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Hidden in our brighter skies

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WASHINGTON — Every year, the night sky gets brighter and the stars look dimmer.

A new study that analyzes data from more than 50,000 amateur astronomers reveals that artificial lighting makes the night sky about 10% brighter every year.

That’s a much faster rate of change than scientists had previously estimated by looking at satellite data. The research, which includes data from 2011 to 2022, is published Thursday in the journal Science.

“We lose, year after year, the possibility of seeing the stars,” said Fabio Falchi, a physicist at the University of Santiago de Compostela, who was not involved in the study.

“If you can still see the darkest stars, you are in a very dark place. But if you only see the brightest ones, you’re in a very light-polluted place,” he said.

As cities get bigger and put in more lights, the “sky glow” or “artificial twilight,” as the study authors call it, becomes more intense.

The 10% annual change “is much larger than I expected – something you will notice clearly over the course of your life,” said Christopher Kyba, study co-author and physicist at the Center for German Research for Geosciences in Potsdam.

Kyba and his colleagues gave this example: A child is born where 250 stars are visible on a clear night. By the time this child turns 18, only 100 stars are still visible.

“This is real pollution, affecting people and wildlife,” said Kyba, who said he hoped policymakers would do more to reduce light pollution. Some localities have set limits.

Survey data from amateur astronomers from the non-profit Globe at Night project was collected in a similar fashion. Volunteers search for the constellation Orion – remember the three stars in its belt – and match what they see in the night sky to a series of maps showing an increasing number of surrounding stars.

Previous studies of artificial lighting, which used satellite images of Earth at night, had estimated the annual increase in sky brightness at around 2% per year.

But the satellites used are not able to detect light with wavelengths towards the blue end of the spectrum, including light emitted by energy-efficient LED bulbs.

According to the researchers, more than half of new outdoor lights installed in the United States over the past decade were LED lights.

Satellites are also better at detecting light that scatters upward, like a spotlight, than light that scatters horizontally, like the glow from an illuminated billboard at night, Kyba said.

Skyglow disrupts human circadian rhythms, as well as other life forms, said Georgetown biologist Emily Williams, who was not part of the study.

“Migratory songbirds normally use starlight to orient themselves in the sky at night,” she said. “And when baby sea turtles hatch, they use light to orient themselves towards the ocean – light pollution is a huge problem for them.”

Part of what is lost is universal human experience, said Falchi, a physicist at the University of Santiago de Compostela.

“The night sky has been, for all generations before ours, a source of inspiration for art, science, literature,” he said.

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