Definition:Celestial Horizon/Pole

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The poles of the celestial horizon are known as the zenith and the nadir.


Let an observer $O$ on the surface of Earth be assumed to be the center of the celestial sphere $C$.

The zenith $Z$ is the point on $C$ which is vertically overhead.


Consider the celestial sphere $C$ with an observer $O$ at the center.

Let $Z$ be the zenith of $C$

Let the straight line $ZO$ be produced from $O$ to meet $C$ at $R$.

The point $R$ is known as the nadir of $C$.