Prime meridian (Greenwich)

This article is about the prime meridian through Greenwich, London. For Earth's current international standard prime meridian, seeIERS Reference Meridian.For the general concept of prime meridians, seeprime meridian.

Tourists queuing to take pictures on the line of the historical prime meridian at theRoyal Observatory, Greenwich

TheGreenwich meridianis aprime meridian,a geographical reference line that passes through theRoyal Observatory,Greenwich,inLondon,England.From 1884 to 1974, the Greenwichmeridianwas the international standard prime meridian, used worldwide for timekeeping and navigation. The modern standard, theIERS Reference Meridian,is based on the Greenwich meridian, but differs slightly from it.This prime meridian (at the time, one ofmany) was first established bySir George Airyin 1851, and by 1884, over two-thirds of all ships andtonnageused it as the referencemeridianon theirchartsand maps. In October of that year, at the invitation of thePresident of the United States,41 delegates from 25 nations met inWashington, D.C.,United States, for theInternational Meridian Conference.This conference selected the meridian passing through Greenwich as the world standard prime meridian due to its popularity.However, France abstained from the vote, and French maps continued to use theParis meridianfor several decades. In the 18th century, London lexicographerMalachy Postlethwaytpublished his African maps showing the "Meridian of London" intersecting theEquatora few degrees west of the later meridian andAccra,Ghana.

Laser projected from theRoyal Observatory,marking the original Greenwich Prime meridian

The plane of the prime meridian is parallel to the local gravity vector at theAirytransit circle(51°28′40.1″N0°0′5.3″W)of the Greenwich observatory. The prime meridian was therefore long symbolised by a brass strip in the courtyard, now replaced by stainless steel, and since 16 December 1999, it has been marked by a powerful greenlasershining north across theLondonnight sky.

TheGlobal Positioning System(GPS) receivers show that the marking strip for the prime meridian at Greenwich is not exactly at zero degrees, zero minutes, and zero seconds but at approximately 5.3 seconds of arc to the west of the meridian (meaning that the meridian appears to be 102 metres east). In the past, this offset has been attributed to the establishment of reference meridians for space-based location systems such asWGS-84(which the GPS relies on) or to the fact that errors gradually crept into theInternational Time Bureautimekeeping process. The actual reason for the discrepancy is that the difference between precise GNSS coordinates and astronomically determined coordinates everywhere remains a localized gravity effect due tovertical deflection;thus, no systematic rotation of global longitudes occurred between the former astronomical system and the current geodetic system.

History

The line down the middle of this 1911 map is the prime meridian, shown passing throughGreenwich.SaxavordandBaltaat the top of the map are in theShetland Islands,the most northerly parts of Scotland and the United Kingdom. Shetland lies 1° W of the prime meridian.

Before the establishment of a common meridian, most maritime countries established their own prime meridian, usually passing through the country in question. In 1721,Great Britainestablished its own meridian passing through an early transit circle at the newly establishedRoyal Observatoryat Greenwich. The meridian was moved around 10 metres or so east on three occasions as transit circles with newer and better instruments were built, on each occasion next door to the existing one. This was to allow uninterrupted observation during each new construction. The final meridian was established as an imaginary line from theNorth Poleto theSouth Polepassing through the Airy transit circle. This became the United Kingdom's meridian in 1851.For all practical purposes of the period, the changes as the meridian was moved went unnoticed.

Transit instruments are installed to beperpendicularto the local level (which is a plane perpendicular to a plumb line). In 1884, theInternational Meridian Conferencetook place inWashington, D.C.to establish an internationally-recognised single meridian. The meridian chosen was that which passed through the Airy transit circle at Greenwich, and it became the prime meridian of the world for a century. In 1984 it was superseded in that role by theIERS Reference Meridianwhich, at this latitude, runs about 102 metres to the east of the Greenwich meridian.

At around the time of the 1884 conference, scientists were making measurements to determine thedeflection of the verticalon a large scale.One might expect that plumb lines set up in various locations, if extended downward, would all pass through a single point, the centre of Earth, but this is not the case, primarily due to Earth being anellipsoid,not a sphere. The downward extended plumb lines don't even all intersect the rotation axis of Earth; this much smaller effect is due to the uneven distribution of Earth's mass. To make computations feasible, scientists defined ellipsoids of revolution, more closely emulating the shape of Earth, modified for a particular zone; a published ellipsoid would be a good base line for measurements. The difference between the direction of a plumb line or vertical, and a line perpendicular to the surface of the ellipsoid of revolution – anormalto said ellipsoid – at a particular observatory, is the deflection of the vertical.

AGPSreceiver at the marking strip of the Greenwich Meridian in front of theRoyal Observatory.The indicated longitude is not exactly zero because thegeodeticzero meridian on a geocentric reference ellipsoid (which is what GPS positioning yields, using theIERS Reference Meridian) is 102 metres east of this strip.

When the Airy transit circle was built, a mercury basin was used to align the telescope to the perpendicular. Thus the circle was aligned with the local vertical or plumb line, which is deflected slightly from the normal, or line perpendicular, to the reference ellipsoid used to define geodetic latitude and longitude in theInternational Terrestrial Reference Frame(which is nearly the same as theWGS84 systemused by theGPS). While Airy's local vertical, set by theapparentcentre of gravity of Earth still points to (aligns with) the modern celestial meridian (the intersection of the prime meridian plane with the celestial sphere), it does not pass through Earth's rotation axis. As a result of this, the ITRF zero meridian, defined by a plane passing through Earth's rotation axis, is 102.478 metres to the east of the prime meridian. A 2015 analysis by Malyset al.shows the offset between the former and the latter can be explained by this deflection of the vertical alone; other possible sources of the offset that have been proposed in the past are smaller than the current uncertainty in the deflection of the vertical, locally. The astronomical longitude of the Greenwich prime meridian was found to be 0.19″ ± 0.47″ E, i.e. the plane defined by the local vertical on the Greenwich prime meridian and the plane passing through Earth's rotation axis on the ITRF zero meridian are effectively parallel. Claims, such as that on the BBC website, that the gap between astronomical and geodetic coordinates means thatanymeasurements of transit time across the IRTF zero meridian will occur precisely 0.352 seconds (or 0.353siderealseconds) before the transit across the "intended meridian"are based on a failure of understanding. The explanation by Malyset al.on the other hand is more studied and correct.

Meridian today

Greenwich meridian and Earth

The Greenwich meridian passes through eight countries inEuropeandAfricafrom north to south:

1. United Kingdom(specifically, onlyEngland)
2. France
3. Spain
4. Algeria
5. Mali
6. Burkina Faso
7. Togo
8. Ghana

It also passes throughAntarctica,only touchingQueen Maud Land,aterritorial claim of Norway,on its way from theNorth Poleto theSouth Pole.

It crosses the maritimeexclusive economic zonesof:

1. Greenland(Denmark)
2. Norway(via proximity toSvalbard,Jan Mayen Island,and the Norwegian mainland in the North Atlantic, andBouvet Islandin the South Atlantic)

See also

• 180th meridian
• Prime meridian
• United Kingdom Ordnance Survey Zero Meridian

External links

• "Where the Earth's surface begins—and ends",Popular Mechanics,December 1930

• International Conference Held at Washington for the Purpose of Fixing a Prime MeridianatProject Gutenberg
• A pictorial catalogue of meridian markers