Definition:Longitude (Terrestrial)

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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 $\bigcirc NKS$.


Longitude-Terrestrial.png


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.


Also known as

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


Also see


Linguistic Note on Longitude

When the word longitude is spoken, an extraneous d is often added: long-di-tude.

This is technically incorrect.


Linguistic Note on Greenwich

The place name Greenwich, in the context of the Greenwich meridian, is pronounced something like Gren-itch or Gren-idge.


Sources

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