74:
335:, 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 – a
412:
263:
168:
241:(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
343:
391:
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 that
390:
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
383:
centre 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
366:
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
294:
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
318:
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
330:
on 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 an
226:) 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
249:
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
144:. 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 the
189:
303:
passing 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.
673:
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".
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Before the establishment of a common meridian, most maritime countries established their own prime meridian, usually passing through the country in question. In 1721,
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624:"International Conference Held at Washington for the Purpose of Fixing a Prime Meridian and a Universal Day. October, 1884. Protocols of the proceedings"
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254:; thus, no systematic rotation of global longitudes occurred between the former astronomical system and the current geodetic system.
311:
141:
30:
This article is about the prime meridian through
Greenwich, London. For Earth's current international standard prime meridian, see
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seconds) before the transit across the "intended meridian" are based on a failure of understanding. The explanation by Malys
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was the international standard prime meridian, used worldwide for timekeeping and navigation. The modern standard, the
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Malys, Stephen; Seago, John H.; Palvis, Nikolaos K.; Seidelmann, P. Kenneth; Kaplan, George H. (1 August 2015).
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850:
282:, the most northerly parts of Scotland and the United Kingdom. Shetland lies 1° W of the prime meridian.
112:, is based on the Greenwich meridian, but differs slightly from it. This prime meridian (at the time, one of
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measurements of transit time across the IRTF zero meridian will occur precisely 0.352 seconds (or 0.353
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to said ellipsoid – at a particular observatory, is the deflection of the vertical.
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zero meridian on a geocentric reference ellipsoid (which is what GPS positioning yields, using the
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117:
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rotation axis, is 102.478 metres to the east of the prime meridian. A 2015 analysis by Malys
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At around the time of the 1884 conference, scientists were making measurements to determine the
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established its own meridian passing through an early transit circle at the newly established
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771:(Technical report) (5th ed.). National Ocean Service. December 1983. pp. 6–10.
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The line down the middle of this 1911 map is the prime meridian, shown passing through
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129:
113:
89:
35:
822:
International
Conference Held at Washington for the Purpose of Fixing a Prime Meridian
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which, at this latitude, runs about 102 metres to the east of the
Greenwich meridian.
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Tourists queuing to take pictures on the line of the historical prime meridian at the
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750:
Geodetic
Surveys in the United States, the Beginning and the Next 100 Years 1807-1940
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Voting took place on 13 October and the resolutions were adopted on 22 October 1884.
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to the local level (which is a plane perpendicular to a plumb line). In 1884, the
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The plane of the prime meridian is parallel to the local gravity vector at the
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245:(which the GPS relies on) or to the fact that errors gradually crept into the
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published his
African maps showing the "Meridian of London" intersecting the
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receiver at the marking strip of the Greenwich Meridian in front of the
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752:(Report). NOAA special publication NOS NGS. National Geodetic Survey.
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783:"Scientists explain why Greenwich Meridian line is in 'wrong place'"
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for several decades. In the 18th century, London lexicographer
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and maps. In October of that year, at the invitation of the
354:. The indicated longitude is not exactly zero because the
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The Greenwich meridian passes through eight countries in
599:"What is the Prime Meridian and why is it in Greenwich?"
529:, and the Norwegian mainland in the North Atlantic, and
92:, a geographical reference line that passes through the
731:"The Greenwich Meridian before the Airy Transit Circle"
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in 1851, and by 1884, over two-thirds of all ships and
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34:. For the general concept of prime meridians, see
403:. on the other hand is more studied and correct.
175:, marking the original Greenwich Prime meridian
156:a few degrees west of the later meridian and
8:
554:United Kingdom Ordnance Survey Zero Meridian
104:, England. From 1884 to 1974, the Greenwich
856:Geography of the Royal Borough of Greenwich
808:"Where the Earth's surface begins—and ends"
379:). While Airy's local vertical, set by the
593:
591:
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295:established as an imaginary line from the
27:Meridian passing through Greenwich, London
876:1851 establishments in the United Kingdom
861:1884 establishments in the British Empire
833:A pictorial catalogue of meridian markers
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369:International Terrestrial Reference Frame
306:Transit instruments are installed to be
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626:. Project Gutenberg. 1884. p. 209
136:, 41 delegates from 25 nations met in
7:
881:1721 establishments in Great Britain
653:. Phillip Wilson. pp. 12, 137.
362:) is 102 metres east of this strip.
690:"Why the Greenwich meridian moved"
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371:(which is nearly the same as the
312:International Meridian Conference
278:at the top of the map are in the
142:International Meridian Conference
649:Greenwich Time and the Longitude
367:latitude and longitude in the
134:President of the United States
1:
886:Royal Observatory, Greenwich
415:Greenwich meridian and Earth
79:Royal Observatory, Greenwich
487:territorial claim of Norway
116:) was first established by
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748:Dracup, Joseph F. (1994).
328:deflection of the vertical
29:
18:Prime Meridian (Greenwich)
715:10.1007/s00190-015-0844-y
247:International Time Bureau
239:Global Positioning System
230:shining north across the
171:Laser projected from the
140:, United States, for the
124:used it as the reference
735:thegreenwichmeridian.org
502:exclusive economic zones
500:It crosses the maritime
59:51.4778111°N 0.0014750°W
603:Royal Museums Greenwich
477:It also passes through
360:IERS Reference Meridian
321:IERS Reference Meridian
110:IERS Reference Meridian
32:IERS Reference Meridian
765:Geodesy for the Layman
533:in the South Atlantic)
489:, on its way from the
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206:51.477806°N 0.001472°W
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64:51.4778111; -0.0014750
645:Howse, Derek (1997).
427:from north to south:
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434:(specifically, only
211:51.477806; -0.001472
150:Malachy Postlethwayt
706:2015JGeod..89.1263M
252:vertical deflection
201: /
55: /
866:English inventions
793:on 17 August 2015.
694:Journal of Geodesy
521:(via proximity to
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86:Greenwich meridian
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827:Project Gutenberg
812:Popular Mechanics
700:(12): 1263–1272.
660:978-0-85667-468-6
352:Royal Observatory
292:Royal Observatory
173:Royal Observatory
94:Royal Observatory
16:(Redirected from
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146:Paris meridian
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47:51°28′40.12″N
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791:the original
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375:used by the
373:WGS84 system
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217:Airy Transit
193:51°28′40.1″N
178:
85:
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50:0°00′05.31″W
40:
630:30 November
608:13 December
234:night sky.
209: /
62: /
845:Categories
580:References
495:South Pole
491:North Pole
479:Antarctica
301:South Pole
297:North Pole
215: (
509:Greenland
333:ellipsoid
268:Greenwich
196:0°0′5.3″W
128:on their
98:Greenwich
787:BBC News
538:See also
523:Svalbard
397:sidereal
381:apparent
356:geodetic
272:Saxavord
126:meridian
106:meridian
702:Bibcode
513:Denmark
493:to the
452:Algeria
436:England
299:to the
258:History
154:Equator
122:tonnage
657:
519:Norway
442:France
425:Africa
421:Europe
387:et al.
337:normal
243:WGS-84
232:London
130:charts
102:London
769:(PDF)
560:Notes
472:Ghana
447:Spain
401:et al
276:Balta
228:laser
162:Ghana
158:Accra
100:, in
88:is a
655:ISBN
632:2012
610:2021
504:of:
485:, a
467:Togo
457:Mali
423:and
274:and
237:The
181:Airy
114:many
84:The
825:at
710:doi
393:any
377:GPS
348:GPS
847::
810:,
785:.
733:.
708:.
698:89
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678:^
601:.
588:^
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346:A
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634:.
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511:(
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186:(
38:.
20:)
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