HOME WEB NEWS IMAGES CLASSIFIEDS YELLOW PAGESPOLLS - SURVEYS WIKI COUNTRIES PHOTOS US UK INDIA
Cities Stocks Weather Movies Coupons Travel Blogs Health Horoscope Maps ASIA EUROPE Classifieds Yellow Pages
 web site, your cookies, cookie central, expiration date, web server | ||
|
new york, resource page, uml profile, agile modeling, this site, this page | ||
|
Spices, Indian Food, Spice Blends herbs, tandoori, Curry | ||
 1
|
Location of the Prime Meridian
The Prime Meridian is the meridian (line of longitude) at which longitude is defined to be 0°.
Unlike the parallels of latitude, which are defined by the rotational axis of the Earth (the poles being 90° and the Equator 0°), the Prime Meridian is arbitrary. By international convention, the modern Prime Meridian is one passing through Greenwich, London, United Kingdom, known as the International Meridian or Greenwich Meridian. Historically, various meridians have been used, including four different ones through Greenwich.
Heading south from the North Pole, the Prime Meridian first passes through the United Kingdom (the most northerly land on the meridian is the shore (53°45′34″N) southeast of the Sand-le-Mere caravan park east of Kingston upon Hull, England). It then passes through France, Spain, Algeria, Mali, Burkina Faso, Togo, and Ghana, and then through Queen Maud Land to the South Pole. The Prime Meridian and the opposite 180th meridian (at 180° longitude), which the International Date Line generally follows, form a great circle that divides the Earth into the Eastern and Western Hemispheres.
Contents |
The Royal Observatory, Greenwich (today a museum)
The Prime Meridian is ultimately arbitrary — a matter of convention — and various conventions have been used or advocated throughout history:
Prime Meridian in Greenwich
The Greenwich Meridian, based at the Royal Observatory, Greenwich, was established by Sir George Airy in 1851. By 1884, over two-thirds of all ships and tonnage used it as the reference meridian on their maps. In October of that year, at the behest of U.S. President Chester A. Arthur, 41 delegates from 25 nations met in Washington, D.C., USA, for the International Meridian Conference. This conference selected the Greenwich Meridian as the official Prime Meridian due to its popularity. However, France abstained from the vote and French maps continued to use the Paris Meridian for several decades.
WGS84 longitudes, which are used by satellite navigation systems, differ slightly from traditional longitudes. The WGS84 zero meridian is 102.5 metres (336.3 feet) to the east of the line marked at Greenwich.History of the Prime Meridian -Past and Present. The offset at other locations can be as much as 30″ east or west.
WGS84 uses the zero meridian as defined by the Bureau International de l\'Heure,European Organisation for the Safety of Air Navigation and IfEN: "WGS 84 Implementation Manual", page 13. 1998 which was defined by compilation of star observations in different countries. The plane of this geodetic meridian passes through the centre of the Earth, unlike the plane of the astronomical meridian which contains the direction of gravity (indicated by a plumb line) which points opposite to the direction of the zenith, to which astronomical instruments are aligned. The angle between these two meridian planes at the Royal Observatory, the east-west component of vertical deflection, is 5.31”. The WGS84 datum is an average of the various continental drifts. As a result, the astronomical meridian between the vertical crosshairs of Airy\'s transit telescope drifts toward the east as it is carried by the European portion of the Eurasian tectonic plate, closer to the geodetic meridian, by about one centimetre per year.
Curiously, whether by accident or design, the location of the WGS84 0° meridian is marked in Greenwich by the presence of a waste basket on the path leading more or less due east from the observatory containing the transit telescope.
The zero meridian used by the Ordnance Survey (OSGB36 datum) is about six metres to the west of the line marked at Greenwich. This was the standard meridian before 1851, and the Ordnance Survey simply continued to use it.
Universal Time is notionally based on the WGS84 meridian. However, the standard international time UTC can differ from the mean observed time on the meridian by up to about one second (equivalent to about 280 metres at Greenwich), because of changes in the Earth\'s rotation. Leap seconds are inserted periodically to keep UTC in sync with the Earth.
The Greenwich Meridian is now marked at night by a laser beam emitted from the observatory.[1]
The prime meridians of the following bodies in the Solar System have been defined:
This article is licensed under the GNU Free Documentation License. It uses material from Wikipedia
