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Prime meridian (Greenwich)

January 19, 2024

This article is about a historical prime meridian. For the general concept and information on modern prime meridians, see prime meridian.

51°28′40.12″N 0°00′05.31″W / 51.4778111°N 0.0014750°W / 51.4778111; -0.0014750

Tourists queuing to take pictures on the line of the historic prime meridian at the Royal Observatory, Greenwich.

The historic prime meridian or Greenwich meridian is a geographical reference line that passes through the Royal Observatory, Greenwich, in London, England.[1] The modern IERS Reference Meridian widely used today is based on the Greenwich meridian, but differs slightly from it.[2] This prime meridian (at the time, one of many) was first established by Sir George Airy in 1851, and by 1884, over two-thirds of all ships and tonnage used it as the reference meridian on their charts and maps. In October of that year, at the behest of US President Chester A. Arthur, 41 delegates from 25 nations met in Washington, D.C., United States, for the International Meridian Conference. This conference selected the meridian passing through Greenwich as the world standard prime meridian due to its popularity.[a] However, France abstained from the vote, and French maps continued to use the Paris meridian for several decades. In the 18th century, London lexicographer Malachy Postlethwayt published his African maps showing the "Meridian of London" intersecting the Equator a few degrees west of the later meridian and Accra, Ghana.[4]

Laser projected from the Royal Observatory, marking the original Greenwich Prime meridian.

The plane of the prime meridian is parallel to the local gravity vector at the Airy transit circle (51°28′40.1″N 0°0′5.3″W / 51.477806°N 0.001472°W / 51.477806; -0.001472 (Airy Transit))[5] 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 green laser shining north across the London night sky.

Global 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 as WGS 84 (which GPS relies on) or to the fact that errors gradually crept into the International Time Bureau timekeeping 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 to vertical deflection; thus, no systematic rotation of global longitudes occurred between the former astronomical system and the current geodetic system.[6]

History edit

 
The line down the middle of this 1911 map is the prime meridian, shown passing through Greenwich. Saxavord and Balta at the top of the map are in the Shetland 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 Britain established its own meridian passing through an early transit circle at the newly established Royal Observatory at 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 the north pole to the south pole passing through the Airy transit circle. This became the United Kingdom's meridian in 1851.[7][better source needed] For all practical purposes of the period, the changes as the meridian was moved went unnoticed.

Transit instruments are installed to be perpendicular to the local level (which is a plane perpendicular to a plumb line). In 1884, the International Meridian Conference took place in Washington, 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 the IERS Reference Meridian which, 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 the deflection of the vertical on a large scale.[8] One might expect that plumb lines set up in various locations, if extended downward, would all pass through a single point, the centre of the Earth, but this is not the case, primarily due to the Earth being an ellipsoid, not a sphere. The downward extended plumb lines don't even all intersect the rotation axis of the Earth; this much smaller effect is due to the uneven distribution of the Earth's mass. To make computations feasible, scientists defined ellipsoids of revolution, more closely emulating the shape of the 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 – a normal to said ellipsoid – at a particular observatory, is the deflection of the vertical.[9]

 
A GPS receiver at the marking strip of the Greenwich Meridian in front of the Royal Observatory. The indicated longitude is not exactly zero because the geodetic zero meridian on a geocentric reference ellipsoid (which is what GPS positioning yields, using the IERS Reference Meridian) is 102 metres east of this strip.[2]

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 the International Terrestrial Reference Frame (which is nearly the same as the WGS-84 system used by GPS). While Airy's local vertical, set by the apparent centre of gravity of the 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 the Earth's rotation axis. As a result of this, the ITRF zero meridian, defined by a plane passing through the Earth's rotation axis, is 102.478 metres to the east of the prime meridian. A 2015 analysis by Malys et 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″ East, i.e. the plane defined by the local vertical on the Greenwich prime meridian and the plane passing through the 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 that any measurements of transit time across the IRTF zero meridian will occur precisely 0.352 seconds (or 0.353 sidereal seconds) before the transit across the "intended meridian"[10] are based on a failure of understanding. The explanation by Malys et al.[5] on the other hand is more studied and correct.

Meridian today edit

 
Greenwich meridian and the earth

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

It also passes through Antarctica (only touching Queen Maud Land, the territorial claim of Norway).

It crosses the maritime Exclusive Economic Zones of:

See also edit

Notes edit

  1. ^ Voting took place on 13 October and the resolutions were adopted on 22 October 1884.[3]

References edit

  1. ^ ROG Learning Team (23 August 2002). "The Prime Meridian at Greenwich". Royal Museums Greenwich. Retrieved 14 June 2012.
  2. ^ a b "What is the Prime Meridian and why is it in Greenwich?". Royal Museums Greenwich. Retrieved 13 December 2021.
  3. ^ Howse, Derek (1997). Greenwich Time and the Longitude. Phillip Wilson. pp. 12, 137. ISBN 978-0-85667-468-6.
  4. ^ Malachy Postlethwayt. (1774) Universal Dictionary of Trade and Commerce. (4th edition) London: W. Strahan, J. & F. Rivington. Vol. 1 "A New and Correct Map of the Coast of Africa".
  5. ^ a b Malys, Stephen; Seago, John H.; Palvis, Nikolaos K.; Seidelmann, P. Kenneth; Kaplan, George H. (1 August 2015). "Why the Greenwich meridian moved". Journal of Geodesy. 89 (12): 1263–1272. Bibcode:2015JGeod..89.1263M. doi:10.1007/s00190-015-0844-y.
  6. ^ Malys, Stephen; Pavlis, Nikolaos; Seidelmann, Kenneth; Kaplan, George; Seago, John H. (2015). "Why the Greenwich meridian moved". Journal of Geodesy. 89 (12): 1263–1272. Bibcode:2015JGeod..89.1263M. doi:10.1007/s00190-015-0844-y.
  7. ^ Dolan, Graham. "The Greenwich Meridian before the Airy Transit Circle". thegreenwichmeridian.org.
  8. ^ Dracup, Joseph F. (8 June 2006). "Geodetic Surveys in the United States, the Beginning and the Next 100 Years". NOAA History: A Science Odyssey.
  9. ^ Geodesy for the Layman (PDF) (Technical report) (5th ed.). National Ocean Service. December 1983. pp. 6–10.
  10. ^ . BBC News. Archived from the original on 17 August 2015.

External links edit

 
Wikimedia Commons has media related to Greenwich prime meridian.
  • "Where the Earth's surface begins—and ends", Popular Mechanics, December 1930
  • International Conference Held at Washington for the Purpose of Fixing a Prime Meridian at Project Gutenberg
  • A pictorial catalogue of meridian markers


January 19, 2024
prime, meridian, greenwich, this, article, about, historical, prime, meridian, general, concept, information, modern, prime, meridians, prime, meridian, 4778111, 0014750, 4778111, 0014750, tourists, queuing, take, pictures, line, historic, prime, meridian, roy. This article is about a historical prime meridian For the general concept and information on modern prime meridians see prime meridian 51 28 40 12 N 0 00 05 31 W 51 4778111 N 0 0014750 W 51 4778111 0 0014750 Tourists queuing to take pictures on the line of the historic prime meridian at the Royal Observatory Greenwich The historic prime meridian or Greenwich meridian is a geographical reference line that passes through the Royal Observatory Greenwich in London England 1 The modern IERS Reference Meridian widely used today is based on the Greenwich meridian but differs slightly from it 2 This prime meridian at the time one of many was first established by Sir George Airy in 1851 and by 1884 over two thirds of all ships and tonnage used it as the reference meridian on their charts and maps In October of that year at the behest of US President Chester A Arthur 41 delegates from 25 nations met in Washington D C United States for the International Meridian Conference This conference selected the meridian passing through Greenwich as the world standard prime meridian due to its popularity a However France abstained from the vote and French maps continued to use the Paris meridian for several decades In the 18th century London lexicographer Malachy Postlethwayt published his African maps showing the Meridian of London intersecting the Equator a few degrees west of the later meridian and Accra Ghana 4 Laser projected from the Royal Observatory marking the original Greenwich Prime meridian The plane of the prime meridian is parallel to the local gravity vector at the Airy transit circle 51 28 40 1 N 0 0 5 3 W 51 477806 N 0 001472 W 51 477806 0 001472 Airy Transit 5 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 green laser shining north across the London night sky Global 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 as WGS 84 which GPS relies on or to the fact that errors gradually crept into the International Time Bureau timekeeping 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 to vertical deflection thus no systematic rotation of global longitudes occurred between the former astronomical system and the current geodetic system 6 Contents 1 History 2 Meridian today 3 See also 4 Notes 5 References 6 External linksHistory edit nbsp The line down the middle of this 1911 map is the prime meridian shown passing through Greenwich Saxavord and Balta at the top of the map are in the Shetland 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 Britain established its own meridian passing through an early transit circle at the newly established Royal Observatory at 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 the north pole to the south pole passing through the Airy transit circle This became the United Kingdom s meridian in 1851 7 better source needed For all practical purposes of the period the changes as the meridian was moved went unnoticed Transit instruments are installed to be perpendicular to the local level which is a plane perpendicular to a plumb line In 1884 the International Meridian Conference took place in Washington 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 the IERS Reference Meridian which 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 the deflection of the vertical on a large scale 8 One might expect that plumb lines set up in various locations if extended downward would all pass through a single point the centre of the Earth but this is not the case primarily due to the Earth being an ellipsoid not a sphere The downward extended plumb lines don t even all intersect the rotation axis of the Earth this much smaller effect is due to the uneven distribution of the Earth s mass To make computations feasible scientists defined ellipsoids of revolution more closely emulating the shape of the 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 a normal to said ellipsoid at a particular observatory is the deflection of the vertical 9 nbsp A GPS receiver at the marking strip of the Greenwich Meridian in front of the Royal Observatory The indicated longitude is not exactly zero because the geodetic zero meridian on a geocentric reference ellipsoid which is what GPS positioning yields using the IERS Reference Meridian is 102 metres east of this strip 2 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 the International Terrestrial Reference Frame which is nearly the same as the WGS 84 system used by GPS While Airy s local vertical set by the apparent centre of gravity of the 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 the Earth s rotation axis As a result of this the ITRF zero meridian defined by a plane passing through the Earth s rotation axis is 102 478 metres to the east of the prime meridian A 2015 analysis by Malys et 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 East i e the plane defined by the local vertical on the Greenwich prime meridian and the plane passing through the 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 that any measurements of transit time across the IRTF zero meridian will occur precisely 0 352 seconds or 0 353 sidereal seconds before the transit across the intended meridian 10 are based on a failure of understanding The explanation by Malys et al 5 on the other hand is more studied and correct Meridian today edit nbsp Greenwich meridian and the earthThe Greenwich meridian passes through eight countries from north to south United Kingdom specifically only England France Spain Algeria Mali Burkina Faso Togo GhanaIt also passes through Antarctica only touching Queen Maud Land the territorial claim of Norway It crosses the maritime Exclusive Economic Zones of Greenland Denmark Norway via proximity to Svalbard Jan Mayen Island and the Norwegian mainland in the North Atlantic and Bouvet Island in the South Atlantic See also edit180th meridian Prime meridian Greenwich Mean Time GMT the local mean or average time at Greenwich There is no such thing as a Greenwich Mean United Kingdom Ordnance Survey Zero MeridianNotes edit Voting took place on 13 October and the resolutions were adopted on 22 October 1884 3 References edit ROG Learning Team 23 August 2002 The Prime Meridian at Greenwich Royal Museums Greenwich Retrieved 14 June 2012 a b What is the Prime Meridian and why is it in Greenwich Royal Museums Greenwich Retrieved 13 December 2021 Howse Derek 1997 Greenwich Time and the Longitude Phillip Wilson pp 12 137 ISBN 978 0 85667 468 6 Malachy Postlethwayt 1774 Universal Dictionary of Trade and Commerce 4th edition London W Strahan J amp F Rivington Vol 1 A New and Correct Map of the Coast of Africa a b Malys Stephen Seago John H Palvis Nikolaos K Seidelmann P Kenneth Kaplan George H 1 August 2015 Why the Greenwich meridian moved Journal of Geodesy 89 12 1263 1272 Bibcode 2015JGeod 89 1263M doi 10 1007 s00190 015 0844 y Malys Stephen Pavlis Nikolaos Seidelmann Kenneth Kaplan George Seago John H 2015 Why the Greenwich meridian moved Journal of Geodesy 89 12 1263 1272 Bibcode 2015JGeod 89 1263M doi 10 1007 s00190 015 0844 y Dolan Graham The Greenwich Meridian before the Airy Transit Circle thegreenwichmeridian org Dracup Joseph F 8 June 2006 Geodetic Surveys in the United States the Beginning and the Next 100 Years NOAA History A Science Odyssey Geodesy for the Layman PDF Technical report 5th ed National Ocean Service December 1983 pp 6 10 Scientists explain why Greenwich Meridian line is in wrong place BBC News Archived from the original on 17 August 2015 External links edit nbsp Wikimedia Commons has media related to Greenwich prime meridian Where the Earth s surface begins and ends Popular Mechanics December 1930International Conference Held at Washington for the Purpose of Fixing a Prime Meridian at Project Gutenberg A pictorial catalogue of meridian markers Retrieved from https en wikipedia org w index php title Prime meridian Greenwich amp oldid 1194715995, wikipedia, wiki, book, books, library,

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