When it comes to which city is the brightest, American cities outshine the rest. Scientists have found that German cities emit several times less light per capita than comparably sized American cities.

"The size and difference in light emission is surprisingly large," said Christopher Kyba, one of the researchers, in a news release. "This work will allow us to identify comparable cities in order to uncover the reasons behind the differences."

In this case, the researchers used the ESA's NightPod instrument, which allowed astronauts to take high resolution images of individual cities. This new imagery made it possible to identify and measure the output of individual light sources of light pollution in the first time.

So what did they find? In large cities in developing countries, the brightest light sources were typically airports or harbors. In contrast, the brightest areas in the capital cities of Europe were associated with leisure, such as stadiums and city centers.

More interesting, though, is how the brightness ranged from city to city. While artificial light is a problem for astronomers and nocturnal animals, it also has the potential to be an important tool in understanding human activity.

"Artificial light is responsible for a sizable portion of all nighttime electricity consumption," said Franz Holker, co-author of the new study. "Identifying areas where light could be more efficiently used will make it possible to save energy, reduce costs and reduce the impact of artificial light on the nighttime environment."

The findings are published in the journal Remote Sensing.

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