Fig. 3. Estimating the sun position with two birefringent calcite sunstones under totally overcast skies. The direction of polarization of skylight is symbolized by a dashed line, the degree of linear polarization of skylight is symbolized by the thickness of the dashed line. A Zone d of high degrees of polarization of skylight is located by scanning along zone b about 20°–30° above the horizon with a sunstone held with its slits enclosing about 45° with the local meridian a. This method provides a fair contrast of the slit images in the sunstone when the sun (S) is not close to the horizon or the zenith (Z). B The sunstone is rotated to reach equal intensities of the slit images to read the direction of polarization of skylight. The sun is located along the celestial great circle c that lies in the plane of scattering and is perpendicular to d. C The zone of highly polarized skylight d is verified. The sunstone is moved along d and then along c with its slit parallel to d. Along d the contrast of slit images is expected to remain perceivable. Along c the contrast of slit images is expected to quickly decrease. D A second patch appropriate for skypolarimetry is chosen along d. The second sun-stone is used to identify the direction of polarization of skylight here to mark out the great circle e. The intersections of c and e mark the positions of the sun and the antisolar point. Both celestial positions can be used to estimate the solar elevation angle ΘS and the direction of the solar meridian sm. The estimation is more reliable, if the intersecting angle κ of c and e is close to 90°.
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