The parallactic angle is the angle between the great circles from an observed direction toward the celestial pole and toward the zenith. Its change gives sky rotation in pupil-stabilized observations.
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The parallactic angle is an important concept in astronomy and astrophysics related to the observation of celestial objects. It is defined as the angle between two lines of sight: one pointing towards an observer from a celestial object and the other pointing from the observer to the point in the sky directly above them, often referred to as the zenith or the meridian.