Lunar skylight polarization signal polluted by urban lighting

Abstract

[1] On clear moonlit nights, a band of highly polarized light stretches across the sky at a 90 degree angle from the moon, and it was recently demonstrated that nocturnal organisms are able to navigate based on it. Urban skyglow is believed to be almost unpolarized, and is therefore expected to dilute this unique partially linearly polarized signal. We found that urban skyglow has a greater than expected degree of linear polarization (p = 8.6 AE 0.3%), and confirmed that its presence diminishes the natural lunar polarization signal. We also observed that the degree of linear polarization can be reduced as the moon rises, due to the misalignment between the polarization angles of the skyglow and scattered moonlight. Under near ideal observing conditions, we found that the lunar polarization signal was clearly visible (p = 29.2 AE 0.8%) at a site with minimal light pollution 28 km from Berlin's center, but was reduced (p = 11.3 AE 0.3%) within the city itself. Daytime measurements indicate that without skyglow p would likely be in excess of 50%. These results indicate that nocturnal animal navigation systems based on perceiving polarized scattered moonlight likely fail to operate properly in highly lightpolluted areas, and that future light pollution models must take polarization into account.

Artificial light alters natural regimes of night-time sky brightness

“…The increase in the magnitude of sky glow by artificial lights in cities is also likely to affect these behaviours and, in addition, may interfere with the navigation of many species which use celestial patterns of light to orientate themselves in the night-time environment4344. For example, artificial light has been shown to disrupt the orientation and therefore migration direction of birds45, and concerns have been raised about the interference of artificial light pollution with celestial polarized sky light patterns28 which many insects4647484950 and birds5152 use to navigate.…”

“…The impacts of direct artificial light include navigational disorientation of birds1415, turtles1617 and invertebrates1819, the displacement and concentration of selected foraging habitat in bats2021 and birds22, the extension of foraging times in birds222324, and changes to the composition of invertebrate communities25. Added to this, the potential effects of sky glow include artificially increasing the amount of ambient light and time available for foraging2627, interfering with celestial light patterns used in navigation28, and the disruption of temporal activity patterns linked to daily, monthly and annual lunar rhythms in sky brightness29303132. Understanding how sky glow alters natural regimes of sky brightness is therefore fundamental to investigating its environmental impacts.…”

Two camera system for measurement of urban uplight angular distribution

“…Although the azimuthal angle has been ignored in almost all past skyglow simulations, it has been suggested that azimuthal variations due to the screening of light not directed along street canyons could be responsible for the generation of linearly polarized skyglow [10]. These histograms were filled with the number of times that a radiometer performed an observatino in the given direction, as well as the observed radiance (in digital numbers).…”

Angular distribution of uplight at 10,000 ft over Berlin

“…This system is able to measure the upwards emission from zenith to an angle of 70 o , for any given azimuthal direction. Testing whether the upward emitted light is azimuthally symmetric is of interest, because it could explain the polarization of skyglow observed by Kyba et al (2011). Azimuthally symmetric emission is generally assumed by skyglow models (e.g.…”