A prime meridian is a meridian (a line of longitude) in a geographic coordinate system at which longitude is defined to be 0°. Together, a prime meridian and its anti-meridian (the 180th meridian in a 360°-system) form a great circle. This great circle divides a spheroid, e.g., Earth, into two hemispheres. 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.
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. The most widely used modern meridian is the IERS Reference Meridian. It is derived but deviates slightly from the Greenwich Meridian, which was selected as an international standard in 1884.
The notion of longitude was developed by the Greek Eratosthenes (c. 276 BC – c. 195 BC) in Alexandria, and Hipparchus (c. 190 BC – c. 120 BC) in Rhodes, and applied to a large number of cities by the geographer Strabo (64/63 BC – c. 24 AD). But it was Ptolemy (c. AD 90 – c. AD 168) who first used 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° to 18°W), although his maps correspond more closely to the Cape Verde islands (22° to 25° W). 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. 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.
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. Christopher Columbus reported (1493) 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 Diogo Ribeiro's 1529 map. São Miguel Island (25.5°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 deviation line did not follow a line of longitude.
In 1541, Mercator produced his famous 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, Ferro, 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. But it was 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, that enabled navigators to use the lunar method of determining longitude more accurately using the octant developed by Thomas Godfrey and John Hadley. 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.
|Locality||Modern longitude||Meridian name||Comment|
|Bering Strait||168°30′ W||Offered in 1884 as possibility for a "neutral prime meridian" by Pierre Janssen at the International Meridian Conference |
|Washington, D.C.||77°03′56.07″ W (1897) or 77°04′02.24″ W (NAD 27) or 77°04′01.16″ W (NAD 83)||New Naval Observatory meridian|
|Washington, D.C.||77°02′48.0″ W, 77°03′02.3″, 77°03′06.119″ W or 77°03′06.276″ W (both presumably NAD 27). If NAD27, the latter would be 77°03′05.194″ W (NAD 83)||Old Naval Observatory meridian|
|Washington, D.C.||77°02′11.56299″ W (NAD 83), 77°02′11.55811″ W (NAD 83), 77°02′11.58325″ W (NAD 83) (three different monuments originally intended to be on the White House meridian)||White House meridian|
|Washington, D.C.||77°00′32.6″ W (NAD 83)||Capitol meridian|
|Philadelphia||75° 10′ 12″ W|||
|Rio de Janeiro||43° 10′ 19″ W|||
|Fortunate Isles / Azores||25° 40′ 32″ W||Used until the Middle Ages, proposed as one possible neutral meridian by Pierre Janssen at the International Meridian Conference|
|El Hierro (Ferro),
|18° 03′ W,
later redefined as
17° 39′ 46″ W
|Tenerife||16° 38' 22" W||Tenerife meridian||Rose to prominence with Dutch cartographers and navigators after they abandoned the idea of a magnetic meridian|
|Cadiz||6° 17' 35.4" W||Cadiz meridian||Royal Observatory in southeast tower of Castillo de la Villa, used 1735–1850 by Spanish Navy.|
|Lisbon||9° 07′ 54.862″ W|||
|Madrid||3° 41′ 16.58″ W|||
|Greenwich||0° 00′ 05.3101″ W||Greenwich meridian||Airy Meridian|
|Greenwich||0° 00′ 05.33″ W||United Kingdom Ordnance Survey Zero Meridian||Bradley Meridian|
|Greenwich||0° 00′ 00.00″||IERS Reference Meridian|
|Paris||2° 20′ 14.025″ E||Paris meridian|
|Brussels||4° 22′ 4.71″ E|||
|Antwerp||4° 24′ E||Antwerp meridian|
|Amsterdam||4° 53′ E||through the Westerkerk in Amsterdam; used to define the legal time in the Netherlands from 1909 to 1937|
|Bern||7° 26′ 22.5″ E|
|Pisa||10° 24′ E|||
|Oslo (Kristiania)||10° 43′ 22.5″ E|||
|Florence||11°15′ E||Florence meridian||used in the Peters projection, 180° from a meridian running through the Bering Strait|
|Rome||12° 27′ 08.4″ E||meridian of Monte Mario||Used in Roma 40 Datum |
|Copenhagen||12° 34′ 32.25″ E||Rundetårn|
|Naples||14° 15′ E|||
|Pressburg||17° 06′ 03″ E||Meridianus Posoniensis||Used by Sámuel Mikoviny|
|Buda||19° 03′ 37″ E||meridianu(s) Budense||Used between 1469–1495; introduced by Regiomontanus, used by Marcin Bylica, Galeotto Marzio, Miklós Erdélyi (1423–1473), Johannes Tolhopff (c. 1445–1503), Johannes Muntz. Set in the royal castle (and observatory) of Buda.|
|Stockholm||18° 03′ 29.8″ E||at the Stockholm Observatory|
|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.|
|Warsaw||21° 00′ 42″ E||Warsaw meridian|||
|Várad||21° 55′ 16″ E||Tabulae Varadienses|| 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 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.|
|Alexandria||29° 53′ E||Meridian of Alexandria||The meridian of Ptolemy's Almagest.|
|Saint Petersburg||30° 19′ 42.09″ E||Pulkovo meridian|
|Great Pyramid of Giza||31° 08′ 03.69″ E||1884 |
|Jerusalem||35° 13′ 47.1″ E|||
|Mecca||39° 49′ 34″ E||see also Mecca Time |
|Ujjain||75° 47′ E||Used from 4th century CE Indian astronomy and calendars(see also Time in India).|
|Kyoto||136° 14′ E||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.|
|~ 180||Opposite of Greenwich, proposed 13 October 1884 on the International Meridian Conference by Sandford Fleming |
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.[note 1] The modern prime meridian, the IERS Reference Meridian, is placed very near this meridian and is the prime meridian that currently has the widest use.
The position of the Greenwich Meridian has been defined by the location of the Airy Transit Circle ever since the first observation was taken with it by Sir George Airy in 1851. 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 0.15 seconds of time. It was Airy's transit circle that was adopted in principle (with French delegates, who pressed for adoption of the Paris meridian abstaining) 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.
Beginning in 1973 the International Time Bureau and later the International Earth Rotation and Reference Systems Service changed from reliance on optical instruments like the Airy Transit Circle to techniques such as lunar laser ranging, satellite laser ranging, and very-long-baseline interferometry. The new techniques resulted in the IERS Reference Meridian, the plane of which passes through the centre of mass of the Earth. This differs from the plane established by the Airy transit, which is affected by vertical deflection (the local vertical is affected by influences such as nearby mountains). The change from relying on the local vertical to using a meridian based on the centre of the Earth caused the modern prime meridian to be 5.3" east of the astronomic Greenwich prime meridian through the Airy Transit Circle. At the latitude of Greenwich, this amounts to 102 metres. This was officially accepted by the Bureau International de l'Heure (BIH) in 1984 via its BTS84 (BIH Terrestrial System) that later became WGS84 (World Geodetic System 1984) and the various ITRFs (International Terrestrial Reference Systems).
Due to the movement of Earth's tectonic plates, the line of 0° longitude along the surface of the Earth has slowly moved toward the west from this shifted position by a few centimetres; that is, towards the Airy Transit Circle (or the Airy Transit Circle has moved toward the east, depending on your point of view) since 1984 (or the 1960s). With the introduction of satellite technology, it became possible to create a more accurate and detailed global map. With these advances there also arose the necessity to define a reference meridian that, whilst being derived from the Airy Transit Circle, would also take into account the effects of plate movement and variations in the way that the Earth was spinning. As a result, the International Reference Meridian was established and is commonly used to denote Earth's prime meridian (0° longitude) by the International Earth Rotation and Reference Systems Service, which defines and maintains the link between longitude and time. Based on observations to satellites and celestial compact radio sources (quasars) from various coordinated stations around the globe, Airy's transit circle drifts northeast about 2.5 centimetres per year relative to this Earth-centred 0° longitude.
It is also the reference meridian of the Global Positioning System operated by the United States Department of Defense, and of WGS84 and its two formal versions, the ideal International Terrestrial Reference System (ITRS) and its realization, the International Terrestrial Reference Frame (ITRF). A current convention on the Earth uses the opposite of the IRM as the basis for the International Date Line.
|Country, territory or sea||Notes|
|United Kingdom||From Tunstall in East Riding to Peacehaven, passing through Greenwich|
|France||From Villers-sur-Mer to Gavarnie|
|Spain||From Cilindro de Marboré to Castellón de la Plana|
|Mediterranean Sea||Gulf of Valencia|
|Spain||From El Verger to Calpe|
|Algeria||From Stidia to Algeria-Mali border near Bordj Mokhtar|
|Togo||For about 600 m|
|Ghana||For about 16 km|
|Togo||For about 39 km|
|Ghana||From the Togo-Ghana border near Bunkpurugu to Tema|
Passing through Lake Volta at
|Atlantic Ocean||Passing through the Equator at ("Null Island")|
|Antarctica||Queen Maud Land, claimed by Norway|
|Antarctica||Amundsen-Scott station, South Pole|
As on the Earth, prime meridians must be arbitrarily defined. Often a landmark such as a crater is used; other times a prime meridian is defined by reference to another celestial object, or by magnetic fields. The prime meridians of the following planetographic systems have been defined:
The 45×90 points are the four points on earth which are halfway between the geographical poles, the equator, the Prime Meridian, and the 180th meridian.Airy-0
Airy-0 is a crater on Mars whose location defined the position of the prime meridian of that planet. It is about 0.5 kilometres (0.31 mi) across and lies within the larger crater Airy in the region Sinus Meridiani. The IAU Working Group on Cartographic Coordinates and Rotational Elements has now recommended setting the longitude of the Viking 1 lander (47°.95137 west) as the standard. This deﬁnition maintains the position of the center of Airy-0 at 0° longitude, within the tolerance of current cartographic uncertainties.Merton Davies tied this crater into an extensive geodetic control network of the planet Mars based on Mariner 9 and earlier photographs. The Mariner 9 Geodesy/Cartography Group proposed that the prime meridian of Mars be defined by the center of Airy-0, which was selected by Harold Masursky, Gerard de Vaucouleurs, and Merton Davies at a Group meeting on 14 August 1972.It was named in honor of the British Astronomer Royal Sir George Biddell Airy (1801-1892), who in 1850 built the transit circle telescope at Greenwich. The location of that telescope was subsequently chosen to define the location of Earth's prime meridian.Airy (Martian crater)
Airy is an impact crater on Mars, named in honor of the British Astronomer, Royal Sir George Biddell Airy (1801–1892). The crater is approximately 43 kilometres (27 mi) in diameter and is located at 0.1°E 5.1°S in the Meridiani Planum region. The much smaller crater Airy-0, which defines the location of Mars' prime meridian, lies within it.Beer (Martian crater)
Beer is a crater lying situated within the Margaritifer Sinus quadrangle (MC-19) region of the planet Mars, named in honor of the German astronomer, Wilhelm Beer. It is located at 14.4°S 351.8°E .
Beer and collaborator Johann Heinrich Mädler produced the first reasonably good maps of Mars in the early 1830s. When doing so, they selected a particular feature for the prime meridian of their charts. Their choice was strengthened when Giovanni Schiaparelli used the same location in 1877 for his more famous maps of Mars. The feature was later called Sinus Meridiani ("Middle Bay"), but following the landing of the NASA probe MER-B Opportunity in 2004 it is perhaps better known as Meridiani Planum. Currently the Martian prime meridian is the crater Airy-0.
Beer lies in the southwest of Meridiani Planum, about 8° from the prime meridian and about 10° west from the crater Mädler. Schiaparelli is also in the region.Dante (crater)
Dante is a lunar impact crater that is located on the far side of the Moon. It lies in the northern hemisphere exactly opposite the prime meridian facing the Earth. The nearest craters of note are Larmor to the north and Morse to the southeast. To the southwest is the oddly shaped Buys-Ballot.
This crater is overlain by part of the ray system radiating from Larmor Q to the northwest. The rim of Dante is circular but somewhat eroded. The fresh crater Dante G is attached to the exterior along the east-southeastern rim. The interior floor of this crater is uneven and marked by several small impacts.
The crater lies within the Freundlich-Sharonov Basin.Davies (crater)
Davies is a crater on Mars located at 46°N 0°E on the fringe of Acidalia Planitia near Arabia Terra. It is approximately 48 km in diameter. The crater's name was formally approved by the IAU in 2006.It was named in honor of Merton Davies (1917-2001), a pioneer in the cartography of planetary bodies. An employee of the RAND Corporation, he assisted NASA in mapping Mars with colleagues Gérard de Vaucouleurs and Harold Masursky and defined the prime meridian of Mars as passing through the crater Airy-0. Davies Crater lies on the prime meridian, appropriate because Davies was responsible for its delineation.Decimal degrees
Decimal degrees (DD) express latitude and longitude geographic coordinates as decimal fractions and are used in many geographic information systems (GIS), web mapping applications such as OpenStreetMap, and GPS devices. Decimal degrees are an alternative to using degrees, minutes, and seconds (DMS). As with latitude and longitude, the values are bounded by ±90° and ±180° respectively.
Positive latitudes are north of the equator, negative latitudes are south of the equator. Positive longitudes are east of Prime meridian, negative longitudes are west of the Prime Meridian. Latitude and longitude are usually expressed in that sequence, latitude before longitude.Eastern Hemisphere
The Eastern Hemisphere is a geographical term for the half of Earth which is east of the prime meridian (which crosses Greenwich, London, UK) and west of the antimeridian (which crosses the Pacific Ocean and relatively little land from pole to pole). It is also used to refer to Afro-Eurasia (Africa and Eurasia) and Australia, in contrast with the Western Hemisphere, which includes mainly North and South America. The Eastern Hemisphere may also be called the "Oriental Hemisphere". In addition, it may be used in a cultural or geopolitical sense as a synonym for the "Old World".Hemispheres of Earth
In geography and cartography, the hemispheres of Earth refer to any division of the globe into two hemispheres (from Ancient Greek ἡμισφαίριον hēmisphairion, meaning "half of a sphere").
The most common such divisions are by latitudinal or longitudinal markers:
Northern Hemisphere, the half that lies north of the Equator
Southern Hemisphere, the half that lies south of the Equator
Eastern Hemisphere, the half that lies east of the prime meridian and west of the 180th meridian
Western Hemisphere, the half that lies west of the prime meridian and east of the 180th meridianThe East–West division can also be seen in a cultural sense, as a division into two cultural hemispheres.
However, other schemes have sought to divide the planet in a way that maximizes the preponderance of one geographic feature or another in each division:
Land–Water Land Hemisphere, the hemisphere on Earth containing the largest possible area of land
Water Hemisphere, the hemisphere on Earth containing the largest area of water
The Earth may also be split into hemispheres of day and night by the terrestrial terminator.Hun Kal (crater)
Hun Kal is a small crater on Mercury that serves as the reference point for the planet's system of longitude. The longitude of Hun Kal's center is defined as being 20° W, thus establishing the planet's prime meridian.
Hun Kal was chosen as a reference point since the actual prime meridian was in shadow when Mariner 10 photographed the region, hiding any features near 0° longitude from view.
Hun Kal is about 1.5 km in diameter.The name "Hun Kal" means '20' in the language of the Maya.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). It passes about 5.3 arcseconds east of George Biddell Airy's 1851 transit circle or 102 metres (335 ft) at the latitude of the Royal Observatory, Greenwich. It is also the reference meridian of the Global Positioning System (GPS) operated by the United States Department of Defense, and of WGS84 and its two formal versions, the ideal International Terrestrial Reference System (ITRS) and its realization, the International Terrestrial Reference Frame (ITRF).Longitude
Longitude (, AU and UK also ), is a geographic coordinate that specifies the east–west position of a point on the Earth's surface, or the surface of a celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek letter lambda (λ). Meridians (lines running from pole to pole) connect points with the same longitude. By convention, one of these, the Prime Meridian, which passes through the Royal Observatory, Greenwich, England, was allocated the position of 0° longitude. The longitude of other places is measured as the angle east or west from the Prime Meridian, ranging from 0° at the Prime Meridian to +180° eastward and −180° westward. Specifically, it is the angle between a plane through the Prime Meridian and a plane through both poles and the location in question. (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 the 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 approximately the angle between the local vertical and the equatorial plane.
If the Earth were perfectly spherical and radially homogeneous, then the longitude at a point would be equal to 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 radially homogeneous and has rugged terrain, which affect 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 the astronomical longitude, calculated 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.Meridian (geography)
A (geographic) meridian (or line of longitude) is the half of an imaginary great circle on the Earth's surface, terminated by the North Pole and the South Pole, connecting points of equal longitude, as measured in angular degrees east or west of the Prime Meridian. The position of a point along the meridian is given by that longitude and its latitude, measured in angular degrees north or south of the Equator. Each meridian is perpendicular to all circles of latitude. Each is also the same length, being half of a great circle on the Earth's surface and therefore measuring 20,003.93 km (12,429.9 miles).Mädler (Martian crater)
Mädler is a crater on Mars named in honor of the German astronomer Johann Heinrich Mädler. It is located at 2.7°E 10.7°S.
Mädler and collaborator Wilhelm Beer produced the first reasonably good maps of Mars in the early 1830s. When doing so, they selected a particular feature for the prime meridian of their charts. Their choice was strengthened when Giovanni Schiaparelli used the same location in 1877 for his more famous maps of Mars. The feature was later called Sinus Meridiani ("Middle Bay" or "Bay of the Meridian"), but following the landing of the NASA probe MER-B Opportunity in 2004 is perhaps better known as Meridiani Planum.
Mädler lies in the south of Meridiani Planum, close to the prime meridian and about 10° east of Beer. Schiaparelli is also in the region.Null Island
Null Island is a name for the area around the point where the prime meridian and the equator cross, located in the Gulf of Guinea (Atlantic Ocean) off the west African coast. In the WGS84 datum, this is at zero degrees latitude and longitude (0°N 0°E), and is the location of a buoy. The name 'Null Island' serves as both a joke based around the suppositional existence of an island there, and also as a name to which coordinates erroneously set to 0,0 are assigned in placenames databases in order to more easily find and fix them. The nearest land is Annobón island, part of Equatorial Guinea, at 1°25′S 5°38′E.Prime meridian (Greenwich)
A prime meridian, based at the Royal Observatory, Greenwich, in London, England, was established by Sir George Airy in 1851. By 1884, over two-thirds of all ships and tonnage used it as the reference meridian on their charts and maps. In October of that year, at the behest of US President Chester A. Arthur, 41 delegates from 25 nations met in Washington, D.C., United States, for the International Meridian Conference. This conference selected the meridian passing through Greenwich as the official prime meridian due to its popularity. However, France abstained from the vote, and French maps continued to use the Paris meridian for several decades. In the 18th century, London lexicographer Malachy Postlethwayt published his African maps showing the "Meridian of London" intersecting the Equator a few degrees west of the later meridian and Accra, Ghana.The plane of the prime meridian is parallel to the local gravity vector at the Airy transit circle (51°28′40.1″N 0°0′5.3″W) of the Greenwich observatory. The prime meridian was therefore long symbolised by a brass strip in the courtyard, now replaced by stainless steel, and since 16 December 1999, it has been marked by a powerful green laser shining north across the London night sky.
Global Positioning System (GPS) receivers show that the marking strip for the prime meridian at Greenwich is not exactly at zero degrees, zero minutes, and zero seconds but at approximately 5.3 seconds of arc to the west of the meridian (meaning that the meridian appears to be 102.478 metres east). In the past, this offset has been attributed to the establishment of reference meridians for space-based location systems such as WGS 84 (which GPS relies on) or that errors gradually crept into the International Time Bureau timekeeping process. The actual reason for the discrepancy is that the difference between precise GNSS coordinates and astronomically determined coordinates everywhere remains a localized gravity effect due to the deflection of the vertical; thus, no systematic rotation of global longitudes occurred between the former astronomical system and the current geodetic system.Royal Borough of Greenwich
The Royal Borough of Greenwich ( (listen), , or ) is a London borough in south-east London, England. Taking its name from the historic town of Greenwich, the London Borough of Greenwich was formed in 1965 by the amalgamation of the former area of the Metropolitan Borough of Greenwich with part of the Metropolitan Borough of Woolwich to the east. The local council is Greenwich London Borough Council which meets in Woolwich Town Hall. The council's offices are also based in Woolwich, the main urban centre in the borough.
Greenwich is world-famous as the traditional location of the Prime Meridian, on which all Coordinated Universal Time is based. The Prime Meridian running through Greenwich and the Greenwich Observatory is where the designation Greenwich Mean Time, or GMT began, and on which all world times are based. In 2012, Greenwich was listed as a top ten global destination by Frommer's – the only UK destination to be listed.
Greenwich was one of six host boroughs for the 2012 London Olympics and events were held at the Royal Artillery Barracks (shooting), Greenwich Park (equestrianism) and The O2 – the former Millennium Dome (gymnastics and basketball). It is also the home borough of professional football club Charlton Athletic.
To mark the Diamond Jubilee of Elizabeth II, Greenwich became a Royal Borough on 3 February 2012, due in part to its historic links with the Royal Family, and to its UNESCO World Heritage Site status as home of the Prime Meridian.Selenographic coordinates
Selenographic coordinates are used to refer to locations on the surface of Earth's moon. Any position on the lunar surface can be referenced by specifying two numerical values, which are comparable to the latitude and longitude of Earth. The longitude gives the position east or west of the Moon's prime meridian, which is the line passing from the lunar north pole through the point on the lunar surface directly facing Earth to the lunar south pole. (See also Earth's prime meridian.) This can be thought of as the midpoint of the visible Moon as seen from the Earth. The latitude gives the position north or south of the lunar equator. Both of these coordinates are given in degrees.
Astronomers defined the fundamental location in the selenographic coordinate system by the small, bowl-shaped satellite crater 'Mösting A'. The coordinates of this crater are defined as:
The coordinate system has become precisely defined due to the Lunar Laser Ranging Experiment.
Anything past 90°E or 90°W would not be seen from Earth, except for libration, which makes 59% of the Moon visible.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. The other half is called the Eastern Hemisphere.
|Time in physics|
|Archaeology and geology|
|Other units of time|