Definition:Longitude (Terrestrial)

Definition
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.

If $\bigcirc NJS$ is on the hemisphere of Earth immediately to the east of the principal meridian, the longitude is defined as longitude $n \degrees$ east, where $n \degrees$ denotes $n$ degrees (of arc).

If $\bigcirc NJS$ is on the hemisphere of Earth immediately to the west of the principal meridian, the longitude is defined as longitude $n \degrees$ west.

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 .

Also known as
Sometimes you see the longitude reported as east (or west) from (or of) Greenwich.

Also see

 * Definition:Longitude (Celestial)
 * Definition:Longitude (Galactic)