\begin{definition}[Definition:Celestial Sphere/Observer]
In the context of astronomy, in particular spherical astronomy, the '''observer''' is considered to be a reference point located at the center of the celestial sphere.
In practice, of course, an actual '''observer''' is physically located on the surface of Earth.
However, the celestial sphere is considered to be so much larger than the radius of Earth that the '''observer''' is considered to be at the center of Earth.
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In the above diagram, the position of the '''observer''' is indicated by $O$.
\end{definition}