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Location of the Prime Meridian

Prime Meridian in Greenwich

A GPS receiver at the Greenwich Meridian

Laser projected from the Royal Observatory in Greenwich marking the Prime Meridian

The Royal Observatory, Greenwich (today a museum)

The 24-hour clock at Greenwich

The Prime Meridian, also known as the International Meridian or Greenwich Meridian, is the meridian (line of longitude) passing through the Royal Greenwich Observatory in London — 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, separate the Eastern and Western Hemispheres. The Prime Meridian and the 180th meridian form a great circle which encircles the Earth like a longitudinal equator.

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 (including four different Greenwich meridians). The Greenwich Meridian established by Sir George Airy in 1851 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 (The most northerly land on the meridian is the shore (53°45′34″N) southeast of the Sand-le-Mere caravan park east of Kingston upon Hull, England.)
• France
• Spain
• Algeria
• Mali
• Burkina Faso
• Togo
• Ghana
• Antarctica to the South Pole

The zero meridian used by satellite navigation systems (on the WGS84 datum) is 102.5 metres (336.3 feet) to the east of the line marked at Greenwich.^[1]. WGS84 uses the zero meridian as defined by the Bureau International de l'Heure,^[2] which was defined by compilation of star observations in different countries. The mean of this data caused a shift of about 100 metres east away from the Prime Meridian at Greenwich, UK.^[3] The plane of this geodetic meridian passes through the centre of the Earth, unlike the plane of the astronomical meridian which contains the direction of gravity (indicated by a plumb line) which points opposite to the direction of the zenith, to which astronomical instruments are aligned. The angle between these two meridian planes at the Royal Observatory, the east-west component of vertical deflection, is 5.31″. The WGS84 datum is an average of the various continental drifts. As a result, the astronomical meridian between the vertical crosshairs of Airy's transit telescope drifts toward the east as it is carried by the European portion of the Eurasian tectonic plate, closer to the geodetic meridian, by about one centimetre per year.

Curiously, whether by accident or design, the location of the WGS84 0° meridian is marked in Greenwich by the presence of a waste basket on the path leading more or less due east from the observatory containing the transit telescope.

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 continued to use it.

Universal Time is notionally based on the WGS84 meridian. However, the standard international time UTC can differ from the mean observed time on the meridian by up to about one second (equivalent to about 280 metres at Greenwich), because of changes in the Earth's rotation. Leap seconds are inserted periodically to keep UTC in sync with the Earth.

The Greenwich Meridian is now marked at night by a laser beam emitted from the observatory.[1]

One degree of longitude is approximately equal to (111.320 + 0.373sin²φ)cosφ km, where φ is latitude.^[4]

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, or advocated, include:

• WGS84 longitude zero - 102.5 metres (336.3 feet) east of the traditional Prime Meridian

WGS84 longitudes have supplanted traditional ones. The offset at other locations can be as much as 30″ east or west.

• Berne (7° 26′ 22.5″ E)
• Brussels (4° 22′ 4.71″ E)
• Copenhagen (12° 34′ 32.25″ east of Greenwich; Rundetårn)
• 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.
• Great Pyramid of Giza (1884) ^[5]
• Alexandria, the meridian of Ptolemy's Almagest.
• Jerusalem (35° 13′ 46″ east of Greenwich for the large dome of the Church of the Holy Sepulchre)
• Lisbon (9° 07′ 54.862″ west of Greenwich)
• Madrid (3° 41′ 16.48″ west of Greenwich)
• Oradea, Romania (21° 55′ 16″ east of Greenwich)
• Oslo (Kristiania) (10° 43′ 22.5″ east of Greenwich)
• Paris (2° 20′ 14.025″ east of Greenwich; see Paris Meridian)
• Philadelphia
• Pisa
• Rome (meridian of Mount Mario 12° 27′ 08.04″ east of Greenwich)
• Saint Petersburg (Pulkovo meridian, 30° 19′ 42.09″ east of Greenwich)
• Stockholm (at the observatory, 18° 3′ 29.8″ east of Greenwich)
• Ujjain (75° 47′ east of Greenwich) Used in Indian astronomy and calendars.
• Warsaw (21° 00’ 42” east of Greenwich; see Warsaw meridian)
• Washington, D.C. (77° 3′ 2.3″ west of Greenwich; see Washington meridian)
• Approximately 59° east of Greenwich ^[6]

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.

1. ^ History of the Prime Meridian -Past and Present
2. ^ European Organisation for the Safety of Air Navigation and IfEN: "WGS 84 Implementation Manual", page 13. 1998
3. ^ National Maritime Museum: "The Longitude of Greenwich"
4. ^ P. Kenneth Seidelmann, ed., Explanatory Supplement to the Astronomical Almanac (Mill Valley, Cal.: University Science Books, 1992) page 700.
5. ^ Wilcomb E. Washburn, "The Canary Islands and the Question of the Prime Meridian: The Search for Precision in the Measurement of the Earth"
6. ^ Maimonides, Hilchot Kiddush Hachodesh 11:17, calls this point אמצע היישוב, "the middle of the habitation", i.e. the habitable hemisphere. Evidently this was a convention accepted by Arab geographers of his day.

• International Conference Held at Washington for the Purpose of Fixing a Prime Meridian, available at Project Gutenberg.
• scanned TIFFs of the conference proceedings
• Prime meridians in use in the 1880s, by country
• Canadian Prime Meridian