\begin{definition}[Definition:Declination]
Consider the celestial sphere $C$ with observer $O$.
Let $P$ and $Q$ be the north celestial pole and south celestial pole respectively.
Let $X$ be a point on $C$.
Let $PXQ$ be the vertical circle through $X$.
Let $D$ be the point where the celestial equator intersects $PXQ$.
The length of the arc $DX$ of $PXQ$ is known as the '''declination''' of $X$.
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\end{definition}