Polarization Pattern of the Moonlit Clear Night Sky at Full Moon: Comparison of Moonlit and Sunlit Skies
The light of the night sky originates from the following main natural sources (in order of brightness): (1) moonlight, (2) stars and planets, (3) the Milky Way, (4) zodiacal light, (5) airglow, and (6) the light from these sources scattered by the earth’s
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The light of the night sky originates from the following main natural sources (in order of brightness): (1) moonlight, (2) stars and planets, (3) the Milky Way, (4) zodiacallight, (5) airglow, and (6) the light from these sources scattered by the earth's atmosphere. Light pollution from artificial city lighting also affects many sites' night skies. Most of these sources are weakly polarized, the airglow is unpolarized (Wolstencroft and Brandt 1974), and the scattered skylight may be strongly polarized. At full moon, in extended regions of the sky around the moon and antimoon, the moonlight scattered and polarized in the atmosphere overwhelms all other sources of the night sky. Moonlight has nearly the same spectral composition as sunlight, but with a shift somewhat toward the red (Kopal 1969). While the sunlight is always unpolarized, the moonlight is slightly partially linearly polarized, and at full moon it is unpolarized (Pellicori 1971). With the increase of the phase angle ß, its polarization is negative, i.e. its direction of polarization is parallel to the plane of sight. The degree p of negative polarization culminates at ß = 11 0 , then it decreases. At ß = 23.51 0 the moonlight is unpolarized again. With the further increase of ß, the polarization of the moonlight becomes positive, i.e. its direction of polarization becomes perpendicular to the plane of the direction of sight. In the positive range of polarization, the maximum p = 8.7 % is reached on the first or second day following the fourth quarter. Then p decreases again until it becomes zero 3 days prior to and after the new moon. Following the new moon, p increases until its maximum of 6.6 % 1 day prior to the first quarter. Following this stage, in the same way as after the full moon, p of the moonlight decreases to zero, then its angle of polarization turns over again. Using full-sky imaging polarimetry, Gal et al. (2001a) compared the polarization patterns of moonlit and sunlit skies (Fig. 8.1) as weIl as the positions of the lunar/solar Arago and Babinet neutral points of clear moonlit/sunlit night/day skies (Fig. 8.2). They confirmed experimentally that the polarizational characteristics of a moonlit night sky are practically identical with
G. Horváth et al., Polarized Light in Animal Vision © Springer-Verlag Berlin Heidelberg 2004
Part II: Polarization Patterns in Nature
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