\begin{definition}[Definition:North Polar Distance]
Consider the celestial sphere $C$ with observer $O$.
Let $X$ be a point on $C$.
Let $X$ have declination of $\delta$.
The '''north polar distance''' of $X$, denoted $\operatorname {N.P.D.} x$, is defined as:
:$\operatorname {N.P.D.} x = 90 \degrees - \delta$
That is, the '''north polar distance''' of $X$ length of the arc from the north celestial pole $P$ along the vertical circle through $X$.
:500px
\end{definition}