Longitude ( /ˈlɒndʒɨtjuːd/ or /ˈlɒŋɡɨtjuːd/)^[1] is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda (λ). Dictionary meaning: The angular distance of a place east or west of the meridian at Greenwich, England, or west of the standard meridian of a celestial...: "at a longitude of 2° W"; "lines of longitude points with the same longitude lie in lines running from the North Pole to the South Pole. By convention, one of these, the Prime Meridian, which passes through the Royal Observatory, Greenwich, England, establishes the position of zero degrees longitude. The longitude of other places is measured as an angle east or west from the Prime Meridian. Specifically, it is the angle between a plane containing the Prime Meridian and a plane containing the North Pole, South Pole and the location in question. If the direction of longitude (east or west) is not specified, positive longitude values are east of the Prime Meridian, and negative values are west of the Prime Meridian. This forms a right-handed coordinate system with the z axis (right hand thumb) pointing from the Earth's center toward the North Pole and the x axis (right hand index finger) extending from Earth's center through the equator at the Prime Meridian. A location's north-south position along a meridian is given by its latitude, which is the angle between the local vertical and the plane of the Equator.

If the Earth were perfectly symmetrical and homogenous, then longitude at a point would just be the angle between a vertical north-south plane through that point and the plane of the Greenwich meridian. Everywhere on Earth the vertical north-south plane would contain the Earth's axis. But the Earth is not homogenous, and has mountains-- which have gravity and so can shift the vertical plane away from the Earth's axis. The vertical north-south plane still intersects the plane of the Greenwich meridian at some angle; that angle is astronomical longitude, the longitude you calculate from star observations. The longitude shown on maps and GPS devices is the angle between the Greenwich plane and a not-quite-vertical plane through the point; the not-quite-vertical plane is perpendicular to the surface of the spheroid chosen to approximate the Earth's sea-level surface, rather than perpendicular to the sea-level surface itself.

Latitude lines run east and west and measure north and south; longitude lines run north and south and measure east and west. The lines measure distances in degrees.

But where do you start? Where is 0 degrees?

Well, that depends on whether you're looking for 0 degrees latitude or 0 degrees longitude. They are different things.

The equator is 0 degree latitude. This imaginary line, which runs through parts of South America, Africa, and Asia, is officially the halfway point between the North Pole and the South Pole.

The prime meridian is 0 degrees longitude. This imaginary line runs through the United Kingdom, France, Spain, western Africa, and Antarctica.

By using the equator and prime meridian, we can divide the world into four hemispheres, north, south, east, and west. For instance, the United States is in the Western Hemisphere (because it is west of the prime meridian) and also in the Northern Hemisphere (because it is north of the equator).

The earliest maps have the equator marked on them, but it wasn't until the late 19th Century that the prime meridian was named. Until that time, as many as 14 different locations were being identified on various maps as 0 degrees longitude. The International Meridian Conference of 1884 changed all that, naming an imaginary line drawn through Greenwhich, England, as the prime meridian.

These lines are drawn only on maps and globes, however. You won't find an "equator line" on the streets of Brazil. These lines are merely for the identification of where things are on a map or a globe.

Graphics courtesy of ArtToday

Brief History of the Prime Meridian

    Since 1870, geographers and scientists of allied disciplines from all nations have been trying the possibility of fixing a common zero for longitude and time reckoning throughout the globe. And it was the Prime Meridian, which was the first to be discussed. Hipparchos was the first astronomer to determine the differences in longitude, of which he used Rhodes. Ptolemy , following the Marinus of Tyre, adopted a meridian through the Canary Islands, which marked the western boundary of the world, whereas, to the east, there seemed to be no such boundary.  
    In 1871 the first International Geographical Congress (IGC) took place at Antwerp. The view expressed was for passage charts for all nations, not necessarily coastal or harbour charts, the Greenwich meridian should be adopted as the common zero for longitude, and that this should become obligatory within fifteen years.  It was also recommended that, whenever ships exchanged longitudes at sea , they should be based on Greenwich. This did not apply to land maps and coastal charts, these should keep its own prime meridian.
    However, the 2nd IGC in Rome in 1875 discussed the whole matter again without coming to any further conclusions. France did express that if we were to accept the metric system, then they would accept the Greenwich meridian. Eventually, it was agreed internationally that a prime meridian was needed, and that it should be Greenwich.
    So after years of discussion, the conference hoped that if the entire world was to accept Greenwich as the prime meridian, Great Britain might be prepared to conform to the metric system.

Prime meridians in use in the early 1880s

    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 - 102.5 metres 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.

• 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.
• Giza
• 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)
• 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)

 

COUNTRY	SEA CHARTS	LAND MAPS
Austria	Greenwich	Ferro
Bavaria	-	Munich
Belgium	Greenwich	Brussels
Brazil	Greenwich and Rio de Janeiro	Rio de Janeiro
Denmark	Greenwich, Copenhagen and Paris	Copenhagen
France and Algeria	Paris	Paris
Germany	Greenwich and Ferro	Ferro
Holland	Greenwich	Amsterdam
India	-	Greenwich
Italy	Greenwich	Rome
Japan	Greenwich	Greenwich
Norway	Greenwich and Christiania	Ferro and Christiania
Portugal	Lisbon	Lisbon
Russia	Greenwich, Pulkowa and Ferro	Ferro, Pulkowa, Warsaw and Paris
Spain	Cadiz (S.Fernando)	Madrid
Sweden	Greenwich, Stockholm and Paris	Ferro and Stockholm
Switzerland	-	Paris
UK and colonies	Greenwich	Greenwich
USA	Greenwich	Greenwich and Washington

The Prime Meridian  

 

 

    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 (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:

 

• The 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 102.5 metres to the east of the line marked at Greenwich.
    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 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. 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 through Airy's transit instrument 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.  

 

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

 

http://www.freewebs.com/manetsute/primemeridian.htm