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Even during the worst of the pandemic in New York City, when the threat of the virus had emptied out the streets, the lights of Times Square stayed on, its many towering advertisements flashing and flickering. The coronavirus had driven millions of people indoors, but the city’s most recognizable plaza was illuminated—a symbol, George Lence, a spokesperson for Times Square’s sign operators, told me, of “New York’s strength and resiliency,” a marker that everything might still be fine. If Times Square—or any other famous monument in a major city—were to go dark, it would send a worrying message.

Outdoor lighting signals security, a bright deterrent against misdeeds that would otherwise flourish under the cover of darkness, and that idea extends to other definitions of security. “We have this idea that light is something positive, and we are in dark times, so we need some light to brighten us up,” says Annette Krop-Benesch, a researcher in Berlin who studies the effects of artificial light on the biological rhythms of humans and animals.

But Times Square was something of an aberration: Everything, of course, was not fine in the United States and beyond.

From a satellite’s perspective, Earth at night, under cloudless conditions, is, in normal times, a navy-blue marble with a dusting of gold. The electric sparks of human activity shimmer in the darkness: a bustling downtown, a well-traveled highway, a fleet of container ships in open water. But when the coronavirus swept across the globe, the glow of civilization shifted from city centers to residential areas. Entire stretches of road, once shiny like strands of tinsel from car headlights, vanished from the nighttime map. As entire populations and industries curtailed their usual movements, pixels of light on satellite images rearranged themselves accordingly—a new bright cluster here, a fresh spot of darkness there.

Some changes have coincided with the implementation of emergency measures meant to slow the transmission of the fast-spreading virus. The effect was stark in China, according to Qian Liu, a doctoral student in geography at George Mason University in Virginia. Liu and her fellow researchers used images from a weather satellite to examine the average nighttime radiance—a measure of artificial light on the ground—across the country’s provinces. They found that radiance levels decreased from December, when the first coronavirus cases were reported, to January and February, when officials put entire cities on strict lockdowns.

Near Wuhan, the city where the virus first emerged, the data showed that residential areas brightened while commercial areas dimmed this spring—a sign that more people were staying home than usual. “In China, people’s commercial areas and living areas are separate,” Liu says. The levels appeared to return to normal as provinces lifted restrictions in March, with the exception of Hubei, where Wuhan is located, which remained under quarantine until April.

Christopher Elvidge, a researcher who specializes in nighttime observations of light sources at the Colorado School of Mines, found similar effects in the U.S. Analyzing data from the same satellite that Liu’s team used, Elvidge and his colleagues found that from February to March, artificial light dimmed in states such as New York and California, which were among the first to introduce widespread stay-at-home orders, but remained unchanged in states such as Florida and Arizona, which took a less stringent approach.

Satellite data might have even captured the result of plummeting oil prices. In late April, as U.S. oil prices dropped below zero for the first time in history, oil fields in Texas appeared significantly dimmer compared with satellite images taken three months earlier. As demand had diminished worldwide, oil companies had sharply cut their operations, which apparently eliminated the need to keep their deserted sites lit.

Such views can tell us only so much, though; weather satellites aren’t spy satellites, and their resolution at night isn’t good enough to resolve small-scale sources of artificial light. When researchers spot dimming in a particular region, “we can’t necessarily say, ‘Okay, this was advertising lighting that turned off,’ or, ‘People went to bed earlier,’ or, ‘There’s less traffic,’” says Christopher Kyba, a researcher at the German Research Center for Geoscience who studies the ecological impacts of nighttime artificial light.

On the ground, different cities managed the electric lights within their borders in different ways. In Spain, for example, authorities in Pamplona and A Coruña decided to turn off lighting in certain public spaces for the duration of the nationwide lockdown this spring, in part to save energy. Alejandro Sánchez de Miguel, a light-pollution researcher at the University of Exeter, says some towns even encouraged residents to turn off their lights at night so that they could see more stars in the sky than usual. (It helped that the atmosphere itself was cleaner; with fewer vehicles on the road and power plants in operation, there were fewer air pollutants to scatter light and magnify its glow.) For a few days in March, thousands of people across Italy turned off all of the lights in their home and aimed their smartphone cameras outside to collect data for researchers studying light pollution.

In these metaphorically dark times, New York wasn’t the only place that tried to keep its most brilliant lights burning; parts of the world even became, for a time, significantly brighter than usual. Several locations in the United Kingdom installed blue light beams meant to pay tribute to health workers battling the pandemic. These beacons happened to turn on during the migratory bird season, when they risked luring birds away from their flight paths and disorienting them to the point of exhaustion. “Light can make people feel good and bring people together, but we need to think carefully about when and where we use it,” Kyba says.

When it’s well deployed, though, light can give a glimmer of hope. For a month this spring, Matterhorn mountain in Switzerland was illuminated in bright projections each night—the flags of other nations, thank-yous written in different languages, and hashtagged advice to stay home. Such luminous shows of support appeared at other landmarks around the world, from the Great Pyramid in Egypt to the Las Vegas Strip, already one of the brightest spots on the planet as seen from space. Even if everything wasn’t fine, there were beacons to illuminate a way forward.

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