Prime meridian

Ptolemy's 1st projection, redrawn under Maximus Planudes around 1300, using a prime meridian through the Canary Islands west of Africa, at the left-hand edge of the map. (The obvious central line shown here is the junction of two sheets).

The notion of longitude for Greeks was developed by the Greek Eratosthenes (c. 276 – 195 BCE) in Alexandria, and Hipparchus (c. 190 – 120 BCE) in Rhodes, and applied to a large number of cities by the geographer Strabo (64/63 BCE – c. 24 CE). But it was Ptolemy (c. 90 – 168 CE) 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.[1] At that time the chief method of determining longitude was by using the reported times of lunar eclipses in different countries.

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 (28°54′N 76°38′E), a city near the Kurukshetra.[4]

William Grigg's facsimile of the 1529 Spanish Padron Real, from the copy made by Diogo Ribeiro and held by the Vatican Library.

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.[lower-alpha 1] This is shown in the copies of Spain's Padron Real made by Diogo Ribeiro in 1527 and 1529. 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.[8]

1571 Africa map by Abraham Ortelius, with Cape Verde as its prime meridian.

1682 map of East Asia by Giacomo Cantelli, with Cape Verde as its prime meridian; Japan is thus located around 180° E.

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.[9] In 1634, Cardinal Richelieu used the westernmost island of the Canaries, El Hierro, 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.[10]

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.[11]

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."[12]

In 1884, at the International Meridian Conference in Washington, D.C., 22 countries voted to adopt the Greenwich meridian as the prime meridian of the world.[13] 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.

The current international standard Prime Meridian is the IERS Reference Meridian. The International Hydrographic Organization adopted an early version of the IRM in 1983 for all nautical charts.[14] It was adopted for air navigation by the International Civil Aviation Organization on 3 March 1989.[15]

Since 1984, the international standard for the Earth's prime meridian is the IERS Reference Meridian. Between 1884 and 1984, the meridian of Greenwich was the world standard. These meridians are physically very close to each other.

Prime meridian at Greenwich

The line of the Greenwich meridian at the Royal Observatory, Greenwich

Main article: Prime meridian (Greenwich)

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.[16][lower-alpha 2]

The position of the historic prime meridian, based at the Royal Observatory, Greenwich, was established by Sir George Airy in 1851. It was defined by the location of the Airy Transit Circle ever since the first observation he took with it.[18] 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.[19] 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.[20][21]

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.

List of places

On Earth, starting at the North Pole and heading south to the South Pole, the IERS Reference Meridian (as of 2016) passes through:

Co-ordinates (approximate)	Country, territory or sea	Notes
90°0′N 0°0′E	Arctic Ocean
85°46′N 0°0′E	Exclusive Economic Zone (EEZ) of Greenland (Denmark)
81°39′N 0°0′E	Greenland Sea
80°29′N 0°0′E	EEZ of Svalbard (Norway)
76°11′N 0°0′E	International waters
73°44′N 0°0′E	EEZ of Jan Mayen (Norway)
72°53′N 0°0′E	Norwegian Sea
69°7′N 0°0′E	International waters
64°42′N 0°0′E	EEZ of Norway
63°29′N 0°0′E	EEZ of Great Britain
61°0′N 0°0′E	North Sea
53°46′N 0°0′E	United Kingdom	From Tunstall in East Riding to Peacehaven, passing through Greenwich
50°47′N 0°0′E	English Channel	EEZ of Great Britain
50°14′N 0°0′E	English Channel	EEZ of France
49°20′N 0°0′E	France	From Villers-sur-Mer to Gavarnie
42°41′N 0°0′E	Spain	From Cilindro de Marboré to Castellón de la Plana
39°56′N 0°0′E	Mediterranean Sea	Gulf of Valencia; EEZ of Spain
38°52′N 0°0′E	Spain	From El Verger to Calp
38°38′N 0°0′E	Mediterranean Sea	EEZ of Spain
37°1′N 0°0′E	Mediterranean Sea	EEZ of Algeria
35°50′N 0°0′E	Algeria	From Stidia to Algeria-Mali border near Bordj Badji Mokhtar
21°52′N 0°0′E	Mali	Passing through Gao
15°00′N 0°0′E	Burkina Faso
11°7′N 0°0′E	Togo	For about 600 m
11°6′N 0°0′E	Ghana	For about 16 km
10°58′N 0°0′E	Togo	For about 39 km
10°37′N 0°0′E	Ghana	From the Togo-Ghana border near Bunkpurugu to Tema Passing through Lake Volta at 7°46′N 0°0′E
5°37′N 0°0′E	Atlantic Ocean	EEZ of Ghana
1°58′N 0°0′E		International waters
0°0′N 0°0′E		Passing through the Equator (see Null Island)
51°43′S 0°0′E		EEZ of Bouvet Island (Norway)
57°13′S 0°0′E		International waters
60°0′S 0°0′E	Southern Ocean	International waters
69°36′S 0°0′E	Antarctica	Queen Maud Land, claimed by  Norway
90°0′S 0°0′E	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:

• 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 the English astronomer Richard Christopher Carrington started his observations of sunspots.[26] The second is the Stonyhurst heliographic coordinates system, originated at Stonyhurst Observatory.
• In 1975 the prime meridian of Mercury was defined[27][28] to be 20° east of the crater Hun Kal.[29]
• Defined[30] in 1992, the prime meridian of Venus passes through the central peak in the crater Ariadne.[31]
• 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 was established in 1971[32] and passes through the center of the crater Airy-0, although it is fixed by the longitude of the Viking 1 lander, which is defined to be 47.95137°W.[33]
• 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.[34] It is unknown whether Jupiter has any internal solid surface that would enable a more Earth-like coordinate system. System I and System II coordinates are based on atmospheric rotation, and System III coordinates use Jupiter's magnetic field. The prime meridians of Jupiter's four Galilean moons were established in 1979.[35]
• Titan is the largest moon of Saturn and, like the Earth's moon, always has the same face towards Saturn, and so the middle of that face is 0 longitude.
• Like Jupiter, Neptune is a gas giant, so any surface is obscured by clouds. The prime meridian of its largest moon, Triton, was established in 1991.[36]
• Pluto's prime meridian is defined as the meridian passing through the center of the face that is always towards Charon, its largest moon, as the two are tidally locked to each other. Charon's prime meridian is similarly defined as the meridian always facing directly toward Pluto.

• 1st meridian east – Line of longitude
• 1st meridian west – Line of longitude
• 180th meridian – Line of longitude
• Null Island – Cartographical (only) marker at 0°N, 0°E
• Geographical center of Earth – Geometric centre of all land surfaces on EarthPages displaying short descriptions of redirect targets

Wikimedia Commons has media related to Prime meridian.

• "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