Prime meridian
A prime meridian is the meridian in a geographic coordinate system at which longitude is defined to be 0°. Together, a prime meridian and its anti-meridian form a great circle. This great circle divides a spheroid 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.
in his Atlas Cosmographicae uses a prime meridian somewhere close to 25°W, passing just to the west of Santa Maria Island in the Atlantic. His 180th meridian runs along the Strait of Anián
A prime meridian is ultimately arbitrary, unlike an equator, which is determined by the axis of rotation. For Earth's prime meridian, various conventions have been used or advocated in different regions 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.
Longitudes for the Earth and Moon are measured from their prime meridian at 0° to 180° east and to 180° west. For all other Solar System bodies, longitude is measured from 0° to 360°. West longitudes are used if the rotation of the body is direct, that is, it follows the right hand rule. East longitudes are used if the rotation is retrograde.
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 that time the chief method of determining longitude was by using the reported times of lunar eclipses in different countries.
's map of 1529; the original is in 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 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. 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 then it had been shown that the zero magnetic deviation line did not follow a line of longitude.
, with Cape Verde as its prime meridian.
, 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 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. In 1634, Cardinal Richelieu used the westernmost island of the Canaries, Ferro, 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.
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.
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."
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. 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
| Locality | Modern longitude | Meridian name | Comment |
| Bering Strait | 168°30' W | Offered in 1884 as possibility for a neutral prime meridian by Pierre Janssen at the International Meridian Conference | |
| Washington, D.C. | 77°03′56.07″ W or 77°04′02.24″ W or 77°04′01.16″ W | New Naval Observatory meridian | |
| Washington, D.C. | 77°02′48.0″ W, 77°03′02.3″, 77°03′06.119″ W or 77°03′06.276″ W. If NAD27, the latter would be 77°03′05.194″ W | Old Naval Observatory meridian | |
| Washington, D.C. | 77°02′11.56299″ W, 77°02′11.55811″ W, 77°02′11.58325″ W | White House meridian | |
| Washington, D.C. | 77°00′32.6″ W | Capitol meridian | |
| Philadelphia | 75° 10' 12″ W | ||
| Rio de Janeiro | 43° 10' 19″ W | ||
| Fortunate Isles / Azores | 25° 40' 32″ W | Used until the Middle Ages, proposed as one possible neutral meridian by Pierre Janssen at the International Meridian Conference | |
| El Hierro, Canary Islands | 18° 03' W, 17° 39' 46″ W | Ferro meridian | |
| Tenerife | 16° 38' 22" W | Tenerife meridian | Rose to prominence with Dutch cartographers and navigators after they abandoned the idea of a magnetic meridian |
| Cadiz | 6° 17' 35.4" W | Cadiz meridian | Royal Observatory in southeast tower of :es:Castillo de la Villa |Castillo de la Villa, used 1735–1850 by Spanish Navy. |
| Lisbon | 9° 07' 54.862″ W | ||
| Madrid | 3° 41' 16.58″ W | ||
| Kew | 0° 00' 19.0″ W | Prime Meridian | Located at King George III's Kew Observatory |
| Greenwich | 0° 00' 05.3101″ W | Greenwich meridian | Airy Meridian |
| Greenwich | 0° 00' 05.33″ W | United Kingdom Ordnance Survey Zero Meridian | Bradley Meridian |
| Greenwich | 0° 00' 00.00″ | IERS Reference Meridian | |
| Paris | 2° 20' 14.025″ E | Paris meridian | |
| Brussels | 4° 22' 4.71″ E | ||
| Antwerp | 4° 24' E | Antwerp meridian | |
| Amsterdam | 4° 53' E | through the Westerkerk in Amsterdam; used to define the legal time in the Netherlands from 1909 to 1937 | |
| Bern | 7° 26' 22.5″ E | ||
| Pisa | 10° 24' E | ||
| Oslo | 10° 43' 22.5″ E | ||
| Florence | 11°15' E | Florence meridian | used in the Peters projection, 180° from a meridian running through the Bering Strait |
| Rome | 12° 27' 08.4″ E | meridian of Monte Mario | Used in Roma 40 Datum |
| Copenhagen | 12° 34' 32.25″ E | Rundetårn | |
| Naples | 14° 15' E | ||
| Pressburg | 17° 06' 03″ E | Meridianus Posoniensis | Used by Sámuel Mikoviny |
| Buda | 19° 03' 37″ E | meridianu Budense | Used between 1469–1495; introduced by Regiomontanus, used by Marcin Bylica, :hu:Galeotto Marzio|Galeotto Marzio, Miklós Erdélyi, Johannes Tolhopff, Johannes Muntz. Set in the royal castle of Buda. |
| Stockholm | 18° 03' 29.8″ E | at the Stockholm Observatory | |
| Kraków | 19° 57' 21.43″ E | Kraków meridian | at the Old Kraków Observatory at the Śniadecki' College; mentioned also in Nicolaus Copernicus's work On the Revolutions of the Heavenly Spheres. |
| Warsaw | 21° 00' 42″ E | Warsaw meridian | |
| Várad | 21° 55' 16″ E | Tabulae Varadienses | Between 1464 and 1667, a prime meridian was set in the Fortress of Oradea by Georg von Peuerbach. In his logbook Columbus stated, he had one copy of Tabulae Varadienses on board to calculate the actual meridian based on the position of the Moon, in correlation to Várad. Amerigo Vespucci also recalled, how was he acquired the knowledge to calculate meridians by means of these tables. |
| Alexandria | 29° 53' E | Meridian of Alexandria | The meridian of Ptolemy's Almagest. |
| Saint Petersburg | 30° 19' 42.09″ E | Pulkovo meridian | |
| 31° 08' 03.69″ E | 1884 | ||
| Jerusalem | 35° 13' 47.1″ E | ||
| Mecca | 39° 49' 34″ E | see also Mecca Time | |
| Ujjain | 75° 47' E | Used from 4th century CE Indian astronomy and calendars. | |
| Kyoto | 136° 14' E | Used in 18th and 19th century Japanese maps. Exact place unknown, but in "Kairekisyo" in Nishigekkoutyou-town in Kyoto, then the capital. | |
| ~ 180 | Opposite of Greenwich, proposed 13 October 1884 on the International Meridian Conference by Sandford Fleming |
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. The modern prime meridian, the IERS Reference Meridian, is placed very near this meridian and is the prime meridian that currently has the widest use.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 2 seconds of longitude. It was Airy's transit circle that was adopted in principle as the Prime Meridian of the world at the 1884 International Meridian Conference.
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 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:
- 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. The second is the Stonyhurst heliographic coordinates system, originated at Stonyhurst Observatory.
- In 1975 the prime meridian of Mercury was defined to be 20° east of the crater Hun Kal.
- Defined in 1992, the prime meridian of Venus passes through the central peak in the crater Ariadne.
- 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 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 west.
- 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. 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.
- 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.
- 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.
- Pluto's prime meridian is defined as the center of the face that is always pointed towards Charon, its largest moon, as the two are tidally locked towards each other.
Citations