The IERS Reference Meridian (IRM), also called the International Reference Meridian, is the
prime meridian
A prime meridian is an arbitrary 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 c ...
(0° longitude) maintained by the
International Earth Rotation and Reference Systems Service (IERS). It passes about 5.3
arcseconds east of
George Biddell Airy
Sir George Biddell Airy (; 27 July 18012 January 1892) 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 E ...
's 1851
transit circle
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 mo ...
which is at the
latitude
In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pol ...
of the
Royal Observatory, Greenwich
The Royal Observatory, Greenwich (ROG; known as the Old Royal Observatory from 1957 to 1998, when the working Royal Greenwich Observatory, RGO, temporarily moved south from Greenwich to Herstmonceux) is an observatory situated on a hill in ...
.
Thus it differs slightly from the historical
Greenwich meridian.
It is the reference meridian of the
Global Positioning System
The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite sy ...
(GPS) operated by the
United States Space Force
The United States Space Force (USSF) is the space service branch of the U.S. Armed Forces, one of the eight U.S. uniformed services, and the world's only independent space force. Along with its sister branch, the U.S. Air Force, the Space ...
, and of
WGS84
The World Geodetic System (WGS) is a standard used in cartography, geodesy, and satellite navigation including GPS. The current version, WGS 84, defines an Earth-centered, Earth-fixed coordinate system and a geodetic datum, and also describ ...
and its two formal versions, the ideal
International Terrestrial Reference System
The International Terrestrial Reference System (ITRS) describes procedures for creating reference frames suitable for use with measurements on or near the Earth's surface. This is done in much the same way that a physical standard might be descri ...
(ITRS) and its realization, the
International Terrestrial Reference Frame (ITRF).
Location
The most important 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
In astronomy, geography, and related sciences and contexts, a '' direction'' or '' plane'' passing by a given point is said to be vertical if it contains the local gravity direction at that point.
Conversely, a direction or plane is said to be ho ...
, while the IERS Reference is a
geodetic longitude, that is, the plane of the meridian contains the center of mass of the Earth.
[
The ]International Hydrographic Organization
The International Hydrographic Organization (IHO) is an intergovernmental organisation representing hydrography. , the IHO comprised 98 Member States.
A principal aim of the IHO is to ensure that the world's seas, oceans and navigable waters a ...
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
The International Civil Aviation Organization (ICAO, ) is a specialized agency of the United Nations that coordinates the principles and techniques of international air navigation, and fosters the planning and development of international a ...
on 3 March 1989. Tectonic plates
Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large ...
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.
The IERS system is not quite fixed to any point attached to the Earth. For example, all points on the European portion of the Eurasian plate, including the Royal Observatory, are moving northeast at about 2.5 cm per year relative to it. The 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
In satellite laser ranging (SLR) a global network of observation stations measures the round trip time of flight of ultrashort pulses of light to satellites equipped with retroreflectors. This provides instantaneous range measurements of milli ...
(SLR) stations, lunar laser ranging (LLR) stations, and the highly accurate very long baseline interferometry
Very-long-baseline interferometry (VLBI) is a type of astronomical interferometry used in radio astronomy. In VLBI a signal from an astronomical radio source, such as a quasar, is collected at multiple radio telescopes on Earth or in space. T ...
(VLBI) stations. 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 (the meridian
Meridian or a meridian line (from Latin ''meridies'' via Old French ''meridiane'', meaning “midday”) may refer to
Science
* Meridian (astronomy), imaginary circle in a plane perpendicular to the planes of the celestial equator and horizon
* ...
at 180° both east and west of the Prime Meridian
A prime meridian is an arbitrary 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 c ...
) is opposite the IERS Reference Meridian and forms a great circle
In mathematics, a great circle or orthodrome is the circular intersection of a sphere and a plane passing through the sphere's center point.
Any arc of a great circle is a geodesic of the sphere, so that great circles in spherical geomet ...
with it dividing the earth into Western Hemisphere
The Western Hemisphere is the half of the planet Earth that 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. Politically, the term We ...
and Eastern Hemisphere
The Eastern Hemisphere is the half of the planet Earth which is east of the prime meridian (which crosses Greenwich, London, United Kingdom) and west of the antimeridian (which crosses the Pacific Ocean and relatively little land from pole to pol ...
.
Universal Time
Universal Time (UT or UT1) is a time standard based on Earth's rotation. While originally it was mean solar time at 0° longitude, precise measurements of the Sun are difficult. Therefore, UT1 is computed from a measure of the Earth's angle wit ...
is notionally based on the prime 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 of a second. Leap second
A leap second is a one- second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time (International Atomic Time (TAI), as measured by atomic clocks) and imprecise observ ...
s are inserted periodically to keep UTC close to Earth's angular position relative to the Sun; see mean solar time
Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of solar time is the day, based on the synodic rotation period. Two types of solar time are apparent solar time (sundial ti ...
.
List of places
Starting at the North Pole
The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is the point in the Northern Hemisphere where the Earth's axis of rotation meets its surface. It is called the True North Pole to distinguish from the Mag ...
and heading south to the South Pole
The South Pole, also known as the Geographic South Pole, Terrestrial South Pole or 90th Parallel South, is one of the two points where Earth's axis of rotation intersects its surface. It is the southernmost point on Earth and lies antipod ...
, the IERS Reference Meridian passes through 8 countries:
See also
* 1st meridian east
The meridian 1° east of Greenwich is a line of longitude that extends from the North Pole across the Arctic Ocean, the Atlantic Ocean, Europe, Africa, the Southern Ocean, and Antarctica to the South Pole.
The 1st meridian east forms a great ci ...
* 1st meridian west
The meridian 1° west of Greenwich is a line of longitude that extends from the North Pole across the Arctic Ocean, the Atlantic Ocean, Europe, Africa, the Southern Ocean, and Antarctica to the South Pole.
The 1st meridian west forms a great c ...
* 180th meridian
* Coordinated Universal Time
Coordinated Universal Time or UTC is the primary time standard by which the world regulates clocks and time. It is within about one second of mean solar time (such as UT1) at 0° longitude (at the IERS Reference Meridian as the currently used ...
* Prime meridian
A prime meridian is an arbitrary 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 c ...
* Prime meridian (Greenwich)
The historic prime meridian or Greenwich meridian is a geographical reference line that passes through the Royal Observatory, Greenwich, in London, England. The modern IERS Reference Meridian widely used today is based on the Greenwich mer ...
References
Notes
Citations
{{geographical coordinates, state=collapsed
Named meridians
Prime meridians