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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 11th Dark Sky Symposium, Osnabruck Oct 6, 2011.

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Lunar Skylight Polarization Signal Polluted by Urban Lighting

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  1. Lunar Skylight Polarization Signal Polluted by Urban Lighting Christopher Kyba1,2, Thomas Ruhtz1, Jürgen Fischer1, Franz Hölker2 1Freie Universität Berlin 2Leibniz-Institute of Freshwater Ecology and Inland Fisheries 11th Dark Sky Symposium, Osnabruck Oct 6, 2011

  2. 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

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

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

  5. 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

  6. How does light become polarized?

  7. 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

  8. 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

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

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

  11. CFW-8 Sigma 24mm f1.8 SC4022 LPF Blue Red Measurement equipment tripod Note: Measure each LPF position (at least) 4x for each filter

  12. Skyglow pollutes lunar polarization signal! Degree of linear polarization Urban moonrise: 3.9 ± 0.2% Urban with moon: 11.3 ± 0.3% Rural moon: 29.2 ± 0.8% Urban daytime: 56.6 ± 1.0% Laboratory (LCD): 98.1 ± 1.2%

  13. Skyglow pollutes lunar polarization signal! Degree of linear polarization Urban no moon: 8.6 ± 0.3% Urban moonrise: 3.9 ± 0.2% Urban with moon: 11.3 ± 0.3% Rural moon: 29.2 ± 0.8% Urban daytime: 56.6 ± 1.0% Laboratory (LCD): 98.1 ± 1.2% ?!?!?

  14. 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

  15. Naively, no

  16. 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

  17. But skyglow can be polarized! • Moonless, clear sky observing conditions • Similar values observed several months apart • Results for one particular direction in one city 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%

  18. How can this be?

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

  20. 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

  21. Acknowledgements Funding BMBF 033L038A 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: CharlieBrown7034 (WC) Light pollution map: WEW/FU Berlin Light pollution model: Martin Aubé (2007)

  22. Thank you!

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

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

  25. Searchlights Visual brightness (blue band) Degree of linear polarization

  26. 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:Migration Mulheim et al. 2006 Cochran et al. 2004

  27. 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

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