Dynamic patterns of skylight polarization as clock and compass

Abstract

Although many animals use the polarization of skylight for directional orientation, most investigations have been limited to consideration of the static relationships between E-vector orientation and the sun's azimuth. Much more useful information is directly available to any animal that could utilize the dynamic properties of skylight polarization, which could not be obtained by simple observation of the static pattern alone. For example the pole point, where the earth's axis intersects the celestial sphere, would provide a fixed “beacon” for an animal which could see the uniform rotation of the entire pattern of skylight polarization around this point at 15 °/h. The azimuth of the pole point provides true compass direction; its altitude is the observer's latitude; and, as Wheatstone pointed out in 1848, the E-vector orientation at the pole point provides true local sun time. Even animals without a highly developed sensitivity to polarization of light might locate the pole point approximately by observing the band of maximum polarization which rotates around this point. At the equinoxes this band passes through the pole point, and at other times within 23.5 °. Such cues may explain some recent, puzzling results of animal navigation experiments. These theoretical possibilities suggest a need for direct measurements of both the threshold rates at which animals can detect E-vector rotation, and capabilities to discriminate between slightly different E-vector orientations.