Definition:North Polar Distance

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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 $X$.