Polarization patterns of the twilight sky
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Author/Creator ORCID
Date
2005-08-18
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Citation of Original Publication
Thomas W. Cronin, Eric J. Warrant, and Birgit Greiner "Polarization patterns of the twilight sky", Proc. SPIE 5888, Polarization Science and Remote Sensing II, 58880R (18 August 2005); doi: 10.1117/12.613053; https://doi.org/10.1117/12.613053
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© (2005) Society of Photo-Optical Instrumentation Engineers (SPIE). One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.
© (2005) Society of Photo-Optical Instrumentation Engineers (SPIE). One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.
Abstract
Although natural light sources produce depolarized light, patterns of partially linearly polarized light appear in the sky
due to scattering from air molecules, dust, and aerosols. Many animals, including bees and ants, orient themselves to
patterns of polarization that are present in daytime skies, when the intensity is high and skylight polarization is strong
and predictable. The halicitid bee Megalopta genalis inhabits rainforests in Central America. Unlike typical bees, it
forages before sunrise and after sunset, when light intensities under the forest canopy are very low, and must find its way
to food sources and return to its nest in visually challenging circumstances. An important cue for the orientation could
be patterns of polarization in the twilight sky. Therefore, we used a calibrated digital camera to image skylight
polarization in an overhead patch of sky, 87.6° across, before dawn on Barro Colorado Island in Panama, where the bees
are found. We simultaneously measured the spectral properties of polarized light in a cloudless patch of sky 15° across
centered on the zenith. We also performed full-sky imaging of polarization before dawn and after dusk on Lizard Island
in Australia, another tropical island. During twilight, celestial polarized light occurs in a wide band stretching
perpendicular to the location of the hidden sun and reaching typical degrees of polarization near 80% at wavelengths
>600 nm. This pattern appears about 45 minutes before local sunrise or disappears 45 minutes after local sunset (about
20 minutes after the onset of astronomical twilight at dawn, or before its end at dusk) and extends with little change
through the entire twilight period. Such a strong and reliable orientation cue could be used for flight orientation by any
animal with polarization sensitivity that navigates during twilight.