\begin{definition}[Definition:Azimuth (Astronomy)]
Let $X$ be a point on the celestial sphere.
The spherical angle between the principal vertical circle and the vertical circle on which $X$ lies is the '''azimuth''' of $X$.
The '''azimuth''' is usually measured in degrees, $0 \degrees$ to $180 \degrees$ either west or east, depending on whether $X$ lies on the eastern or western hemisphere of the celestial sphere.
\end{definition}