Effect of light pollution on sky polarization

These false-color images compare the sky polarization on a night with the moon almost full outside of Berlin (left) and inside of the city (right). Outside of the city there is an obvious pattern, and inside of the city the pattern is gone, due to light pollution. Just like the sun, the moon produces a celestial compass that stretches across the sky, and produces the broad bands in the left hand image. While the celestial compass is invisible to humans, some nocturnal animals are able to perceive it, and use it for navigation.

Urban skyglow (light pollution) is mostly unpolarized, and its presence can overwhelm the weak moon signal, as is shown in the right hand image. The two images are of the same patch of the sky, taken within an hour and a half of each other. The images are centered on the North Star, and star trails have been removed. These results were published in the Journal of Geophysical Research and the paper can be downloaded for free.

Proving conclusively that an animal navigates using the polarization of the moonlit sky is understandably very difficult. Based on their visual organs, however, it is believed that species of nocturnal moths, bees, crickets and spiders all make use of this signal.

Even though the rural site appeared dark to our eyes when we were making the measurement, the polarization at the rural site was nevertheless considerably less than we would have expected to see if there was no light pollution in Berlin (approximately 55%). These data were taken on a night with a relatively high, almost full winter moon. On most nights, especially in the summer when the moon is usually lower, the celestial compass would be much dimmer, and probably invisible to nocturnal insects even at the rural location.

You can read more about our interdisciplinary research project at the "Verlust der Nacht" (Loss of the Night) webpage.

What can I do to help?

Skyglow can extend up to almost 200 kilometers away from its urban source. Much (if not most) of this skyglow is due to light directly radiated into the sky by poorly designed light fixtures. It is possible (and more cost effective) to light streets and walkways with light fixtures that send no direct radiation into the sky (0% ULOR). If you want to reduce skyglow where you live, contact your local government and ask them to adopt legislation requiring the installation of 0% ULOR lamps for all new and replacement public lighting. If you want to help reduce skyglow the world over, become a member of the International Dark Sky Association.

You can also help by participating in the GLOBE at Night campaign, in which citizen scientists all over the world collaborate in making estimates of the night sky brightness where they live. You can see the observations made by members of your own community here.