A prime meridian is a meridian, i.e., a line of longitude, at which longitude is defined
to be 0°. A prime meridian and its opposite in a 360°-system, the 180th meridian, form
a great circle. This great circle divides the sphere, e.g.,
the Earth, into two hemispheres. If one uses directions of East and West from a defined
prime meridian, then they can be called Eastern Hemisphere and 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.
History
The notion of longitude was developed by the Greek Eratosthenes in Alexandria and Hipparchus
in Rhodes and applied to a large number of cities by the geographer Strabo. But it was
Ptolemy 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, although his maps correspond more closely
to the Cape Verde islands. The main point is to be comfortably west of the western tip
of Africa as negative numbers were not yet in use. His prime meridian corresponds to
18° 40' west of Winchester today. At this 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 sixteenth century followed his lead. But there was still a hope that a "natural"
basis for a prime meridian existed. Christopher Columbus reported that the compass pointed
due north somewhere in mid-Atlantic and this fact was used in the important Tordesillas
Treaty 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 in the Azores
was still used for the same reason as late as 1594 by Christopher Saxton, although by
this time it had been shown that the zero deviation line did not follow a line of longitude.
In 1541, Mercator produced his famous forty-one centimetre terrestrial globe and drew his
prime meridian precisely through Fuertaventura 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 common today. This
practice was followed by navigators well into the eighteenth century. In 1634, Cardinal
Richelieu used the westernmost island of the Canaries, Ferro, 19° 55' west of Paris, as
the choice of meridian. Unfortunately, the geographer Delisle decided to round this off
to 20°, so that it simply became the meridian of Paris disguised.
In the early eighteenth century the battle was on to improve the determination of longitude
at sea, leading to the development of the 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 held 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.
List of prime meridians on Earth International prime meridian
In October 1884 the Greenwich Meridian was selected by delegates 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.
Prime meridian at Greenwich
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 0.15 seconds of time. It was Airy's transit circle that was adopted in principle
as the Prime Meridian of the world in 1884. 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. IERS Reference Meridian
Satellites changed the reference from the surface of the Earth to its center of mass
around which all satellites orbit regardless of surface irregularities. The first satellite
navigation system, TRANSIT, selected in the 1960s as its reference meridian on an Earth-centered
ellipsoid the longitude on the NAD27 ellipsoid of its development laboratory halfway between
Washington, D.C. and Baltimore, Maryland. These identical numeric longitudes at a location
remote from Greenwich caused 0° of longitude on an Earth-centered ellipsoid to be 5.3"
east of the astronomic Greenwich prime meridian through the Airy transit circle. At the latitude
of Greenwich, this amounts to 102.5 metres. This was officially accepted by the Bureau
International de l'Heure in 1984 via its BTS84 that later became WGS84 and the various ITRFs.
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 since 1984. 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 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 from various coordinated stations around the globe, Airy's transit circle drifts
northeast about 2.5 centimetres per year relative to this Earth-centered 0° longitude. Circa
1999 the international reference meridian passed 5.31 arcseconds east of Airy's meridian
or 102.5 metres at the latitude of the Royal Observatory, Greenwich, London. 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 and its realization, the International Terrestrial Reference Frame.
A current convention on the Earth uses the opposite of the IRM as the basis for the International
Date Line. List of places
On Earth, starting at the North Pole and heading south to the South Pole, the IERS Reference
Meridian passes through:
Prime meridian on other planetary bodies 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 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 its cloud tops—the only part of the planet
visible from space—rotate at different rates depending on latitude. It is unknown whether
Jupiter has any internal solid surface that would enable a more Earth-like coordinate
system. Scientific Astronomer uses System II coordinates, based on the mean atmospheric
rotation of the north and south Equatorial belts. System III coordinates use Jupiter's
magnetic field. Titan, like the Earth's moon, always has the
same face towards Saturn, and so the middle of that face is 0 longitude.
See also 1st meridian east
1st meridian west Maps by prime meridian Maps in Wikimedia Commons
by prime meridian Footnotes
References Burgess, Ebenezer [1860], "Translation of
the Surya-Siddhanta", Journal of the American Oriental Society 6, Google, p. 185
Dolan, Graham. "The Greenwich Meridian before the Airy Transit Circle". The Greenwich Meridian.
Dolan, Graham. "WGS84 and the Greenwich Meridian". The Greenwich Meridian.
Hooker, Brian, A multitude of prime meridians, retrieved 13 Jan 2013
Norgate, Jean and Martin, Prime meridian, retrieved 13 Jan 2013
Sobel, Dava; Andrewes, William J. H., The Illustrated Longitude, Fourth Estate, London
External links "Where the Earth's surface begins—and ends",
Popular Mechanics, December 1930 scanned TIFFs of the conference proceedings
Prime meridians in use in the 1880s, by country Canadian Prime Meridian