Prime meridian

A prime meridian is a meridian in a geographic coordinate system at which longitude is defined to be 0°. On a spheroid, a prime meridian and its anti-meridian form a great ellipse. This divides the body into two hemispheres: the Eastern Hemisphere and the Western Hemisphere. Unlike the equator, which also divides a spherical celestial body into two hemispheres, the prime meridian is astronomically arbitrary. For each of Earth's historic prime meridians, various conventions have been used or advocated in different regions throughout history, but none have a basis in physical geography. Earth's current international standard prime meridian is the IERS Reference Meridian. It is derived, but differs slightly, from the Greenwich Meridian, the previous standard.
File:Atlas Cosmographicae 033.jpg|thumb|upright=1.3|right|Gerardus Mercator in his Atlas Cosmographicae used a prime meridian somewhere close to 25°W, passing just to the west of Santa Maria Island in the Azores in the Atlantic Ocean. His 180th meridian runs along the Strait of Anián
Longitudes for the Earth and Moon are measured from their prime meridian to 180° east and west. For all other Solar System bodies, longitude is measured from 0° to 360°. West longitudes are used if the rotation of the body is prograde, meaning that its direction of rotation is the same as that of its orbit. East longitudes are used if the rotation is retrograde.

History

The notion of longitude for Greeks was developed by the Greek Eratosthenes in Alexandria, and Hipparchus in Rhodes, and applied to a large number of cities by the geographer Strabo. Ptolemy was the first geographer to use a consistent meridian for a world map, in his Geographia.
Ptolemy used as his basis the "Fortunate Isles", a group of islands in the Atlantic, which are usually associated with the Canary Islands, although his maps correspond more closely to the Cape Verde islands. The main point is to be comfortably west of the western tip of Africa as negative numbers were not yet in use. His prime meridian corresponds to 18°40′ west of Winchester today. At that time the chief method of determining longitude was by using the reported times of lunar eclipses in different countries.
One of the earliest known descriptions of standard time in India appeared in the 4th century CE astronomical treatise Surya Siddhanta. Postulating a spherical Earth, the book described the thousands years old customs of the prime meridian, or zero longitude, as passing through Avanti, the ancient name for the historic city of Ujjain in Central India, and Rohitaka, the ancient name for Rohtak, a city north.
File:Propaganda Map.jpg|thumb|right|upright=1.4|William Grigg's facsimile of the 1529 Spanish Padron Real, from the copy made by Diogo Ribeiro and held by the Vatican Library
Ptolemy's Geographia was first printed with maps at Bologna in 1477, and many early globes in the 16th century followed his lead, but there was still a hope that a "natural" basis for a prime meridian existed. In 1493, Christopher Columbus reported that the compass pointed due north somewhere in mid-Atlantic, and this fact was used in the important Treaty of Tordesillas of 1494, which settled the territorial dispute between Spain and Portugal over newly discovered lands. The Tordesillas line was eventually settled at 370 leagues west of Cape Verde. This is shown in the copies of Spain's Padron Real made by Diogo Ribeiro in 1527 and 1529. São Miguel Island in the Azores was still used for the same reason as late as 1594 by Christopher Saxton, although by then it had been shown that the zero magnetic declination line did not follow a line of longitude.
File:Atlas Ortelius KB PPN369376781-006av-006br.jpg|thumb|upright=1.3|1571 Africa map by Abraham Ortelius, with Cape Verde marking its prime meridian
File:CEM-36-Regno-della-China-2355.jpg|thumb|upright=1.3|1682 map of East Asia by Giacomo Cantelli, with Cape Verde originating its prime meridian; Japan is thus located around 180° E.
In 1541, Mercator produced his 41 cm terrestrial globe and drew his prime meridian precisely through Fuerteventura in the Canaries. His later maps used the Azores, following the magnetic hypothesis, but by the time that Ortelius produced the first modern atlas in 1570, other islands such as Cape Verde were coming into use. In his atlas longitudes were counted from 0° to 360°, not 180°W to 180°E as is usual today. This practice was followed by navigators well into the 18th century. In 1634, Cardinal Richelieu used the westernmost island of the Canaries, El Hierro, 19°55′ west of Paris, as the choice of meridian. The geographer Delisle decided to round this off to 20°, so that it simply became the meridian of Paris disguised.
In the early 18th century, the battle was on to improve the determination of longitude at sea, leading to the development of the marine chronometer by John Harrison. The development of accurate star charts, principally by the first British Astronomer Royal, John Flamsteed between 1680 and 1719 and disseminated by his successor Edmund Halley, enabled navigators to use the lunar method of determining longitude more accurately using the octant developed by Thomas Godfrey and John Hadley.
In the 18th century most countries in Europe adapted their own prime meridian, usually through their capital, hence in France the Paris meridian was prime, in Prussia it was the Berlin meridian, in Denmark the Copenhagen meridian, and in United Kingdom the Greenwich meridian.
Between 1765 and 1811, Nevil Maskelyne published 49 issues of the Nautical Almanac based on the meridian of the Royal Observatory, Greenwich. "Maskelyne's tables not only made the lunar method practicable, they also made the Greenwich meridian the universal reference point. Even the French translations of the Nautical Almanac retained Maskelyne's calculations from Greenwich – in spite of the fact that every other table in the Connaissance des Temps considered the Paris meridian as the prime."
In 1884, at the International Meridian Conference in Washington, D.C., 22 countries voted to adopt the Greenwich meridian as the prime meridian of the world. The French argued for a neutral line, mentioning the Azores and the Bering Strait, but eventually abstained and continued to use the Paris meridian until 1911.
The current international standard Prime Meridian is the IERS Reference Meridian. The International Hydrographic Organization adopted an early version of the IRM in 1983 for all nautical charts. It was adopted for air navigation by the International Civil Aviation Organization on 3 March 1989.

International prime meridian

Since 1984, the international standard for the Earth's prime meridian is the IERS Reference Meridian. Between 1884 and 1984, the meridian of Greenwich was the world standard. These meridians are very close to each other.

Prime meridian at Greenwich

In October 1884 the Greenwich Meridian was selected by delegates to the International Meridian Conference held in Washington, D.C., United States to be the common zero of longitude and standard of time reckoning throughout the world.
The position of the historic prime meridian, based at the Royal Observatory, Greenwich, was established by Sir George Airy in 1851. It was defined by the location of the Airy Transit Circle ever since the first observation he took with it. Prior to that, it was defined by a succession of earlier transit instruments, the first of which was acquired by the second Astronomer Royal, Edmond Halley in 1721. It was set up in the extreme north-west corner of the Observatory between Flamsteed House and the Western Summer House. This spot, now subsumed into Flamsteed House, is roughly 43 metres to the west of the Airy Transit Circle, a distance equivalent to roughly 2 seconds of longitude. It was Airy's transit circle that was adopted in principle as the Prime Meridian of the world at the 1884 International Meridian Conference.
All of these Greenwich meridians were located via an astronomic observation from the surface of the Earth, oriented via a plumb line along the direction of gravity at the surface. This astronomic Greenwich meridian was disseminated around the world, first via the lunar distance method, then by chronometers carried on ships, then via telegraph lines carried by submarine communications cables, then via radio time signals. One remote longitude ultimately based on the Greenwich meridian using these methods was that of the North American Datum 1927 or NAD27, an ellipsoid whose surface best matches mean sea level under the United States.

List of places

On Earth, starting at the North Pole and heading south to the South Pole, the IERS Reference Meridian passes through 8 countries, 4 seas, 3 oceans and 1 channel:

Co-ordinates	Country, territory or sea	Notes
	North Pole and Arctic Ocean
	Exclusive Economic Zone of Greenland
	Greenland Sea
	EEZ of Svalbard
	International waters
	EEZ of Jan Mayen
	Norwegian Sea
	International waters
	EEZ of Norway
	EEZ of Great Britain
	North Sea
		From Tunstall in East Riding to Peacehaven, passing through Greenwich
	English Channel	EEZ of Great Britain
	English Channel	EEZ of France
		From Villers-sur-Mer to Gavarnie
		From Cilindro de Marboré to Castellón de la Plana
	Mediterranean Sea	Gulf of Valencia; EEZ of Spain
		From El Verger to Calp
	Mediterranean Sea	EEZ of Spain
	Mediterranean Sea	EEZ of Algeria
		From Stidia to Algeria-Mali border near Bordj Badji Mokhtar
		Passing through Gao
		For about, running through Cinkassé.
		For about
		For about
		For about
		From the Togo-Ghana border near Bunkpurugu to Tema Passing through Lake Volta at
	Atlantic Ocean	EEZ of Ghana
	Atlantic Ocean	International waters
	Atlantic Ocean	Passing through the Equator
	Atlantic Ocean	EEZ of Bouvet Island
	Atlantic Ocean	International waters
	Southern Ocean	International waters
	Antarctica	Queen Maud Land, claimed by
	Antarctica	Amundsen–Scott South Pole Station, South Pole