Since the invention and spread of artificial light, few locations on Earth appear entirely dark after sundown anymore. Even in a place like Tucson, Arizona—located near premiere “dark-sky” locations for astronomers and stargazers—the night sky is illuminated to a certain extent by artificial light sources. Using satellites, scientists are starting to better understand the specific sources of this light pollution.
In a 2020 study led by Christopher Kyba of the German Research Center for Geosciences, scientists conducted an experiment with Tucson’s lights. They brightened then dimmed some streetlights for a few nights and used satellite images to observe changes in Tucson’s radiance. The results are not what you might expect.
A typical night for Tucson is captured in this image, acquired around 2:30 a.m. local time on April 24, 2021, by the Visible Infrared Imaging Radiometer Suite (VIIRS) on the NOAA-NASA Suomi NPP satellite. The VIIRS sensor has a day-night band (DNB) that detects light in a range of wavelengths from green to near-infrared and uses filtering techniques to observe signals such as city lights, wildfires, and reflected moonlight.
Notice that the city appears grid-like, with streets that primarily run east-west, avenues that run north-south, and a few stravenues (unique to Tucson) running diagonally. Artificial light comes from businesses, homes, the airport, advertisement signs, sports complexes, and vehicle traffic. It turns out that just a relatively small amount of the total light comes from streetlights.
Tucson’s streetlights operate on a control system that typically provides 90 percent illumination in the evening and then dims to 60 percent after midnight, saving energy and extending the lifetime of the lights. For the experiment, the researchers worked with the city to temporarily brighten the lights to 100 percent for a few nights, then dim the lights to 30 percent on other nights, so that the change could easily be observed with nightly satellite overpasses.
In their analysis of the VIIRS DNB images, the scientists accounted for differences in the angle that the satellite observes the city each night. They made corrections for things like airglow and the amount of atmosphere through which the light traveled. They timed the experiment so moonlight would not interfere. Taking these factors into account, they then made simple calculations of how much light came from streetlights relative to other urban sources.
Kyba and colleagues found that streetlights accounted for just 13 percent of the city’s total light visible after midnight. That number would jump to 18 percent if the city did not dim the lights. This means most of the light is coming from other types of artificial lighting.
“Light-pollution activists and governments have been very focused on street lighting, and that makes sense because it’s probably the biggest single source and the government has direct control over it,” Kyba said. “But my worry is that most of the growth in light is coming from other applications. If we want to reduce the environmental impact of outdoor lighting, it’s not good enough to change to LED streetlights and then stop. We need to think critically about all the different types of light sources there are.”
NASA Earth Observatory image by Joshua Stevens, using VIIRS day-night band data from the Suomi National Polar-orbiting Partnership. Story by Kathryn Hansen.