Prime Meridian

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Image:Primemeridian.jpg Image:Prime meridian.jpg The Prime Meridian, also known as the International Meridian or Greenwich Meridian, is the meridian (line of longitude) passing through the Royal Greenwich Observatory, Greenwich, England — it is the meridian at which longitude is defined to be 0 degrees. The prime meridian, and the opposite 180th meridian (at 180° longitude), which the International Date Line generally follows, separates the eastern and western hemispheres.

Unlike the parallels of latitude, which are defined by the rotational axis of the Earth (the poles being 90° and the Equator, 0°), the prime meridian is arbitrary, and multiple meridians have been used through history as the prime meridians of various mapmaking systems. The Greenwich Meridian was agreed upon as the international standard in October 1884. At the behest of U.S. President Chester A. Arthur, 41 delegates from 25 nations met in Washington, D.C., USA, for the International Meridian Conference. France abstained when the vote was taken, and French maps continued to use the Paris Meridian for several decades.

Heading south from the North Pole, the Prime Meridian passes through the following countries:

United Kingdom (note London is in both the Western Hemisphere and Eastern Hemisphere due to the proximity of the Prime Meridian)
France
Spain
Algeria
Mali
Burkina Faso
Togo
Ghana

and finally joins all other longitude lines in Antarctica at the South Pole.

The zero meridian used by satellite navigation systems (on the WGS84 datum) is about 100 metres to the east of the line marked at Greenwich. The shift was due firstly to errors in determining its position from other observatories, secondly to recalculations of the position of the Earth's centre, and thirdly to the basing of calculations on where the meridian crosses the Equator, not where it passes through Greenwich. The Royal Observatory's lack of modern instruments prevented accurate measurements there in any case. The meridian is not fixed on the ground as the continents are moving because of continental drift, and the WGS84 datum is an average of the various continental movements - the datum stays where it is and the continents slide around underneath it.

The zero meridian used by the Ordnance Survey (OSGB36 datum) is about six metres to the west of the line marked at Greenwich. This was the standard meridian before 1851, and the Ordnance Survey simply carried on using it.

Universal Time is notionally based on the WGS84 meridian. However, the standard international time UTC can be discrepant from the observed time on the meridian by up to about one second, because of changes in the earth's rotation. Leap seconds are inserted periodically to keep UTC in sync with the earth. One second theoretically corresponds to a variation in longitude of roughly 300 metres either way on the ground at Greenwich.

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Other reference meridians

The meridian through Greenwich was selected as the Prime Meridian because over two-thirds of all ships and tonnage already used it as the reference meridian on their maps in 1884. Other reference meridians used previously include:

WGS84 longitude zero - about 100 metres east of the traditional Prime Meridian

WGS84 longitudes will eventually supplant traditional ones. The offset at other locations can be as much as 30" east or west.

Copenhagen
El Hierro (Ferro) (Canary Islands) Ancient, later redefined 17° 39' 46" W of Greenwich to be exactly 20° W of Paris. French "submarin" at Washington 1884.
Giza
Jerusalem
Madrid (3° 41' 16.48" west of Greenwich)
Oslo (Kristiania) (10° 43' 22.5" east of Greenwich)
Paris (2° 20' 14" east of Greenwich; see Paris Meridian)
Philadelphia
Pisa
Rome (12° 27' 08.04" east of Greenwich)
Saint Petersburg (Pulkovo meridian, 30° 19' 42.09" east of Greenwich)
Ujjain (75° 47' east of Greenwich) Used in Indian astronomy and calendars.
Washington, D.C. (77° 3' 2.3" west of Greenwich; see Washington meridian)

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Other planetary bodies

The prime meridians of the following bodies in the Solar System have been defined:

The prime meridian of the Moon lies directly in the middle of the face of the moon visible from earth and passes near the crater Bruce.
The 20th meridian of Mercury is defined by a special small crater known as Hun Kal, which is Mayan for 20.
The prime meridian of Mars is defined by the crater Airy-0.

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