Prime meridian

GreenwichGreenwich meridianGreenwich Prime Zero meridianzero meridianMeridian0° longitude100 Years Greenwich MeridianCarrington heliographic coordinate systemEarth's prime meridian
A prime meridian is a meridian (a line of longitude) in a geographic coordinate system at which longitude is defined to be 0°.wikipedia
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180th meridian

180°antimeridianAnti-Meridian
Together, a prime meridian and its anti-meridian (the 180th meridian in a 360°-system) form a great circle.
The 180th meridian or antimeridian is the meridian 180° both east and west of the Prime Meridian, with which it forms a great circle dividing the earth into the Western and Eastern Hemispheres.

Longitude

WestlongitudinalE
A prime meridian is a meridian (a line of longitude) in a geographic coordinate system at which longitude is defined to be 0°.
By convention, one of these, the Prime Meridian, which passes through the Royal Observatory, Greenwich, England, was allocated the position of 0° longitude.

Western Hemisphere

WesternhemisphereAmericas
If one uses directions of East and West from a defined prime meridian, then they can be called the Eastern Hemisphere and the Western Hemisphere.
The Western Hemisphere is a geographical term for the half of Earth which lies west of the prime meridian (which crosses Greenwich, London, United Kingdom) and east of the antimeridian.

IERS Reference Meridian

Prime Meridian0° longitudeIERS meridian
The most widely used modern meridian is the IERS Reference Meridian.
The IERS Reference Meridian (IRM), also called the International Reference Meridian, is the prime meridian (0° longitude) maintained by the International Earth Rotation and Reference Systems Service (IERS).

Geographic coordinate system

Coordinatesgeographic coordinateslatitude and longitude
A prime meridian is a meridian (a line of longitude) in a geographic coordinate system at which longitude is defined to be 0°.
In the 1st or 2nd century, Marinus of Tyre compiled an extensive gazetteer and mathematically-plotted world map using coordinates measured east from a prime meridian at the westernmost known land, designated the Fortunate Isles, off the coast of western Africa around the Canary or Cape Verde Islands, and measured north or south of the island of Rhodes off Asia Minor.

Africa

African continentAfricanAfrican politics
The main point is to be comfortably west of the western tip of Africa (17.5° W) as negative numbers were not yet in use.
A definite line was drawn between the two continents by the geographer Ptolemy (85–165 AD), indicating Alexandria along the Prime Meridian and making the isthmus of Suez and the Red Sea the boundary between Asia and Africa.

Geography (Ptolemy)

GeographyGeographiaPtolemy
AD 90 – c. AD 168) who first used a consistent meridian for a world map in his Geographia.
Latitude was expressed in degrees of arc from the equator, the same system that is used now, though Ptolemy used fractions of a degree rather than minutes of arc. His Prime Meridian ran through the Fortunate Isles, the westernmost land recorded, at around the position of El Hierro in the Canary Islands.

Equator

equatorial planeThe Equator
A prime meridian is ultimately arbitrary, unlike an equator, which is determined by the axis of rotation—and various conventions have been used or advocated in different regions and throughout history.
Starting at the Prime Meridian and heading eastwards, the equator passes through:

Eastern Hemisphere

EasternEastEast Hemisphere
If one uses directions of East and West from a defined prime meridian, then they can be called the Eastern Hemisphere and the Western Hemisphere.
The prime meridian at 0° longitude and the antimeridian, at 180° longitude, are the conventionally accepted boundaries, since they divide eastern longitudes from western longitudes.

International Meridian Conference

1884 prime meridian systemabandonedcontemporaneously
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 International Meridian Conference was a conference held in October 1884 in Washington, D.C., in the United States, to determine a prime meridian for international use.

Paris meridian

Meridian of ParisArago medallionFrench meridian arc
"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."
It was a long-standing rival to the Greenwich meridian as the prime meridian of the world.

John Harrison

H-4H4H4 and H5
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.
Longitude fixes the location of a place on Earth east or west of a north-south line called the prime meridian.

Hemispheres of Earth

hemispherehemisphereshemisphere of Earth
This great circle divides a spheroid, e.g., Earth, into two hemispheres.

Tenerife meridian

The Tenerife meridian was the prime meridian of choice for Dutch cartographers and navigators from the 1640s until the beginning of the 19th century.

United Kingdom

BritishUKBritain
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 Germany it was the Berlin meridian, in Denmark the Copenhagen meridian, and in United Kingdom the Greenwich meridian.
The Royal Greenwich Observatory in London was chosen as the defining point of the Prime Meridian in Washington in 1884, though it no longer is.

Meridian (geography)

meridianmeridiansmeridian line
A prime meridian is a meridian (a line of longitude) in a geographic coordinate system at which longitude is defined to be 0°.

Ferro Meridian

The line of longitude running through El Hierro (Ferro), the westernmost of the Canary Islands, was known in European history as the prime meridian in common use outside of the future British Empire.

Florence meridian

Meridian 11°15' East
The Meridian 11°15' East was proposed as prime meridian by Arno Peters in the Peters World Map.

Fortunate Isles

Fortunate IslandsIsles of the BlessedIsle of the Blessed
Ptolemy used as his basis the "Fortunate Isles", a group of islands in the Atlantic, which are usually associated with the Canary Islands (13° to 18°W), although his maps correspond more closely to the Cape Verde islands (22° to 25° W).
Ptolemy used these islands as the reference for the measurement of geographical longitude and they continued to play the role of defining the prime meridian through the Middle Ages.

United Kingdom Ordnance Survey Zero Meridian

Ordnance Survey Zero Meridian
The United Kingdom Ordnance Survey Zero Meridian is the prime meridian used by the Ordnance Survey (OSGB36 datum).

Meridian of Antwerp

Antwerp meridian
The meridian of Antwerp is one of several prime meridians that have been used for geographic referencing.

Royal Observatory, Greenwich

Royal Greenwich ObservatoryRoyal ObservatoryGreenwich Observatory
Between 1765 and 1811, Nevil Maskelyne published 49 issues of the Nautical Almanac based on the meridian of the Royal Observatory, Greenwich.
The basis of longitude, the meridian that passes through the Airy transit circle, first used in 1851, was adopted as the world's Prime Meridian at the International Meridian Conference on 22 October 1884 (voting took place on 13 October).

El Hierro

HierroHierro IslandCanary Islands
In 1634, Cardinal Richelieu used the westernmost island of the Canaries, Ferro, 19° 55' west of Paris, as the choice of meridian.
El Hierro was used in various parts of Europe for more than 500 years as the prime meridian, especially outside of the future British Empire.

Greenwich

Greenwich, LondonGreenwich, EnglandEast Greenwich
Ultimately it was because the palace and its grounds were a royal possession that it was chosen as the site for Charles II's Royal Observatory, from which stemmed Greenwich's subsequent global role as originator of the modern Prime Meridian.