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 (terrestrial) longitude of $J$, and of the meridian $\bigcirc NJS$ itself, is the (spherical) angle that $\bigcirc NJS$ makes with the prime 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 angle), 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 prime meridian itself, the longitude is defined as $0 \degrees$ longitude.

If $\bigcirc NJS$ is the other half of the great circle that contains the prime meridian, the longitude is defined as $180 \degrees$ longitude.


Also defined as

Many sources define the (terrestrial) longitude as so many degrees east (or west) from (or of) the Greenwich meridian.

However, the Greenwich meridian has been superseded as the prime meridian by the IERS reference meridian.


Also known as

Terrestrial longitude is usually known in common parlance as just longitude.

However, this term also encompasses other measurements of longitude, for example celestial longitude and galactic longitude.

Hence on $\mathsf{Pr} \infty \mathsf{fWiki}$ the full term terrestrial longitude is preferred, so as to forestall ambiguity, unless the context is clear.


Also see

  • Results about terrestrial longitude can be found here.


Linguistic Note on Longitude

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

This is technically incorrect.


Sources

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