# 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 $\bigcirc NKS$. 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.

## 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.