) at which the longitude is defined to be 0°. (zero degrees)
.
0°), the Prime Meridian is arbitrary. By international convention, the modern Prime Meridian passes through the
.
Historically, various meridians have been used, including four different ones through Greenwich.
History
The Royal Observatory, Greenwich (today a museum)
Prime Meridian in Greenwich
The Prime Meridian is ultimately arbitrary — a matter of convention — and various conventions have been used or advocated throughout history:
- Rome (12° 27' 08.4” E), meridian of Mount Mario
- Stockholm (18° 3' 29.8” E), at the observatory
- Approximately 59° east of Greenwich
- Ujjain (75° 47' E), Used in Indian astronomy and calendars.
- Kyoto (135° 46' E?), Used in 18th and 19th century Japanese maps.
Laser projected from the Royal Observatory in Greenwich marking the Prime Meridian
The modern Greenwich Meridian, based at the
Royal Observatory, Greenwich, 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 maps. In October of that year, at the behest of
U.S. President Chester A. Arthur, 41 delegates from 25 nations met in
Washington, D.C., USA, for the
International Meridian Conference. This conference selected the Greenwich Meridian 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.
The Greenwich Meridian is now marked at night by a
laser beam emitted northwards from the observatory.
IERS Reference Meridian
A
GPS receiver at the Greenwich Meridian. This does not indicate a longitude of zero because the prime meridian defined by the WGS84 system is about 100 metres to the east.
The
International Earth Rotation and Reference Systems Service (IERS) maintains the IERS Reference Meridian (IRM), also called the International Reference Meridian, which is the reference meridian (prime meridian, 0° longitude) of the
Global Positioning System operated by the
United States Department of Defense, and is the reference meridian in
WGS84 and in its formal versions, the ideal
International Terrestrial Reference System (ITRS) and its realization, the
International Terrestrial Reference Frame (ITRF). It is 5.31″ east of Airy's
transit circle or 102.5 metres (336.3 feet) at the
latitude of the Royal Observatory, Greenwich. This shift is a legacy of the first satellite navigation system, the
Doppler based
TRANSIT system developed by the
Applied Physics Laboratory of
Johns Hopkins University in
Howard County, Maryland. This was the location of its first ground station, so the surveyed coordinates of that ground station in the
North American Datum 1927 (NAD27), a non-Earth centered ellipsoid, became its coordinates in an Earth-centered ellipsoid, such as the
World Geodetic System. This caused the coordinates of any other location on an Earth-centered ellipsoid to shift, especially those far away. When the antenna of a TRANSIT ground station was mounted directly above Airy's transit instrument in June 1969, its longitude on an Earth-centered ellipsoid was 5.64″ west of TRANSIT's reference meridian. Further improvement in gravitational models, such as the Earth Geopotential Model 1996 (
EGM96), and a dramatic increase in the number of ground stations from only four to over 500, as well as the use of time based GPS caused a small additional shift before it reached its present position.
The
International Hydrographic Organization adopted an early version in 1983 for all nautical charts. It was adopted for air navigation by the
International Civil Aviation Organization on 3 March 1989. Because
tectonic plates slowly move over the surface of the Earth, most countries have adopted a version of the IRM for their maps that is fixed relative to their own tectonic plate as it existed at the beginning of a specific year, such as 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. 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, including GPS stations,
Satellite Laser Ranging (SLR) stations,
Lunar Laser Ranging (LLR) stations, and the highly accurate
Very Long Baseline Interferometry (VLBI) stations. The coordinates of all of these stations are adjusted annually until there is no net rotation relative to the major tectonic plates. If Earth had only two hemispherical plates moving relative to each other around any axis from that intersecting their centres to one intersecting their junction, the longitudes (around any other rotation axis) of any two stations diametrically opposite each other must move in opposite directions by the same amount.
Universal Time is notionally based on the WGS84 meridian. However, the standard international time
UTC can differ from the mean observed time on the meridian by up to 0.9 second (equivalent to about 260 metres at Greenwich), because of changes in the Earth's rotation.
Leap seconds are inserted periodically to keep UTC close to Earth's angular position relative to the Sun —
mean solar time.
United Kingdom Ordnance Survey Zero Meridian
The zero meridian used by the
Ordnance Survey (
OSGB36 datum) is about six metres to the west of the Airy meridian marked at Greenwich. When the first Ordnance Survey map was published in 1801, the official Prime Meridian of Great Britain was the one established by the third
Astronomer Royal,
James Bradley. When Airy's new Prime Meridian ('new' by virtue of Sir George Airy's instrument being placed in a room next to that housing James Bradley's instrument) superseded it fifty years later in 1851, the Ordnance Survey simply continued to use Bradley's.
Other planetary bodies
Like on 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 prime meridian of the Moon lies directly in the middle of the face of the moon visible from Earth and passes near the crater Bruce.
- The prime meridian of Mars is defined by the crater Airy-0.
- The prime meridian of Venus passes through the central peak in the crater Ariadne.
- Two different heliographic coordinate systems are used on the Sun. The first is the Carrington heliographic coordinate system. In this system, the prime meridian passes through the center of the solar disk as seen from the Earth on 9 November 1853, which is when Richard Christopher Carrington started his observations of sunspots. The second is the Stonyhurst heliographic coordinates system.
- Jupiter has several coordinate systems because different latitudes rotate at different rates. Scientific Astronomer uses System II coordinates, based on the mean atmospheric rotation of the north and south equatorial belts. System II coordinates use Jupiter's magnetic field.
- Titan, like the earth's moon, always has the same face towards Saturn, and so that face is 0 longitude.
See also
