Streetlights not the only bright lights

When satellites take pictures of Earth at night, how much of the light they see comes from streetlights? In the case of Tucson, Arizona, during March and April last year, the answer is only around 20%.

Over 10 days, officials changed the brightness settings for about 14,000 of the city’s 19,500 streetlights and a team of scientists from Germany, the US and Ireland monitored how this altered what they saw from space.

Usually, most streetlights in Tucson start out at 90% of their maximum possible illumination, and dim to 60% at midnight. During the experiment, they were dimmed to 30% on some nights and brightened to 100% on others.

Christopher Kyba from the GFZ German Research Centre for Geosciences led the work, making use of NASA’s Suomi National Polar-orbiting Partnership (NPP) satellite, which is famous for its global maps of light at night.

The satellite took cloud-free images of Tucson on four nights during the test, and on two other nights with regular lighting after the test. A complementary experiment measured sky brightness from ground level – with the same findings.

Those findings are informative, but also present challenges. If most of the light is coming from sources such as bright shop windows, lit signs, facades and sport fields, then – as the researchers note in their paper in the journal Lighting Research & Technology – authorities need to think about more than just street lighting when trying to reduce light pollution.

There were some positives from the ground study, however. The researchers found that the difference in streetlight brightness on different nights was barely perceptible to the people on the street, as their eyes quickly adapted.

The city received no comments or complaints about the changed lighting during the test and there is no evidence or suggestion that reducing lighting levels any adverse effect on public safety.