A prime meridian, based at the Royal Observatory, Greenwich, in London, 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 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 meridian passing
through Greenwich 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. 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. The prime meridian passes through the
Airy transit circle of the Greenwich observatory. It was long marked by a
brass strip in the courtyard, now replaced by stainless steel, and, since
16 December 1999, has been marked by a powerful green laser shining north
across the London night sky. Global Positioning System receivers show
that the marked 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. In the past this offset has been attributed to the
establishment of reference meridians for space-based location systems such as WGS
84 or that errors gradually crept into the International Time Bureau
timekeeping process. History
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 un-noticed.
Transit instruments are installed to be perpendicular to the local level. In
1884 the International Meridian Conference took place to establish an
internationally recognised single meridan. 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, due to 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 and a line perpendicular to the surface of the
ellipsoid of revolution at a particular observatory is the deflection of the
vertical. The International Meridian Conference did not take this into
account. The phenomenon meant that when the Airy
transit circle was built, that because they used a plumb line to align the
telescope to the perpendicular, the plumb line is deflected slightly from a
line perpendicular to the modern ellipsoid used to define latitude and
longitude, the International Terrestrial Reference Frame. This in turn meant that
the Airy transit circle points very slightly to the east of the modern
celestial meridian. As a result of this, any measurements of transit time across
the view of the transit telescope occurs 0.352 seconds before the transit across
the intended meridian. The practical upshot of this is that the modern prime
meridian does not correspond to the Airy transit, but to a meridian that is 102
metres to the east of the transit. For all practical navigational purposes of
the time, the difference was insignificant, and sailors certainly
would not have noticed. A recent analysis by Malys et al. shows
the offset between the Airy transit circle and the ITRF/WGS 84 meridians can
be explained by deflection of the vertical alone; other possible sources
of the offset that have been proposed in the past are smaller than the current
uncertainty of the deflection of the vertical near the observatory.
The meridian today The Greenwich meridian passes through:
United Kingdom, France,
Spain, Algeria,
Mali, Burkina Faso,
Togo, Ghana,
Queen Maud Land in Antarctica. See also
Greenwich Mean Time IERS Reference Meridian
United Kingdom Ordnance Survey Zero Meridian
References 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 Meridian at Project Gutenberg A pictorial catalogue of meridian
markers