Prime meridian
A prime meridian is a meridian in a geographic coordinate system at which longitude is defined to be 0°. On a spheroid, a prime meridian and its anti-meridian form a great ellipse. This divides the body into two hemispheres: the Eastern Hemisphere and the Western Hemisphere. Unlike the equator, which also divides a spherical celestial body into two hemispheres, the prime meridian is astronomically arbitrary. For each of Earth's historic prime meridians, various conventions have been used or advocated in different regions throughout history, but none have a basis in physical geography. Earth's current international standard prime meridian is the IERS Reference Meridian. It is derived, but differs slightly, from the Greenwich Meridian, the previous standard.
Longitudes for the Earth and Moon are measured from their prime meridian to 180° east and 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 prograde, meaning that its direction of rotation is the same as that of its orbit. East longitudes are used if the rotation is retrograde.
History
The notion of longitude for Greeks was developed by the Greek Eratosthenes in Alexandria, and Hipparchus in Rhodes, and applied to a large number of cities by the geographer Strabo. Ptolemy was the first geographer to use 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.
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 in Central India, and Rohitaka, the ancient name for Rohtak, a city north.
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. In 1493, 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 the copies of Spain's Padron Real made by Diogo Ribeiro in 1527 and 1529. 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 declination line did not follow a line of longitude.
In 1541, Mercator produced his 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, 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.
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. 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, 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 Prussia 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.
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. It was adopted for air navigation by the International Civil Aviation Organization on 3 March 1989.
International prime meridian
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 very close to each other.Prime meridian at Greenwich
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 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. 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 8 countries, 4 seas, 3 oceans and 1 channel:Prime meridian on other celestial 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 Lancashire, England.
- In 1975 the prime meridian of Mercury was defined to be 20° east of the crater Hun Kal. This meridian was chosen because it runs through the point on Mercury's equator where the average temperature is highest.
- Defined in 1992, the prime meridian of Venus passes through the central peak in the crater Ariadne, chosen arbitrarily.
- 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°W.
- The prime meridian on Ceres runs through the Kait crater, which was arbitrarily chosen because it is near the equator.
- The prime meridian on 4 Vesta is 4 degrees east of the crater Claudia, chosen because it is sharply defined.
- 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.
- *Europa's prime meridian is defined such that the crater Cilix is at 182° W. The 0° longitude runs through the middle of the face that is always turned towards Jupiter.
- *Io's prime meridian, like that of Earth's moon, is defined so that it runs through the middle of the face that is always turned towards Jupiter.
- *Ganymede's prime meridian is defined such that the crater Anat is at 128° W, and the 0° longitude runs through the middle of the subjovian hemisphere.
- *Callisto's prime meridian is defined such that the crater Saga is at 326° W.
- Titan is the largest moon of Saturn and, like the Earth's moon, is tidally locked and always has the same face towards Saturn. 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.
- 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.
List of historic prime meridians on Earth
| Locality | Modern longitude | Meridian name | Image | 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°0356.07 W or 77°0402.24 W or 77°0401.16 W | New Naval Observatory meridian | ||
| Washington, D.C. | 77°0248.0 W, 77°0302.3, 77°0306.119 W or 77°0306.276 W. If NAD27, the latter would be 77°0305.194 W | Old Naval Observatory meridian | ||
| Washington, D.C. | 77°0211.56299 W, 77°0211.55811 W, 77°0211.58325 W | White House meridian | ||
| Washington, D.C. | 77°0032.6 W | Capitol meridian | ||
| Philadelphia | 75° 10 12 W | |||
| Rio de Janeiro | 43° 10 19 W | |||
| Azores | 25° 40 32 W | Proposed as one possible neutral meridian by Pierre Janssen at the International Meridian Conference | ||
| El Hierro (Ferro), Canary Islands | 18° 03 W, 17° 39 46 W | Ferro meridian | ||
| Tenerife | 16°3822 W | Tenerife meridian | Rose to prominence with Dutch cartographers and navigators after they abandoned the idea of a magnetic meridian | |
| Lisbon | 9° 07 54.862 W | |||
| Cádiz | 6° 17 35.4" W | Cádiz meridian | Royal Observatory in southeast tower of Castillo de la Villa, used 1735–1850 by Spanish Navy. | |
| 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 05.33 W | United Kingdom Ordnance Survey Zero Meridian | Bradley Meridian | |
| Greenwich | 0° 00 05.3101 W | Greenwich meridian | Airy 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 | ||
| Pisa | 10° 24 E | |||
| Oslo (Kristiania) | 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 | |
| Stockholm | 18° 03 29.8 E | At the Stockholm Observatory | ||
| Buda | 19° 03 37 E | Meridianu Budense | Used between 1469 and 1495; introduced by Regiomontanus, used by Marcin Bylica, Galeotto Marzio, Miklós Erdélyi, Johannes Tolhopff, Johannes Muntz. Set in the royal castle of Buda. | |
| 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 | ||
| Approx. 59° E | Maimonides calls this point אמצע היישוב, "the middle of the habitation", i.e. the habitable hemisphere. Evidently this was a convention accepted by Arab geographers of his day. | |||
| Ujjain | 75° 47 E | Used from 4th century CE Indian astronomy and calendars. | ||
| Beijing | 116° 24 E | Used in Qing dynasty for astronomical and cartographical purposes. | ||
| 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 |