What’s in a Watt? Interpreting nightlights satellite data via citizen science observations
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The image of Earth at Night from space has become iconic, but we are still in the dark when it comes to the question of to what extent different light source types contribute to city light emissions. Our team of citizen scientists used the Nachtlichter app to count and classify 234,044 light sources over a total area of 22 km2 during 2021. We find that a space-based radiance observation of 1 nW/cm2sr in the 500-900 nm range corresponds to 219 lights/km2 on the ground at midnight. This suggests that 78.0±1.6 million individual light sources are still turned on at midnight in Germany. We additionally examined the properties of the lights, and their relation to land cover. We find that private windows are the most abundant light type, and that for every streetlight in German urban areas, there is an illuminated sign and shop window.
