geographic zenith
Quick Summary
The zenith is the point on the celestial sphere directly above an observer on Earth. It is defined by gravity, using a plumb line. The point directly opposite the zenith, underfoot, is called the nadir.1
The geocentric zenith differs slightly from the geographic zenith due to Earth's rotation and imperfect shape, except at the equator and poles. The celestial sphere is the apparent surface of the heavens, used for mapping celestial bodies. The zenith is a key reference point on this sphere. A zenith telescope is a specialized instrument designed for precise time and latitude measurements by observing stars near the zenith.123
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