Lunar Skylight Polarization Signal Polluted by Urban Lighting

Lunar Skylight Polarization Signal Polluted by Urban Lighting Christopher Kyba 1, 2, Thomas Ruhtz 1, Jürgen Fischer 1, Franz Hölker 2 1 Freie Universität Berlin 2 Leibniz-Institute of Freshwater Ecology and Inland Fisheries 11 th Dark Sky Symposium, Osnabruck Oct 6, 2011

Verlust der Nacht (Loss of the Night) • Collaboration of 9 Universities / Institutes • 14 Integrated Subprojects • Many aspects of light pollution considered, from measurement to ecology to sociology

http: //userpage. fu-berlin. de/~kyba/ Light pollution conferences Twitter: @skyglowberlin Youtube: skyglowberlin

Outline • • • Polarization of light Nocturnal insect navigation Experiment setup Results and interpretation Ecological consequences

How does light become polarized? • Polarization is a characteristic of light • Most sources produce unpolarized light • Horizontally polarized light generated by reflections • Linearly polarized light generated through Rayleigh scattering

How does light become polarized?

Nocturnal Navigation • Dung beetles navigate using the polarized sky light of the moon • Signal strength seven orders of magnitude smaller than in daytime Dacke et al. 2003

Properties of sky polarization • Degree of linear pol. is strongest at twilight • Twilight band of maximum polarization runs approximately North/South • Moonlight polarization pattern almost identical to sunlight p. p. (Gál et al. 2001) • Polarization pattern is visible in partly cloudy skies (Pomozi et al. 2001) => Polarization is a more robust directional signal than the sun, moon, or stars

Experimental Setup • Berlin is at 52 o. N • North Star is 38 o from zenith • Moon’s max deviation from ecliptic is 18 o-28 o • Moon is always 62 o to 118 o from North Star (66 o to 114 o in 2010) • North Star is always near peak of Rayleigh polarization • Geometry is independent of position on Earth, and changes a few degrees/day

Measurement Locations N • Compare lunar skylight polarization at urban and rural location • Brandenburg is very dark compared to Berlin 50 km

Measurement equipment LPF Sigma 24 mm f 1. 8 Blue CFW-8 SC 4022 Red tripod Note: Measure each LPF position (at least) 4 x for each filter

Skyglow pollutes lunar polarization signal! Degree of linear polarization Urban moonrise: Urban with moon: Rural moon: Urban daytime: Laboratory (LCD): 3. 9 ± 0. 2% 11. 3 ± 0. 3% 29. 2 ± 0. 8% 56. 6 ± 1. 0% 98. 1 ± 1. 2%

Skyglow pollutes lunar polarization signal! Degree of linear polarization Urban no moon: Urban moonrise: Urban with moon: Rural moon: Urban daytime: Laboratory (LCD): 8. 6 ± 0. 3%? !? !? 3. 9 ± 0. 2% 11. 3 ± 0. 3% 29. 2 ± 0. 8% 56. 6 ± 1. 0% 98. 1 ± 1. 2%

Should skyglow be polarized? • Most sources of light pollution are unpolarized • Horizontally polarized light scattered upwards • Rayleigh scattering can direct light downwards • Sources of light pollution are spatially distributed • Light pollution is generally uncollimated

Naively, no

What do simulations say? • Skyglow simulations describe propagation of light from sources to observer • Most do not take polarization into account • The simulation that does (Kerola 2006) predicts that skyglow is almost unpolarized (~2%) Aubé 2007

But skyglow can be polarized! Degree of linear polarization 370 -510 nm: 10. 1 ± 0. 5% 490 -580 nm: 9. 4 ± 0. 7% 590 -690 nm: 8. 5 ± 1. 4% 370 -700 nm: 8. 6 ± 0. 3% • Moonless, clear sky observing conditions • Similar values observed several months apart • Results for one particular direction in one city

How can this be?

Ecological consequences • Nocturnal insect navigation – – dung beetles crickets? moths? bees? • Bird attraction to searchlights? Dacke et al. 2003

Conclusions • Skyglow pollutes the natural polarization signal of the moon • Extremely likely to affect navigational abilities of some nocturnal insects • Skyglow itself can be weakly polarized • Skyglow polarization could be used for remote sensing of aerosols at night

Acknowledgements Funding BMBF 033 L 038 A MILIEU (FU Berlin) Photo Credits Table: Briho (Wikimedia Commons) Sky: Christopher Kyba Dragonfly: Andreas Trepte (WC) Waggle dance: Jüppsche (WC) Skyglow: Jeremy Stanley Glacier National Park: Ray Stinson New York City: Charlie. Brown 7034 (WC) Light pollution map: WEW/FU Berlin Light pollution model: Martin Aubé (2007)

Thank you!

Polarization of daytime skylight Rayleigh scattering (Pomozi et al. 2001)

Measuring Stokes Vector When this is done for every pixel, you have imaging polarimetry

Searchlights Visual brightness (blue band) Degree of linear polarization

Animal use of polarized light: Migration Mulheim et al. 2006 Cochran et al. 2004 • Birds use polarization cues to re-calibrate their magnetic compass daily • Average of sunset and sunrise is true North, and independent of latitude and time of year • Experiments hold birds in altered magnetic field during twilight • Birds look for this cue preferably at the horizon

Animal use of polarized light: Material detection • Water detection is most well known use • Leads to “polarized light pollution” from artificial surface reflections (Horváth et al. 2009) • Acquatic animals also use it in hunting