\begin{definition}[Definition:Longitude (Terrestrial)]
Let $J$ be a point on Earth's surface that is not one of the two poles $N$ and $S$.
Let $\bigcirc NJS$ be a meridian passing through $J$, whose endpoints are by definition $N$ and $S$.
The '''longitude''' of $J$, and of the meridian $\bigcirc NJS$ itself, is the (spherical) angle that $\bigcirc NJS$ makes with the principal meridian $\bigcirc NKS$.
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If $\bigcirc NJS$ is on the eastern hemisphere, the '''longitude''' is defined as '''longitude $n \degrees$ east''', where $n \degrees$ denotes $n$ degrees (of arc), written $n \degrees \, \mathrm E$.
If $\bigcirc NJS$ is on the western hemisphere, the '''longitude''' is defined as '''longitude $n \degrees$ west''', written $n \degrees \, \mathrm W$.
If $\bigcirc NJS$ is the principal meridian itself, the '''longitude''' is defined as '''$0 \degrees$ longitude'''.
If $\bigcirc NJS$ is the other half of the great circle that contains the principal meridian, the '''longitude''' is defined as '''$180 \degrees$ longitude'''.
\end{definition}