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A zenith is a point in the sky which appears directly above the observer. Further precisely, these are a point on the sky sustaining an altitude of +90 degrees, and these are a pole of the horizontal coordinate system. Geometrically, these are a point on the celestial sphere intersected by a line drawn from either a center of the Globe across the location on the Globe's surface. A point opposite a zenith is the nadir.
a zenith is, by definition, a point along the local meridian.
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