\begin{definition}[Definition:Nadir]
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 $C$.
:480px
In the above diagram, the '''nadir''' is indicated by $R$.
\end{definition}