Most light pollution isn’t coming from streetlights

Light pollution is the arch nemesis of astronomy, spoiling both the enjoyment of the night sky and the professional study of our universe. For years we’ve assumed that streetlights are the main culprit behind light pollution, but a recent study has shown that streetlights contribute no more than 20% of all the pollution, and if we want to solve this vexing astronomical problem, we have to think harder.

Streetlights, car headlights, billboards, sports stadiums, store displays and signs. The trappings of modern civilization, making for safer (not to mention more entertaining) nighttime hours. But the bounty of modern civilization makes for polluted night skies. The more we turn on our lights at night, the less we can access the skies that our ancestors knew and loved.

What’s more, professional astronomy is getting harder and harder. The greatest observatories of the last century are now beleaguered by city lights, and astronomers are forced to build telescopes in some of the most remote and hostile environments on Earth.

What to do about it? For decades the assumption has been that the main culprits behind light pollution are streetlights. After all, they’re pretty much everywhere, and they lazily spray their light in all directions – including up into the sky.

But new research appearing in the October 2020 issue of the journal Lighting Research & Technology paints a very different picture.

Using the Suomi National Polar-orbiting Partnership (NPP) satellite, the research team took several night-time pictures of the city of Tucson, Arizona. Tucson recently upgraded their streetlights to work in a more intelligent manner, allowing city officials to adjust their brightness over the course of each night.

Working with the researchers, city officials allowed the streetlights to dim to only 30% of their maximum brightness, and on following nights crank them up all the way.

By comparing the amount of light pollution appearing in the satellite pictures, the researchers found that, unfortunately, not much changed night to night. Instead, only about 20% of the city’s overall brightness could be attributed to streetlights. These measurements were confirmed by a ground-based team who also took measurements during the experiment.

While smart streetlights are certainly a wise money-saving choice, and they do lessen the amount of light pollution, we’re going to have to work even harder to preserve the night sky for generations to come.