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The Milky Way Is Now Invisible to One Third of Humanity

A global atlas finds that light pollution is obscuring our view of the cosmos.

The Milky Way, that amazing cluster of stars in space is now invisible to one third of humanity, including 80 percent of Americans.

In an updated interactive map created to accompany a study published in the journal Science Advances, researchers from Italy and the US show how light pollution—which emanates from dense urban areas—is the main culprit. The Milky Way has fascinated humans for centuries, often starring in legends and immortalised in photographs, but with increasing urbanisation it's becoming eclipsed from view.

"The stars are still there, but when the artificial sky brightness exceeds the brightness of the stars, we can no longer see them," Chris Elvidge, one of the paper's authors and a researcher at the National Oceanic and Atmospheric Administration (NOAA), told me over the phone.

The team compiled its global atlas of light pollution by using data from the Visible Infrared Imaging Radiometer Suite (VIIRS), a high-resolution radiometer at NOAA. This, said Elvidge, has a specialised spectral band that allows it to collect low-light imaging data. The researchers used VIIRS to plot a map of the light that escapes Earth. They then input this data into a computer model that allowed them to estimate where in the world light pollution was being emitted.

Map of light pollution's visual impact on the night sky. Image: Falchi et al., Sci. Adv., Jakob Grothe/NPS contractor, Matthew Price/CIRES.

"The purpose of it is to enable the detection of moonlight. Moonlight is a half-millionth as bright as sunlight and so if you build an instrument that can see moonlit clouds, you will also detect lights at the Earth's surface," said Elvidge.

The team took a year's worth of data and processed it through algorithms developed over several years to form average figures on which they based their global atlas.

"Light pollution isn't a new phenomenon, it has been going on for many decades"

Across the world, light pollution is most prevalent in countries like Singapore, Canada, Italy, and South Korea. Almost half of the US experiences light pollution. According to Elvidge, light pollution also occurs close to gas flares—a gas combustion contraption used in petroleum refineries and natural gas processing plants to get rid of gas that won't be used.

"Some of the worst light pollution is close to gas flares in Nigeria and Russia, where there is no shielding," said Elvidge, explaining how the lack of a protective barrier allowed light to go straight up into sky. "Most of our urban areas have some shielding against that—though not all of them."

Ultimately, the researchers have created this map to raise awareness on light pollution. They want it to act as a data source, firstly for astronomers who want to know which parts of the world are light pollution-free; secondly, for scientists studying the biological impacts of light pollution; and lastly for governments, so that they can, for example, make better street lights that prevent too much light from seeping into the night sky.

"Light pollution isn't a new phenomenon, it has been going on for many decades and there are multiple generations of people who have been in the same situation," said Elvidge. "The best thing we can do is to try and reduce it as it's not a problem that will go away."