Prime Meridian

Location of the Prime Meridian

Prime Meridian in Greenwich

The Prime Meridian is the meridian (line of longitude) passing through the Royal Greenwich Observatory, Greenwich, England; it is the meridian at which longitude is 0 degrees. It is often referred to as the Greenwich Meridian. The meridian was agreed upon 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 the French would cling to the Paris Meridian for several decades.

The International Date Line is on the opposite side of the world from the Prime Meridian. 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. It 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. (Otherwise a minor earthquake in Greenwich could change longitudes throughout the world.) 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 calculated for a notional meridian which is near the WGS84 meridian.

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
caution:WGS84 longitudes may supplant traditional ones and the offset is not constant with longitude
•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)
•Washington, D.C. (77°3′2.3″ west of Greenwich)
•Ujjain (23.09°N, 75.43°E of Greenwich). Used by Hindu geographers since about 4th century BCE. Still used sometimes in Hindu Astronomy and timekeeping.
With respect to other solid celestial bodies, prime meridians are likewise arbitrarily 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.