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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.

This can be determined by use of a plumb line.


In the above diagram, the zenith is indicated by $Z$.

Also see

Linguistic Note

The word zenith derives from Medieval Latin cenit, a transliteration of the Arabic سَمْت‎ (samt), meaning direction or path) which is itself anabbreviation of سَمْت اَلرَّأْس‎ (samt ar-raʾs), meaning direction of the head).

The US English pronunciation of zenith is zee-nith, while British and other English pronunciation is zen-nith.