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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 $C$.


In the above diagram, the nadir is indicated by $R$.

Also see

Linguistic Note

The word nadir derives from the Arabic نظير‎ (naẓīr), meaning counterpart.

It is usually pronounced nad-deer or nay-deer, although the stress can sometimes be heard on the second syllable: nad-deer.