Prime meridian (Greenwich)

The Prime Meridian is the one meridian ( perpendicular to the Earth's equator, one standing and North to South Pole running semicircle ) from which is counted from the longitude east and west.

Its definition is arbitrary in itself, but was defined by international agreement (the International Meridian Conference of 1884 ) in the meridian plane of the London Observatory Greenwich and is therefore often referred to as Greenwich Meridian ( meridian of Flamsteed House at the Royal Greenwich Observatory). Until then, zero different meridians were in use.

By default, the longitude is counted positive to the east of the prime meridian (0 ° to 360 ° ), but is instead more common east longitude ( 0-180 ° east) and west longitude ( 0-180 ° W ). As abbreviations O or E for " Eastern " and W are used for the "West". The symbol E ( English for " East ", französ. " Est " ) is partially also in the German standard to prevent confusion with zero. On the western hemisphere (especially in the U.S.) and Western counting from 0-360 ° is common, contrary to the international standard.

Today's standard, the zero meridian of WGS84 and ETRS89/GRS80-Bezugsystems is not bound to the surface, but a modeled geodetic datum, because the meridians in a more accurate account of non-surface - fixed are ( continental drift, tidal forces ). It runs in Greenwich Park, about 102 meters east of the historic Greenwich "tourist line " of the old Observatory Greenwich.

Introduction

The length of the prime meridian is the measured half times around the world over the poles of the international earth ellipsoid, ie 20003.9 km. With its opposite meridian cuts, for example, at 180 ° Wrangel (without additive E or W ), the zero meridian supplemented to form a great circle on the earth. The International Date Line runs partly (near the poles and north of the equator) exactly to 180 ° Greenwich ( deviations or exceptions to the 180th degree are: in the Bering Strait, in the Aleutian Islands (Alaska), in Kiribati, in the Fiji Islands, in Tuvalu, Tonga and in New Zealand at the Kermadec Islands and Chatham ).

Before committing to an international prime meridian in 1884, almost every European country had its own prime meridian, usually the longitude of the respective capital or of an astronomical observatory.

With the increasing international travel - especially by the railroad - but a unification of the existing systems was necessary. A large-scale rail transport required for the smooth and safe operation schedule a single time, rather than the sufficiently precise local time, which was different in almost every city. In addition, it has become increasingly important, an exact time to have international ( world time ) is available. It is defined as the local time of the prime meridian.

The International Meridian Conference, Washington 1884

English original protocols, see International Meridian Conference (Protocol )

At the International Meridian Conference on October 14, 1884 in Washington, DC with representatives from 25 nations of the line passing through Greenwich Meridian was introduced as the basis of the international system of coordinates.

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

• The Paris meridian of the Observatory of Paris: 2 ° 20 ' 14.025 "east of Greenwich
• Of since ancient times known Ferro meridian on the Canary island of Ferro ( now called El Hierro ) on day 17 ° 40 '00 "West
• A possible prime meridian in the Azores at about 28 ° 0 ' W
• A possible prime meridian in the Pacific Ocean today 180 ° ( reverse curve to the Greenwich meridian is roughly equivalent to the current date line)
• Of the then- contemporary charts used mostly Greenwich Meridian (which frequent use was decisive for his choice).

In the course of the conference it became clear very soon that the Paris Zero Meridian would not find a majority. The old Ferro meridian was regarded for its several decades previously carried down to exactly 20 ° W of Paris as " French submarine". The Azores and the Bering Strait dropped out mainly because they had no observatory and were also telegraph at that time not connected to the rest of the world.

So finally won the Greenwich meridian as zero meridian International by a large majority - with the abstention of France - by.

History of the Greenwich Meridian

The Greenwich prime meridian passes through eight states on today's land area and has in each country following length:

• United Kingdom ( 319 km )
• France ( 735 km )
• Spain (336 miles)
• Algeria ( 1555 km )
• Mali (760 miles)
• Burkina Faso (430 miles)
• Togo ( 39 km)
• Ghana ( 569 km )
• And the stateless Antarctic Neuschwabenland and further south the Queen Maud Land. The length of the prime meridian on the mainland of the continent Antarctica is 2331 km.

In addition, the zero meridian passes through the following waters:

• Arctic Ocean ( 3217 km - from 90 ° North to 61 ° North )
• North Sea ( 977 km )
• Mediterranean (424 miles)
• Lake Volta in Ghana (78 miles)
• Atlantic Ocean, including the Southern Ocean ( 8278 km )

Prime meridian and time measurement

The mean solar time at the prime meridian was decisive for the World Time (GMT, Greenwich Mean Time), which was replaced in 1972 by the Coordinated Universal Time (UTC). This is based no longer on a local, but - as the wording - coordinates the continuous atomic time with the astronomically measured, the irregularities of the earth's rotation and the positions of the sun reflecting Universal Time. The adjustment is made on leap seconds. At the prime meridian, the difference between UTC and local mean time today is not precisely zero.

Historical concepts of the reference line of the Erdmessnetze

• First division of the world into lengths and widths by Hipparchus of Nicaea ( 190-120 BC), Rhodes ( his astronomical observation )
• Claudius Ptolemy moved him to 150 at the western boundary 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 used until the 20th century.
• Arab astronomers placed the prime meridian first through the western tip of Africa, in 1075 then 10 ° west of Baghdad.
• It was after repeated attempts a transfer, for example, as in 1427 the Azores and America in 1492 were discovered.
• In April 1634 a scholarly conference of all maritime nations, the island Ferro was confirmed.
• As of 1718 in France was the meridian of Paris, from 1738 to the meridian of Greenwich was applied in England.
• In Germany the meridian of Greenwich was adopted in 1885, in France until about 1900. Austria - Hungary used it until 1918, parallel with the old Ferro meridian.
• Since the 1980s, is the prime meridian - and 0 ° latitude, ie the equator, as well as geographical north and south poles - no longer erdoberflächenfest, but aligned to a reference ellipsoid of the earth's shape, thus avoiding effects of continental drift and the tidal forces.