What is the Prime Meridian?

Tricia Christensen
Tricia Christensen

The prime meridian is the vertical line that marks the 0° longitude measurement on the globe of Earth. The line intersects the latitude lines that are horizontal. Not every globe has the same prime meridian measurement, but most measure it as a line that passes through England, France, Spain, Togo, and down to Antarctica.

Prime Meridian at the Royal Observatory in Greenwich, United Kingdom.
Prime Meridian at the Royal Observatory in Greenwich, United Kingdom.

Unlike latitude measurements, any longitude line begins at the North Pole and ends at the South Pole. Latitudinal measurements combined with longitudinal measurements can help a person locate a specific place on a map. They are of particular use in air trafficking, and in determining shipping locations.

Sculpture located at the Prime Meridian in Greenwich.
Sculpture located at the Prime Meridian in Greenwich.

Longitude measurements are also a way to evaluate time. Each space between a longitude measurement represents approximately one hour’s time. If someone was to travel from the prime meridian to the next longitude line, which advances 15°, there would be a difference of approximately one hour in terms of measurements by the sun. Time differences may or may not be accounted for, depending upon the country.

Tablet marking the Prime Meridian in Canada.
Tablet marking the Prime Meridian in Canada.

There has been some debate about where the prime meridian properly belongs. Currently, the measurement is based on a decision made in 1884, which was greatly influenced by shipping lines that already had marked the location. All such measurements are arbitrary, and theoretically, the prime meridian could be anywhere on the globe. Cooperation is needed, however, so international systems use a single longitude line as the source for giving appropriate directions and for determining such things as flight paths or shipping lines.

A global map of time zone divisions, which are based on the prime meridian.
A global map of time zone divisions, which are based on the prime meridian.

Some people confuse the prime meridian with the international dateline. This line is in fact directly opposite on a globe. The international dateline has a measurement of 180° and passes roughly through the middle of the Pacific Ocean.

Earth is not the only planet with a prime meridian. In fact, scientists have created such a line on both the moon and on Mars. These lines are again arbitrary, and should humans ever make use of these planets, they might change according to the needs of the people using them for map making and information.

The prime meridian passes through Spain.
The prime meridian passes through Spain.
Tricia Christensen
Tricia Christensen

Tricia has a Literature degree from Sonoma State University and has been a frequent wiseGEEK contributor for many years. She is especially passionate about reading and writing, although her other interests include medicine, art, film, history, politics, ethics, and religion. Tricia lives in Northern California and is currently working on her first novel.

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Discussion Comments

anon343890

What exactly is the prime meridian? Please someone break it down for me.

Mary Ann Mischke

Does anyone know where I can get a digital file (shapefile format) of the US Meridians? Preferably free.

anon104369

It's not a set thing.

anon46439

the prime meridian goes over land and water. the equator and prime meridian meet just below africa on water. The prime meridian passes through greenwich, England. The U.S. is east of the meridian.

anon42926

the united states is east or west of the prime meridian?

anon28523

what has Greenwich have to do with the prime meridian?

anon27076

i have homework on this. where do the equator and prime meridian meet, on land or on water?

anon22642

prime meridian was created by ptolemy.

anon18291

who made the prime meridian?

dfrum32

How is it useful if not every globe has the same prime meridian marking? I thought that the prime meridian was a set thing that was the same on all globes.

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