Coordinates: 51°28′40.12″N 0°00′05.31″W /
51.4778111°N 0.0014750°W / 51.4778111; -0.0014750
Tourists taking pictures with the Prime Meridian monument
Laser projected from the Royal Observatory in
Greenwich marking the
A prime meridian, based at the Royal Observatory, Greenwich, in
London, England, 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 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
Greenwich as the official prime meridian due to its
popularity.[note 1] 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"
Equator a few degrees west of the later meridian and
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)) 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
Global Positioning System
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 of this line). 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 that errors
gradually crept into the
International Time Bureau timekeeping
process. The actual reason for the discrepancy is that the marking
strip is indeed at astronomical longitude zero degrees, zero minutes,
and zero seconds[note 2]—but
GPS receivers show geodetic longitude
(specifically ITRF/WGS 84).
2 The meridian today
3 See also
6 External links
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 Great Britain'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 be perpendicular to the local
level (which is a plane perpendicular to a plumb line). In 1884, the
International Meridian Conference
International Meridian Conference took place 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.
At around the time of this conference, scientists were making
measurements to determine the deflection of the vertical 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 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; a given ellipsoid would be a good compromise
for measurements in a given area, such as a country or continent. 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.
GPS receiver at the marking strip of the prime meridian. The
indicated longitude is not exactly zero because the geodetic zero
meridian on a geocentric reference ellipsoid (which is what GPS
positioning yields) is located 102 metres to the 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 the International
Terrestrial Reference Frame (which is nearly the same as the WGS-84
system used by GPS). While the local vertical defined at the Airy
transit circle still points to 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 metres to the east of the prime
meridian. A 2015 analysis by Malys et al. shows the offset between the
Airy transit circle and the ITRF/
WGS 84 meridians 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 near the
observatory. The astronomical longitude of the
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. However, the claim, found, e.g., in
a BBC article that this difference between astronomical and geodetic
coordinates means that any measurements of transit time across the
IRTF zero meridian will occur 0.352 seconds (or 0.353 sidereal
seconds) before the transit across the "intended meridian" is based
on a failure of understanding. The explanation by Malys et al. on
the other hand is both lucid and correct.
The meridian today
As of 2018 the
Greenwich meridian passes through:
Queen Maud Land
Queen Maud Land in Antarctica.
Greenwich Mean Time
IERS Reference Meridian
United Kingdom Ordnance Survey Zero Meridian
^ Voting took place on 13 October and the resolutions were adopted on
22 October 1884.
^ Astronomical longitude is influenced by local gravity anomalies.
^ ROG Learning Team (23 August 2002). "The Prime Meridian at
Greenwich". Royal Museums Greenwich. Royal Museums Greenwich.
Retrieved 14 June 2012.
^ Howse 1997, pp. 12, 137.
^ 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".
^ 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.
^ Dracup, Joseph F. (8 June 2006). "Geodetic Surveys in the United
States, the Beginning and the Next 100 Years". NOAA History: A Science
^ 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 2015-08-17.
Howse, Derek (1997).
Greenwich Time and the Longitude. Phillip Wilson.
ISBN 0-85667-468-0. ASIN 0948065265.
Wikimedia Commons has media related to Prime meridian.
"Where the Earth's surface begins—and ends", Popular Mechanics,
International Conference Held at Washington for the Purpose of Fixing
a Prime Meridian at Project Gutenberg
A pictorial catalogue o