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.[1][2][3] 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).

Line across the Earth
Prime Meridian
Equator and Prime Meridian
Nations that touch the Equator (red) and the Prime Meridian (blue)


The reason for the 5.3 arcsecond offset between the IERS Reference Meridian and the Airy transit circle is that the observations with the transit circle were based on the local vertical, while the IERS Reference is a geodetic longitude, that is, the plane of the meridian contains the center of mass of the Earth.[1]

The International Hydrographic Organization adopted an early version of the IRM in 1983 for all nautical charts.[4] The IRM was adopted for air navigation by the International Civil Aviation Organization on 3 March 1989.[5] Tectonic plates slowly move over the surface of Earth, so most countries have adopted for their maps an IRM version fixed relative to their own tectonic plate as it existed at the beginning of a specific year. Examples include the North American Datum 1983 (NAD83), the European Terrestrial Reference Frame 1989 (ETRF89), and the Geocentric Datum of Australia 1994 (GDA94). Versions fixed to a tectonic plate differ from the global version by at most a few centimetres.

However, the IRM is not fixed to any point on Earth. Instead, all points on the European portion of the Eurasian plate, including the Royal Observatory, are slowly moving northeast about 2.5 cm per year relative to it. Thus this IRM is the weighted average (in the least squares sense) of the reference meridians of the hundreds of ground stations contributing to the IERS network. The network includes GPS stations, Satellite Laser Ranging (SLR) stations, Lunar Laser Ranging (LLR) stations, and the highly accurate Very Long Baseline Interferometry (VLBI) stations.[6] All stations' coordinates are adjusted annually to remove net rotation relative to the major tectonic plates. If Earth had only two hemispherical plates moving relative to each other around any axis which intersects their centres or their junction, then the longitudes (around any other rotation axis) of any two, diametrically opposite, stations must move in opposite directions by the same amount. The 180th meridian is opposite the IERS Reference Meridian and forms a great circle with it dividing the earth into Western Hemisphere and Eastern Hemisphere.

Universal Time is notionally based on the WGS84 meridian. Because of changes in the rate of Earth's rotation, standard international time UTC can differ from the mean observed solar time at noon on the prime meridian by up to 0.9 second. Leap seconds are inserted periodically to keep UTC close to Earth's angular position relative to the Sun; see mean solar time.

List of places

Starting at the North Pole and heading south to the South Pole, the IERS Reference Meridian passes through 8 countries:

Country, territory or sea Notes
90°0′N 0°0′E / 90.000°N 0.000°E Arctic Ocean
81°39′N 0°0′E / 81.650°N 0.000°E Greenland Sea
72°53′N 0°0′E / 72.883°N 0.000°E Norwegian Sea
61°0′N 0°0′E / 61.000°N 0.000°E North Sea
53°45′N 0°0′E / 53.750°N 0.000°E  United Kingdom From Tunstall in East Riding to Peacehaven, passing through Greenwich
50°47′N 0°0′E / 50.783°N 0.000°E English Channel
49°19′N 0°0′E / 49.317°N 0.000°E  France From Villers-sur-Mer to Gavarnie
42°41′N 0°0′E / 42.683°N 0.000°E  Spain From Cilindro de Marboré to Castellón de la Plana
39°56′N 0°0′E / 39.933°N 0.000°E Mediterranean Sea Gulf of Valencia
38°52′N 0°0′E / 38.867°N 0.000°E  Spain From El Verger to Calpe
38°38′N 0°0′E / 38.633°N 0.000°E Mediterranean Sea
35°50′N 0°0′E / 35.833°N 0.000°E  Algeria From Stidia to Algeria-Mali border near Bordj Mokhtar
21°50′N 0°0′E / 21.833°N 0.000°E  Mali
14°59′N 0°0′E / 14.983°N 0.000°E  Burkina Faso
11°6′N 0°0′E / 11.100°N 0.000°E  Togo For about 600 m
11°6′N 0°0′E / 11.100°N 0.000°E  Ghana For about 16 km
10°57′N 0°0′E / 10.950°N 0.000°E  Togo For about 39 km
10°36′N 0°0′E / 10.600°N 0.000°E  Ghana From the Togo-Ghana border near Bunkpurugu to Tema
Passing through Lake Volta at 7°48′N 0°0′E / 7.800°N 0.000°E
5°37′N 0°0′E / 5.617°N 0.000°E Atlantic Ocean Passing through the Equator at 0°0′N 0°0′E / 0.000°N 0.000°E ("Null Island")
60°0′S 0°0′E / 60.000°S 0.000°E Southern Ocean
68°54′S 0°0′E / 68.900°S 0.000°E Antarctica Queen Maud Land, claimed by  Norway
90°0′S 0°0′E / 90.000°S 0.000°E Antarctica Amundsen-Scott station, South Pole

See also


  1. ^ a b Malys, Stephen; Seago, John H.; Palvis, Nikolaos K.; Seidelmann, P. Kenneth; Kaplan, George H. (1 August 2015). "Why the Greenwich meridian moved". Journal of Geodesy. 89 (12): 1263–1272. doi:10.1007/s00190-015-0844-y.
  2. ^ IRM on grounds of Royal Observatory from Google Earth Accessed 30 March 2012
  3. ^ The astronomic latitude of the Royal Observatory is 51°28'38"N whereas its latitude on the European Terrestrial Reference Frame (1989) datum is 51°28'40.1247"N.
  4. ^ "A manual on the technical aspects of the United Nations Convention on the Law of the Sea – 1982" (PDF). (4.89 MB) Section 2.4.4.
  5. ^ WGS 84 Implementation Manual page i, 1998
  6. ^ "IERS Conventions (2003): Conventional Terrestrial Reference System and Frame" (PDF). (419 KB)
Abraj Al Bait

The Abraj Al-Bait (Arabic: ابراج البيت‎ "Towers of the House (of God, i.e. the Kaaba)") is a government-owned megatall complex of seven skyscraper hotels in Mecca, Saudi Arabia. These towers are a part of the King Abdulaziz Endowment Project that strives to modernize the city in catering to its pilgrims. The central hotel tower, the Makkah Royal Clock Tower, A Fairmont Hotel, has the world's largest clock face and is the third-tallest building and fifth-tallest freestanding structure in the world.

The building complex is metres away from the world's largest mosque and Islam's most sacred site, the Great Mosque of Mecca. The developer and contractor of the complex is the Saudi Binladin Group, the Kingdom's largest construction company. It is the world's most expensive building with the total cost of construction equaling US$15 billion. The complex was built after the demolition of the Ajyad Fortress, the 18th-century Ottoman citadel on top of a hill overlooking the Grand Mosque. The destruction of the historically significant site in 2002 by the Saudi government sparked international outcry and a strong response from Turkey.

Coordinated Universal Time

Coordinated Universal Time (abbreviated to UTC) is the primary time standard by which the world regulates clocks and time. It is within about 1 second of mean solar time at 0° longitude, and is not adjusted for daylight saving time. In some countries where English is spoken, the term Greenwich Mean Time (GMT) is often used as a synonym for UTC and predates UTC by nearly 300 years.The first Coordinated Universal Time was informally adopted on 1 January 1960 and was first officially adopted as CCIR Recommendation 374, Standard-Frequency and Time-Signal Emissions, in 1963, but the official abbreviation of UTC and the official English name of Coordinated Universal Time (along with the French equivalent) were not adopted until 1967.The system has been adjusted several times, including a brief period where time coordination radio signals broadcast both UTC and "Stepped Atomic Time (SAT)" before a new UTC was adopted in 1970 and implemented in 1972. This change also adopted leap seconds to simplify future adjustments. This CCIR Recommendation 460 "stated that (a) carrier frequencies and time intervals should be maintained constant and should correspond to the definition of the SI second; (b) step adjustments, when necessary, should be exactly 1 s to maintain approximate agreement with Universal Time (UT); and (c) standard signals should contain information on the difference between UTC and UT."A number of proposals have been made to replace UTC with a new system that would eliminate leap seconds. A decision whether to remove them altogether has been deferred until 2023.The current version of UTC is defined by International Telecommunications Union Recommendation (ITU-R TF.460-6), Standard-frequency and time-signal emissions, and is based on International Atomic Time (TAI) with leap seconds added at irregular intervals to compensate for the slowing of the Earth's rotation. Leap seconds are inserted as necessary to keep UTC within 0.9 seconds of the UT1 variant of universal time. See the "Current number of leap seconds" section for the number of leap seconds inserted to date.

Earth orientation parameters

In geodesy, earth orientation parameters (EOP) are a collection of parameters that describe irregularities in the rotation of the Earth.

The Earth's rotational velocity is not constant over time. Any motion of mass in or on the Earth causes a slowdown or speedup of the rotation speed, or a change of rotation axis. Small motions produce changes too small to be measured, but movements of very large mass, like sea currents or tides, can produce discernible changes in the rotation and can change very precise astronomical observations. Global simulations of atmosphere, ocean, and land dynamics are used to create effective angular momentum (EAM) functions that can be used to predict changes in EOP.The collection of earth orientation parameters is fitted to describe the observed rotation irregularities. Technically, they provide the rotational transform from the International Terrestrial Reference System (ITRS) to the International Celestial Reference System (ICRS), or vice versa, as a function of time.

East India DLR station

East India is a station on Docklands Light Railway (DLR) in Blackwall area of Poplar in London, England. The station is situated in the East End of London that takes its name from the nearby former East India Docks of the Port of London, where ships trading with the Indian subcontinent used to dock.

It is on the Beckton and Woolwich Arsenal branches of the DLR, and is in Travelcard Zones 2 and 3. It opened, with the Beckton Branch, on 28 March 1994.

The historic Greenwich Prime Meridian crosses the DLR at the eastern end of the platforms, which is marked by an illuminated blue line underneath the tracks at street level. The modern IERS Reference Meridian used by GPS crosses the tracks approximately 117 metres further east between Neutron Tower and Switch House, but is unmarked.


An equator of a rotating spheroid (such as a planet) is its zeroth circle of latitude (parallel). It is the imaginary line on the spheroid, equidistant from its poles, dividing it into northern and southern hemispheres. In other words, it is the intersection of the spheroid with the plane perpendicular to its axis of rotation and midway between its geographical poles.

On Earth, the Equator is about 40,075 km (24,901 mi) long, of which 78.8% lies across water and 21.3% over land. Indonesia is the country straddling the greatest length of the equatorial line across both land and sea.


IRM may refer to:

IERS Reference Meridian

Illinois Railway Museum

Illinois Reserve Militia, state defense force active during World War II

Bureau of Information Resource Management, responsible for information technology of the U.S. Department of State

Information Rights Management

L'Institut Royal Météorologique de Belgique

Interference reflection microscopy, a microscopy method used to image adherent cells

Internal Revenue Manual, an official compendium of internal guidelines for personnel of the United States Internal Revenue Service

International Review of Mission, a quarterly academic journal

International Roaming MIN assigned by IFAST

IRM (album), by Charlotte Gainsbourg

Iron Mountain Incorporated, a data storage company based in Boston, Massachusetts

ICAO designator for Mahan Air, an Iranian airline

International Earth Rotation and Reference Systems Service

The International Earth Rotation and Reference Systems Service (IERS), formerly the International Earth Rotation Service, is the body responsible for maintaining global time and reference frame standards, notably through its Earth Orientation Parameter (EOP) and International Celestial Reference System (ICRS) groups.

International Meridian

May refer to

any of the Prime Meridians that have been used, are used, or are proposed

Greenwich Meridian established by the International Meridian Conference

IERS Reference Meridian

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°. 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.

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.

Sidereal time

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".

Viewed from the same location, a star seen at one position in the sky will be seen at the same position on another night at the same sidereal time. This is similar to how the time kept by a sundial can be used to find the location of the Sun. Just as the Sun and Moon appear to rise in the east and set in the west due to the rotation of Earth, so do the stars. Both solar time and sidereal time make use of the regularity of Earth's rotation about its polar axis, solar time following the Sun while sidereal time roughly follows the stars.

More exactly, sidereal time is the angle, measured along the celestial equator, from the observer's meridian to the great circle that passes through the March equinox and both celestial poles, and is usually expressed in hours, minutes, and seconds. Common time on a typical clock measures a slightly longer cycle, accounting not only for Earth's axial rotation but also for Earth's orbit around the Sun.

A sidereal day is approximately 23 hours, 56 minutes, 4.0905 SI seconds (24 hours - 4 minutes + 4 seconds). The March equinox itself precesses slowly westward relative to the fixed stars, completing one revolution in about 26,000 years, so the misnamed sidereal day ("sidereal" is derived from the Latin sidus meaning "star") is 0.0084 seconds shorter than the stellar day, Earth's period of rotation relative to the fixed stars.

The slightly longer "true" sidereal period is measured as the Earth Rotation Angle (ERA), formerly the stellar angle. An increase of 360° in the ERA is a full rotation of the Earth.

Because Earth orbits the Sun once a year, the sidereal time at any given place and time will gain about four minutes against local civil time, every 24 hours, until, after a year has passed, one additional sidereal "day" has elapsed compared to the number of solar days that have gone by.

Spherical coordinate system

In mathematics, a spherical coordinate system is a coordinate system for three-dimensional space where the position of a point is specified by three numbers: the radial distance of that point from a fixed origin, its polar angle measured from a fixed zenith direction, and the azimuth angle of its orthogonal projection on a reference plane that passes through the origin and is orthogonal to the zenith, measured from a fixed reference direction on that plane. It can be seen as the three-dimensional version of the polar coordinate system.

The radial distance is also called the radius or radial coordinate. The polar angle may be called colatitude, zenith angle, normal angle, or inclination angle.

The use of symbols and the order of the coordinates differs between sources. In one system frequently encountered in physics (r, θ, φ) gives the radial distance, polar angle, and azimuthal angle, whereas in another system used in many mathematics books (r, θ, φ) gives the radial distance, azimuthal angle, and polar angle. In both systems ρ is often used instead of r. Other conventions are also used, so great care needs to be taken to check which one is being used.

A number of different spherical coordinate systems following other conventions are used outside mathematics. In a geographical coordinate system positions are measured in latitude, longitude and height or altitude. There are a number of different celestial coordinate systems based on different fundamental planes and with different terms for the various coordinates. The spherical coordinate systems used in mathematics normally use radians rather than degrees and measure the azimuthal angle counterclockwise from the x-axis to the y-axis rather than clockwise from north (0°) to east (+90°) like the horizontal coordinate system. The polar angle is often replaced by the elevation angle measured from the reference plane. Elevation angle of zero is at the horizon.

The spherical coordinate system generalizes the two-dimensional polar coordinate system. It can also be extended to higher-dimensional spaces and is then referred to as a hyperspherical coordinate system.

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. It lies at 16° 38′ 22″ W of the current international prime meridian, the IERS Reference Meridian through Greenwich.

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.

World Geodetic System

The World Geodetic System (WGS) is a standard for use in cartography, geodesy, and satellite navigation including GPS. This standard includes the definition of the coordinate system's fundamental and derived constants, the ellipsoidal (normal) Earth Gravitational Model (EGM), a description of the associated World Magnetic Model (WMM), and a current list of local datum transformations.The latest revision is WGS 84 (also known as WGS 1984, EPSG:4326), established in 1984 and last revised in 2004. Earlier schemes included WGS 72, WGS 66, and WGS 60. WGS 84 is the reference coordinate system used by the Global Positioning System.

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