# Prime Meridian

Prime meridian (black)
The historic prime meridian is marked on the floor and on the building of the Transit House in Greenwich.
A GPS receiver does not show exactly zero longitude over the prime meridian marked in Greenwich.
Northernmost point of the prime meridian on mainland Europe in Villers-sur-Mer , France

The prime meridian is that meridian (a semicircle perpendicular to the earth's equator and running from north to south pole ) from which the geographical longitude to east and west is counted.

Its determination is arbitrary. It was placed in the meridian plane of the London observatory Greenwich by international agreement during the International Meridian Conference in 1884 and is therefore often referred to as the Greenwich meridian (meridian of the passage instrument at the Royal Greenwich Observatory ). Until then, different zero meridians were in use.

By convention, the longitude from the prime meridian to the east (i.e. in the sense of the earth's rotation ) is counted as positive (0 ° to + 180 °) and negative to the west (0 ° to −180 °). But more common are eastern longitude (0–180 ° east, algebraically positive) and western longitude (0–180 ° west, algebraically negative). The abbreviations used are O or E for "east" and W for "west". The symbol E (English for "East", French "Est") is also used in some parts of German to avoid confusion with the number 0. In the western hemisphere (especially in the USA), contrary to the international norm, western counting of 0 ° –360 ° is also used.

## introduction

The length of the prime meridian is half the circumference of the earth's international ellipsoid , measured over the poles , i.e. 20,003.9 km. With its opposite so-called antimeridian, which intersects Wrangel Island at 180 ° (without the addition of E or W), the prime meridian complements itself to form a great circle on Earth. The date line runs partly (near the poles and north of the equator) exactly at 180 ° Greenwich (deviations or exceptions from the 180th degree of longitude are: in the Bering Strait , near the Aleutian Islands (Alaska), in Kiribati , near the Fiji Islands , at Tuvalu , near Tonga and the Kermadec Islands and Chatham Islands belonging to New Zealand ).

Before the establishment of an international prime meridian in 1884, almost every European country had its own prime meridian, usually the geographical longitude of the respective capital or its observatory. With increasing international travel - especially by rail - it became necessary to standardize the existing systems. A large-scale traffic plan required a uniform time for smooth and safe operation according to the timetable instead of the previously sufficiently accurate solar time , which differed from city to city. In addition, it became more and more important to have accurate international time ( world time ) available. It is defined as the mean local time of the prime meridian.

## The International Meridian Conference, Washington 1884

At the International Meridian Conference in Washington, DC with representatives from 25 nations on October 13, 1884, the meridian running through Greenwich was introduced as the basis of the international coordinate system.

As a possible international prime meridian, five main options were discussed at the Washington Conference:

• the Parisian meridian of the Paris Observatory : 2 ° 20 ′ 14.025 ″ east of Greenwich
• the Ferro meridian on the Canary Island Ferro (today's name: El Hierro ) at 17 ° 40 ′ west today, which has been known since ancient times
• a possible prime meridian in the Azores at about 28 ° 0 ′ west
• a possible prime meridian in the Pacific Ocean at 180 ° today (opposite arc to the Greenwich meridian, corresponds approximately to today's date line )
• the Greenwich meridian most commonly used on the modern nautical charts of that time (this frequent use was the decisive factor in its choice).

In the course of the conference it soon became apparent that the Paris Prime Meridian would not find a majority. The old Ferro-Meridian was regarded as a "French submarine" because it was fixed a few decades earlier at exactly 20 ° west of Paris. The Azores and the Bering Strait were ruled out mainly because they did not have an observatory and were not connected to the rest of the world by telegraph at that time.

So the Greenwich Meridian finally prevailed as the International Prime Meridian with a large majority - with France abstaining.

## Corrected position of the prime meridian

Visitors to the Greenwich Observatory are often astonished that their GPS receiver does not show exactly zero at the zero meridian marked there. It runs about 102 meters east of the historic meridian through Greenwich Park. The cause of this difference is a measurement error caused by local gravity fields ( gravitational anomalies ) at the time of the agreement made in 1884, which led to the incorrect determination of the astronomical reference points. These still valid reference points and the center of the earth lie in a (meridian) plane that cuts the earth's surface a little further to the east.

Apart from that: The currently valid prime meridian of the WGS84 and ETRS89 / GRS80 reference system is no longer firmly tied to the earth's surface, but a modeled geodetic datum , since meridians are not surface-fixed due to tidal forces , polar movement and continental drift .

## Course of the prime meridian

The Greenwich Prime Meridian crosses eight present-day states on land and has the following length in the individual countries:

The prime meridian also crosses the following waters:

## Prime meridian and time measurement

The mean solar time at the prime meridian became decisive for universal time ( GMT , Greenwich Mean Time ), which was only replaced by coordinated universal time (UTC) in 1972 . Today, this is no longer based on local time, but - as it is called - coordinates continuous atomic time with astronomically measured Universal Time, which reflects the irregularities of the earth's rotation and the positions of the sun . The adjustment takes place via leap seconds . At the prime meridian, the difference between mean local time and UTC is not precisely zero today.

## Historical concepts of the reference line of the earth measuring networks

• First division of the world into longitudes and latitudes by Hipparchus of Nikaia (190-120 BC): Rhodes (his astronomical observation site)
• Claudius Ptolemaeus moved it around 150 to the western border of the known world: Isla del Meridiano ( El Hierro or Ferro, the westernmost of the Canary Islands , the ancient Hesperides), and thus created the Ferro meridian, which was used well into the 20th century .
• Arab astronomers first laid the prime meridian through the western tip of Africa, then in 1075 10 ° west of Baghdad .
• Thereafter there were repeated attempts at relocation, e.g. B. When the Azores were discovered in 1427 and America in 1492.
• In April 1634 the island of Ferro was confirmed by a scholars' congress of all seafaring nations.
• The geographer and polymath Johann Gottfried Gregorii alias Melissantes proposed in 1708 the international unification of the prime meridian with the help of a multilateral political agreement.
• From 1718 the meridian of Paris was used in France , from 1738 the meridian of Greenwich was used in England.
• In the 19th century, the cartographer Philippe Vandermaelen used a prime meridian through Brussels, the seat of his publishing house, e.g. B. the first world atlas on a uniform scale Atlas universel de geographie physique, politique, statistique et mineralogique .
• The meridian of Rome running through the Torre des Meridiano on Monte Mario in Rome was used for the Italian military maps from 1870 to 1974. It runs through Rome as well as through the Vatican .
• In Germany the meridian was taken over from Greenwich in 1885, in France only around 1900. Austria-Hungary used it until 1918 in parallel with the old Ferro meridian.
• Since the 1980s, the prime meridian - and also the 0 °  latitude , i.e. the equator , as well as the geographic north and south poles - is no longer fixed on the surface of the earth , but aligned with a reference ellipsoid over the earth's shape, thus avoiding the effects of continental drift and tidal forces .

## Zero meridians of other celestial bodies

Commons : Prime Meridian  - collection of images, videos and audio files
Commons : International Meridian Conference  - Scans of the original English-language minutes
Wiktionary: Prime Meridian  - explanations of meanings, word origins, synonyms, translations

## Individual evidence

1. A. Schödlbauer: Geodetic Astronomy. de Gruyter 2000, p. 3: The geographical longitude L is the directional angle that the meridian plane of P forms with the meridian plane [...] of Greenwich. The counting of this angle begins as agreed at the reference meridian and is counted positively to the east . (Note: whether in the west negative or> 180 ° does not matter computationally)
2. ^ Motion (minutes of the meeting, page 98, below ) and vote (minutes of the meeting, page 99 ) on October 13, 1884
3. Spektrum.de: Why the prime meridian has shifted
4. See Stephen Malys, John H. Seago, Nikolaos K. Pavlis, P. Kenneth Seidelmann, George H. Kaplan: Why the Greenwich meridian moved. In: Journal of Geodesy 89 (2015), No. 8.doi: 10.1007 / s00190-015-0844-y .
5. Melissantes: Geographia novissima . 1st chapter. Frankfurt am Main and Leipzig 1708, pp. 38/39.
6. Gerald Sammet: The world of maps: Historical and modern cartography in dialogue (=  Atlantica: Earth experience ). 1st edition. Bertelsmann Lexikon Institut, Gütersloh 2008, ISBN 978-3-577-07251-9 , p. 259 ( limited preview in Google Book Search [accessed August 2, 2018]).
7. Observation and Practice . In: Günter D. Roth (Ed.): Handbook for Star Friends . tape 2 , p. 61 ( limited preview in Google Book Search [accessed August 5, 2018]).
8. See Gazetteer of Planetary Nomenclature , Planetary Names: Crater, craters: Kait on Ceres.

Coordinates: 0 ° WGS84 wayside cross in Greenwich Park Coordinates: 51 ° 28 ′ 39.6 ″  N , 0 ° 0 ′ 0 ″  E , Flamsteed House 0 ° 0 ′ 6 ″ W