens or in planetariums," one scientist lamented at a 2012 AAAS conference on the subject. "The lack of the night sky may therefore affect their sense of the scale of the universe and their place in it."

To Duriscoe, the loss of the night sky is a profound one. "I don’t think it’s a coincidence that astronomy is the oldest science," he says. "It’s because people have been looking at these lights in the sky and these comets and planets and wondering what’s going on up there. It’s really stimulating. When you see it for yourself, when you see the real universe for yourself, there’s nothing like that firsthand experience."

More cities are trying to cut back on artificial lighting

Now here’s the good news: As light pollution becomes a bigger issue, some countries are starting to cut back on excessive lighting. France, for instance, recently passed a national law cracking down on light pollution,with steps like restricting the hours of storefront lighting.

In the United States, the best-known example is Los Angeles, which is replacing its old bulbous street lamps that scattered light in every direction with newer, more efficient LEDs that only send light downward. As Ucilia Wang illustrated in Forbes, the reduction in glare has been dramatic, and the skies overhead are now noticeably darker. (That said, cities interested in reducing light pollution need to be careful about color selection. Many LEDs largely emit blue light, which actually brightens the night sky more than any other color.)

There are other lighting technologies cities could adopt, too. "We now have the ability to dim lights in ways that we couldn't before," Kardel told me. "And we can even equip streetlights with sensors so that they only turn on when traffic or people are nearby."

Kardel noted that more and more cities are taking a closer look at these technologies. The catch is that this is usually done to save money, and not necessarily to bring back the night sky. And that can have unexpected consequences: Kyba, for his part, warns of the "rebound effect": if businesses or municipal governments save money by installing more-efficient lighting, they may end up ploughing the savings into lighting more structures overall.

There's also a big push to preserve "dark-sky places"

Obviously a city like Los Angeles is always going to be too big and bright for people to be able to see all of the stars or the Milky Way. For that, people will still have to travel out to rural areas. But there's also increasing interest in protecting these "dark-sky" regions for astronomers, nocturnal wildlife researchers, and the growing number of tourists who want to see what the night sky actually looks like.

A number of parks and communities — both in the United States and elsewhere — have applied to be "international dark-sky places." For communities that apply — like Sedona, Arizona — this typically means following stringent lighting codes at night (like shielding lights under a canopy, dimming them when possible, and even turning them off after a certain hour.)

The National Park Service, too, is now working with communities around the Colorado Plateau to try to preserve the nighttime sky in this region — which is both a popular tourist destination and an important area to astronomers.

As Megan Finnerty reported in the Arizona Republic in 2014, however, this isn't always easy to do. In theory, northern Arizona has plenty of reason to protect its dark skies, since its astronomy and tourism programs bring in millions of dollars and hundreds of thousands of tourists each year.

But the responses from different cities has been mixed. On the one hand, Tucson implemented lighting codes 30 years ago and hasn't gotten any brighter at night since, despite growing rapidly. On the other hand, light from Phoenix remains a problem, since it can travel 200 miles in every direction. And the city has been slow to update its lighting codes — or enforcing the ones on the books.

Kardel, for his part, notes that most of the biggest dark-sky preservation efforts are still taking place within the Colorado Plateau — campaigners haven't yet focused on how to deal with light from surrounding metropolises like Phoenix and Las Vegas. "This place is already world-recognized for its dark skies, and many places are taking an increased interest in ways to preserve that resource."

Further reading

This before-and-after photo by Todd Carlson of Sky News offer a vivid look at the effects of light pollution. When a massive blackout hit Ontario, Canada, in 2003, the Milky Way was suddenly visible in the night sky. When the power returned, the stars disappeared.

Earlier this year, Joseph Stromberg wrote a wonderful essay about his quest to find the darkest skies left.

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