A prime meridian is the 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 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 for a body not tidally locked (or at least not synchronous) is ultimately arbitrary, unlike an equator, which is determined by the axis of rotation. For celestial objects that are tidally locked (more specifically, synchronous), however, their prime meridians are determined by the face always inward of the orbit (a planet facing its star, or a moon facing its planet), just as equators are determined by rotation. For Earth's prime meridian, various conventions have been used or advocated in different regions 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.
Longitudes for the Earth and Moon are measured from their prime meridian at 0° to 180° east and to 180° 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 direct, that is, it follows the right hand rule. East longitudes are used if the rotation is retrograde.However, °E that are greater than 180 can be converted to °W by subtracting the value from 360. The same is true of °W greater than 180, converting to °E.
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, and Rohitaka, the ancient name for Rohtak (), a city near the Kurukshetra.
The notion of longitude for Greeks 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 (2,193 kilometers, 1,362 statute miles, or 1,184 nautical miles)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.
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.
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 Greenwichmeridian 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||Image||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)[ clarification needed ] 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|
|Kew||0° 00' 19.0″ W||Prime Meridian (prior to Greenwich)||Located at King George III's Kew Observatory|
|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|
|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 and 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 (Tabula Varadiensis or Tabulae directionum) 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.[ citation needed ]|
|~ 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.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 modern prime meridian, based at the Royal Observatory, Greenwich, was established by Sir George Airy in 1851.
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 2 seconds of longitude. 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.
On Earth, starting at the North Pole and heading south to the South Pole, the IERS Reference Meridian (as of 2016) passes through:
|Country, territory or sea||Notes|
|Exclusive Economic Zone (EEZ) of Greenland (Denmark)|
|EEZ of Svalbard (Norway)|
|EEZ of Jan Mayen (Norway)|
|EEZ of Norway|
|EEZ of Great Britain|
|United Kingdom||From Tunstall in East Riding to Peacehaven, passing through Greenwich|
|English Channel||EEZ of Great Britain|
|English Channel||EEZ of France|
|France||From Villers-sur-Mer to Gavarnie|
|Spain||From Cilindro de Marboré to Castellón de la Plana|
|Mediterranean Sea||Gulf of Valencia; EEZ of Spain|
|Spain||From El Verger to Calp|
|Mediterranean Sea||EEZ of Spain|
|Mediterranean Sea||EEZ of Algeria|
|Algeria||From Stidia to Algeria-Mali border near Bordj Badji Mokhtar|
|Mali||Passing through Gao|
|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||EEZ of Ghana|
|Passing through the Equator (see Null Island)|
|EEZ of Bouvet Island (Norway)|
|Southern Ocean||International waters|
|Antarctica||Queen Maud Land, claimed by Norway|
|Antarctica||Amundsen–Scott South Pole 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 range of longitudes shall extend from 0° to 360°. Thus, west longitudes are used when the rotation is direct, and east longitudes are used when the rotation is retrograde. ... The Earth, Sun, and Moon do not traditionally follow this definition. Their rotations are direct and longitudes run both east and west 180°, or positive to the east 360°.
|Wikimedia Commons has media related to Prime meridian .|
Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London, reckoned from midnight. At different times in the past, it has been calculated in different ways, including being calculated from noon; as a consequence, it cannot be used to specify a precise time unless a context is given.
Longitude, 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 connect points with the same longitude. The prime meridian, which passes near the Royal Observatory, Greenwich, England, is defined as 0° longitude by convention. Positive longitudes are east of the prime meridian, and negative ones are west.
A time standard is a specification for measuring time: either the rate at which time passes; or points in time; or both. In modern times, several time specifications have been officially recognized as standards, where formerly they were matters of custom and practice. An example of a kind of time standard can be a time scale, specifying a method for measuring divisions of time. A standard for civil time can specify both time intervals and time-of-day.
Universal Time (UT) is a time standard based on Earth's rotation. There are several versions of Universal Time, which differ by up to a few seconds. The most commonly used are Coordinated Universal Time (UTC) and UT1. All of these versions of UT, except for UTC, are based on Earth's rotation relative to distant celestial objects, but with a scaling factor and other adjustments to make them closer to solar time. UTC is based on International Atomic Time, with leap seconds added to keep it within 0.9 second of UT1.
The Royal Observatory, Greenwich is an observatory situated on a hill in Greenwich Park in east London, overlooking the River Thames to the north. It played a major role in the history of astronomy and navigation, and because the Prime Meridian passes through it, it gave its name to Greenwich Mean Time, the precursor to today's Coordinated Universal Time (UTC). The ROG has the IAU observatory code of 000, the first in the list. ROG, the National Maritime Museum, the Queen's House and Cutty Sark are collectively designated Royal Museums Greenwich.
A geographic coordinate system (GCS) is a coordinate system associated with positions on Earth. A GCS can give positions:
Sidereal time is a timekeeping system that astronomers use to locate celestial objects. Using sidereal time, it is possible to easily point a telescope to the proper coordinates in the night sky. Briefly, sidereal time is a "time scale that is based on Earth's rate of rotation measured relative to the fixed stars".
In astronomy and celestial navigation, the hour angle is one of the coordinates used in the equatorial coordinate system to give the direction of a point on the celestial sphere. The hour angle of a point is the angle between two planes: one containing Earth's axis and the zenith, and the other containing Earth's axis and the given point.
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. The conference was held at the request of U.S. President Chester A. Arthur. The subject to discuss was the choice of "a meridian to be employed as a common zero of longitude and standard of time reckoning throughout the world". It resulted in the recommendation of the Greenwich Meridian as the international standard for zero degrees longitude.
Sir George Biddell Airy was an English mathematician and astronomer, and the seventh Astronomer Royal from 1835 to 1881. His many achievements include work on planetary orbits, measuring the mean density of the Earth, a method of solution of two-dimensional problems in solid mechanics and, in his role as Astronomer Royal, establishing Greenwich as the location of the prime meridian. His reputation has been tarnished by allegations that, through his inaction, Britain lost the opportunity to discover the planet Neptune.
The prime meridian is a geographical reference line that passes through the Royal Observatory, Greenwich, in London, England. It was first established by Sir George Airy in 1851, and 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.
Airy-0 is a crater inside the larger Airy Crater on Mars, whose location defines the position of the prime meridian of that planet. It is about 0.5 km (0.3 mile) across and lies within 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 as the standard. This deﬁnition maintains the position of the center of Airy-0 at 0° longitude, within the tolerance of current cartographic uncertainties.
In geodesy, a reference ellipsoid is a mathematically defined surface that approximates the geoid, which is the truer, imperfect figure of the Earth, or other planetary body, as opposed to a perfect, smooth, and unaltered sphere, which factors in the undulations of the bodies' gravity due to variations in the composition and density of the interior, as well as the subsequent flattening caused by the centrifugal force from the rotation of these massive objects . Because of their relative simplicity, reference ellipsoids are used as a preferred surface on which geodetic network computations are performed and point coordinates such as latitude, longitude, and elevation are defined.
A (geographic) meridian is the half of an imaginary great circle on the Earth's surface, a coordinate line 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. Meridians are half of a great circle on the Earth's surface. The length of a meridian on a modern ellipsoid model of the earth has been estimated at 20,003.93 km.
The Eastern Hemisphere is a geographical term for the half of Earth which is east of the prime meridian and west of the antimeridian. It is also used to refer to Afro-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".
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. 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.
The meridian circle is an instrument for timing of the passage of stars across the local meridian, an event known as a culmination, while at the same time measuring their angular distance from the nadir. These are special purpose telescopes mounted so as to allow pointing only in the meridian, the great circle through the north point of the horizon, the north celestial pole, the zenith, the south point of the horizon, the south celestial pole, and the nadir. Meridian telescopes rely on the rotation of the sky to bring objects into their field of view and are mounted on a fixed, horizontal, east–west axis.
The 25th meridian of longitude west from Washington is a line of longitude approximately 102.05 degrees west of the Prime Meridian of Greenwich. In the United States of America, the meridian 25 degrees west of the Washington Meridian defines the eastern boundary of the State of Colorado, the western boundary of the State of Kansas, and the western boundary of the State of Nebraska south of the 41st parallel north.
The 32nd meridian of longitude west from Washington is a line of longitude approximately 109°02′48″ west of the Prime Meridian of Greenwich. In the United States of America, the meridian 32 degrees west of the Washington Meridian defines the western boundaries of the State of Colorado and the State of New Mexico and the eastern boundaries of the State of Utah and the State of Arizona.
The IERS Reference Meridian (IRM), also called the International Reference Meridian, is the prime meridian 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 which is 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).