Brightness and Polarization of Twilight at the Celestial Pole
The method of determining the tropospheric component from observations at two symmetric points on the solar vertical circle  cannot be used for the early period of twilight, when ζ⊙ > 6–7°, when the primary twilight segment is not yet detached from the horizon on the antisolar vertical circle or is at a negligible distance from it . If it is assumed that during bright twilight the degree of higher-order polarization is negligible or entirely absent , the tropospheric component can be determined from observations of the brightness and polarization of the twilight sky at points not on the solar vertical circle far from the zenith. With this in mind, we chose for observations the region near the celestial pole. At the latitude of Dushanbe, this region of the sky is fairly far removed from the solar vertical circle and from the zenith. Therefore, the observations were made in a region even closer to the horizon, but besides taking into account the effect of the tropospheric component for the period of early twilight, we studied the variation of the degree of polarization, brightness, and angle of polarization at the celestial pole.
KeywordsVertical Circle Polarization Drop Celestial Pole Solar Direction Evening Twilight
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