thumb|300px|Countries that touch the [[Equator (red) and that touch the IERS Reference Meridian (blue)]]
A prime meridian is a meridian (a line of longitude) in a geographic coordinate system at which longitude is defined to be 0°. On a spheroid, a prime meridian and its anti-meridian (the 180th meridian in a 360-degree system) form a great ellipse. This divides the body (e.g. Earth) into two hemispheres: the Eastern Hemisphere and the Western Hemisphere (for an east-west notational system). 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 (unlike the prime meridian of Mars) 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.
thumb|upright=1.3|right|[[Gerardus Mercator in his Atlas Cosmographicae (1595) 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 (Bering Strait)]]
Longitudes for the Earth and Moon are measured from their prime meridian (at 0°) to 180° east and west. For all other Solar System bodies, longitude is measured from 0° (their prime meridian) to 360°. West longitudes are used if the rotation of the body is prograde (or 'direct', like Earth), meaning that its direction of rotation is the same as that of its orbit. East longitudes are used if the rotation is retrograde.
History
right|thumb|upright=1.3|[[Ptolemy's 1st projection, redrawn under Maximus Planudes around 1300, using a prime meridian through the Canary Islands west of Africa, at the left-hand edge of the map (The obvious central line shown here is the junction of two sheets.)]]
The notion of longitude for Greeks was developed by the Greek Eratosthenes (c.276195BCE) in Alexandria, and Hipparchus (c.190120BCE) in Rhodes, and applied to a large number of cities by the geographer Strabo (64/63BCEc.24CE). Ptolemy (c.90168CE) 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 (13°W to 18°W), although his maps correspond more closely to the Cape Verde islands (22°W to 25°W). The main point is to be comfortably west of the western tip of Africa (17°30′W) as negative numbers were not yet in use. His prime meridian corresponds to 18°40′ west of Winchester (about 20°W) 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.
thumb|right|upright=1.4|[[William Griggs (inventor)|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 (about ) 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 (25°30′W) 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.
thumb|upright=1.3|1571 Africa map by [[Abraham Ortelius, with Cape Verde marking its prime meridian]]
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 (14°1′W) 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
thumb|upright=1.2|The line of the [[Prime meridian (Greenwich)|Greenwich meridian at the Royal Observatory, Greenwich, England]]
In October 1884 the Greenwich Meridian was selected by delegates (forty-one delegates representing twenty-five nations) 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.
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| Kraków || 19° 57 21.43 E
| Kraków meridian
| at the Old Kraków Observatory at the Śniadecki' College; mentioned also in Nicolaus Copernicus's work On the Revolutions of the Heavenly Spheres.
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| Warsaw || 21° 00 42 E
| Warsaw meridian
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| Between 1464 and 1667, a prime meridian was set in the Fortress of Oradea (Varadinum at the time) by Georg von Peuerbach. In his logbook Columbus stated, he had one copy of Tabulae Varadienses ( or ) on board to calculate the actual meridian based on the position of the Moon, in correlation to Várad. Amerigo Vespucci also recalled, how was he acquired the knowledge to calculate meridians by means of these tables.
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| Alexandria || 29° 53 E
| Meridian of Alexandria
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| The meridian of Ptolemy's Almagest.
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| Saint Petersburg || 30° 19 42.09 E
| Pulkovo meridian
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| || 31° 08 03.69 E
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| 1884
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| Jerusalem || 35° 13 47.1 E
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| calls this point (24 degrees east of Jerusalem) אמצע היישוב, "the middle of the habitation", i.e. the habitable hemisphere. Evidently this was a convention accepted by Arab geographers of his day.
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| Ujjain || 75° 47 E
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| Used from 4th century CE Indian astronomy and calendars(see also Time in India).
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| Beijing || 116° 24 E
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| Used in Qing dynasty for astronomical and cartographical purposes.
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| Kyoto || 136° 14 E
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| Used in 18th and 19th (officially 1779–1871) century Japanese maps. Exact place unknown, but in "Kairekisyo" in Nishigekkoutyou-town in Kyoto, then the capital.
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| || ~ 180
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|thumb|300px|180th meridian
| Opposite of Greenwich, proposed 13 October 1884 on the International Meridian Conference by Sandford Fleming