Lunar Skylight Polarization Signal Polluted by Urban Lighting

Christopher Kyba1,2, Thomas Ruhtz1, Jürgen Fischer1, Franz Hölker2

1Freie Universität Berlin2Leibniz-Institute of Freshwater Ecology and

Inland Fisheries

11th 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: @skyglowberlinYoutube: 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

Dacke et al. 2003

• Dung beetles navigate using the polarized sky light of the moon

• Signal strength seven orders of magnitude smaller than in daytime

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 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

Measurement Locations

50 km

• Compare lunar skylight polarization at urban and rural location

• Brandenburg is very dark compared to Berlin

N

Measurement equipmentSigma 24mm f1.8

tripod

SC4022LPFCFW-8

Blue Red

Note: Measure each LPF position (at least) 4x for each filter

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%

Skyglow pollutes lunar polarization signal!

Degree of linear polarizationUrban 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%

?!?!?

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!

• Moonless, clear sky observing conditions

• Similar values observed several months apart

• Results for one particular direction in one city

Degree of linear polarization370-510 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%

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

Photo CreditsTable: Briho (Wikimedia Commons)

Sky: Christopher KybaDragonfly: Andreas Trepte (WC)Waggle dance: Jüppsche (WC)

Skyglow: Jeremy StanleyGlacier National Park: Ray Stinson

New York City: CharlieBrown7034 (WC)Light pollution map: WEW/FU Berlin

Light pollution model: Martin Aubé (2007)

FundingBMBF 033L038A

MILIEU (FU Berlin)

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

SearchlightsVisual brightness

(blue band)Degree of

linear polarization

Animal use of polarized light:Migration

• 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

Mulheim et al. 2006Cochran et al. 2004

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