Much more information is contained in the booklet `The Greenwich
Stuart Malin and Carole Stott, first published by Ordnance Survey in 1984. Although
currently out of print, the publication is held in many libraries.
In the same way that the Equator separates the Northern and
hemispheres, the Greenwich Meridian divides the East from the West. The zero
degrees longitude line runs from the North Pole to the South Pole, passing directly
through the Old Royal Observatory building at Greenwich in South East London. It is
the basis for timekeeping and navigation throughout the world.
It is, in fact, one of a countless number of meridians in the
world - every possible
line of longitude is one - and until a little over a century ago, many different ones
were adopted by different countries for map-making, navigation and timekeeping.
Even today, it can be confusing as there are four Meridians
all passing through the
Old Royal Observatory.
The earliest is Flamsteed's, named after the first Astronomer
Royal, which was
established in 1675. In 1725, Edmund Halley, the second Astronomer Royal
established a second Meridian.
The third was defined by another Astronomer Royal, James Bradley,
in the mid
18th century, and is still used as the basis for map-making in Britain today.
The fourth Meridian was established in 1851 by yet another
Astronomer Royal, Sir
George Airy, who set up new measuring equipment in a room alongside Bradley's
original equipment. It is the positioning of this neighbouring equipment, just 5.79
metres (19ft) away, which eventually became the basis for international time. Due to
the convergence of meridians of longitude towards the poles, Bradley's Meridian is
5.9m west of Airy's where they cross the South Coast of England, and 5.5m west
where they cross the East Coast.
As the pace of development and travel accelerated in the 19th
century, it became
clear there would have to be a common, world-wide standard for timekeeping. In
1884, 25 countries reached agreement at a conference in Washington, USA, that
Airy's Greenwich Meridian would be adopted as the `Prime Meridian' - zero
degrees - from which time could be set and from which other points of longitude
could be calculated. Over a period of many years, countries which had not
necessarily been party to this original agreement accepted and adopted the
So since 1884, the Airy line has been The Greenwich Meridian,
practical mapping purposes in Britain (excepting hydrographic charts) the Bradley
line continues in use as the zero meridian. The difference between the two is known
and well defined - and is important scientifically - but for most day-to-day purposes
has no real consequence.
The Meridian Line in Britain
The Greenwich Meridian runs for more than 200 miles through
Britain - from near
Withernsea in East Yorkshire to Peacehaven in East Sussex - but generally
speaking, it cannot be seen. It is an invisible line for there is simply no need for it to
be physically marked out on the ground.
However, there are places where it can be visibly identified.
The best known of
these is at Greenwich itself, where the Prime Meridian, as defined by Airy, is
marked by the brass strip at the Observatory site - the spot where people are often
photographed straddling the eastern and western hemispheres. Several other
features, plaques or markers have also been placed by individuals, societies and
authorities at various other points along the route.
One meridian marker, The Chingford Pillar, was erected in 1824
on the edge of
Epping Forest on the earlier Bradley line. A plaque on that pillar indicates that
Airy's later definition set the Meridian 19ft (5.79m) to the east - a point also marked
by an obelisk. Like the Chingford Pillar, significant features marking the Meridian at
Cleethorpes and Peacehaven are also based on the Bradley line, as are many
other smaller `Meridian markers' in the country. However, in recent years, new
markers have tended to be based on the Airy line.
The route of the Line
In very simple terms, from the North Pole the Greenwich Meridian
crosses ice and
water until it `enters' Britain just north of Withernsea. The line then heads south over
the Humber Estuary, passing just east of Cleethorpes before continuing down
through Louth and Boston in Lincolnshire. It then passes just west of both March
and Cambridge before running just east of Ware, Cheshunt and Enfield and through
Walthamstow and Leyton before crossing the Thames to Greenwich. It continues
south through Oxted, East Grinstead and Lewes, reaching the English Channel at
Peacehaven. The line then continues across France, Spain and part of the African
continent until it reaches Antarctica and the South Pole.
The route on Ordnance Survey maps
Although the Greenwich Meridian is not printed on Ordnance
Survey maps as a
specific feature, it is easy to identify the route in detail as long as you are in
possession of a pencil and ruler. The instructions which follow explain how.
Technically, the zero meridian on Ordnance Survey maps is Bradley's
Ordnance Survey was mapping Britain before Airy became Astronomer Royal) and
for practical mapping purposes, the difference defined in 1851 was insufficient to
change the basis of the mapping. In fact, at its true scale, a pencil line drawn today
on a map at 1:50000 or 1:25000 scale will actually cover both the Bradley and Airy
lines! It is only on very large scale maps that the differences between the two can
How to find the Line
In the top and bottom margins of all small-scale Ordnance Survey
maps, degrees of
longitude are clearly marked (as are degrees of latitude in the side margins).
Where 0 degrees longitude is marked, that is the zero meridian. Link the precise
marks for 0 degrees at the top and bottom of a map for any part of the route and the
resulting line is a section of the meridian.
This is particularly easy to do on the popular Landranger®
series of Ordnance
Survey maps, because at intervals along the meridian there are also small blue
crosses positioned on each map. By ensuring the line linking the top and bottom of
the map passes through the appropriate crosses (officially known as `graticule
intersections') - ideally by using a flexible rule - the route of the meridian will be
The meridian crosses just ten of the 204 maps in the Landranger
series and these
are all widely available in High Street shops or direct from Ordnance Survey
HelpLine. Copies are also held by many libraries for reference purposes.
The ten relevant Landranger maps (listed from north to south) are:
Sheet 107 - Kingston upon Hull and surrounding area
Sheet 113 - Grimsby, Louth and Market Rasen
Sheet 122 - Skegness area
Sheet 131 - Boston and Spalding area
Sheet 142 - Peterborough and surrounding area
Sheet 154 - Cambridge, Newmarket and surrounding area
Sheet 166 - Luton, Hertford and surrounding area
Sheet 177 - East London, Billericay and Gravesend
Sheet 187 - Dorking, Reigate and Crawley area
Sheet 198 - Brighton and The Downs
Landranger maps are produced at the 1:50000 scale - that is,
2 cm to 1 km or 11/4
inches to 1 mile.
People wanting to trace the line on a larger scale map for
greater detail in a
specific locality should carry out the same process but use the appropriate
Ordnance Survey maps at the 1:25000 scale, published in the Pathfinder® and
ExplorerTM series. Around 30 different titles cover the entire route of the meridian
Although each covers a smaller area than a Landranger map,
much greater detail
is shown, including field boundaries and specific groups of buildings, as they are
drawn to a scale of 4 cm to 1 km or 21/2 inches to 1 mile. A free mapping index is
available to help identify the relevant map for any particular area - simply ring
Ordnance Survey HelpLine for a copy.
For even greater detail, Ordnance Survey 1:10000 scale maps
(around 6 inches to
1 mile) may be referred to. Again, some major libraries and local authority offices
hold reference copies for their local areas, although maps at this large scale - which
identify many individual buildings - tend to be available for sale only through larger
stockists. The margins of these maps also contain the necessary longitude
markings which can be joined together in the manner previously described.
Anyone requiring even more detailed mapping than this should
seek further advice
from the HelpLine about the availability of very large scale Ordnance Survey
SuperplanTM mapping and customised services which are available to calculate
and trace the meridian route on such mapping.
As the year 2000 approaches, interest in the Greenwich Meridian
is likely to
increase still further. Ordnance Survey is already considering the possibilities for
appropriate publications which could be issued to assist and inform people of the
significance and route of the line. Details will be announced as decisions are
Ordnance Survey, Britain's national mapping agency
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The Greenwich Meridian in the Space Age
This paper has been prepared by Carl Calvert, Geodetic Advisor,
to provide answers to some of the more technical queries relating to The
Greenwich Meridian. A more general customer information paper is also available.
Time itself has a scale and an origin. For most of us the scale
is Atomic Time
(TAI) and Universal Time (UT) is a defined time, synonymous with Greenwich
Mean Time (GMT) which it has replaced - but Greenwich has not been forgotten.
Time can be kept with reference to the rotation of the earth
and this type of time is
called Sidereal time. Universal Time 1 (UT1) is related to the earth rotation time
which is still called Greenwich Mean Sidereal Time (GMST). GMST is now referred
to the International Reference Meridian (IRM), defined within the International
Celestial Reference System (ICRF) rather than the Greenwich Meridian.
Civilian time (Broadcast) as recommended by the International
Committee (CCIR) is Universal Time Coordinated (UTC). This is the time that we
hear on radio transmissions.
UTC is related to Atomic Time (TAI) by a correction defined
by the International
Earth Rotation Service (IERS). The correction is given as a whole second. So while
the difference between UT1 and TAI has fractional parts of a second the difference
between TAI and UTC does not.
The Global Positioning System, GPS, has its own GPS time, kept
by atomic clocks
so it uses the TAI timescale, which was identical to UTC on 5 January 1980. GPS
observations enable positions and time to be determined any where in or around
The link between longitude and time in an earth-fixed is defined
by the IERS
Terrestrial Reference Frame (ITRF). ITRF is based on observations to satellites
and celestial compact radio sources (quasars) from various coordinated stations
around the globe.
In Europe ITRF is realised as ETRF, the European Terrestrial
Reference Frame. In
1989 they were identical but are slowly moving apart due to tectonic plate
movements and inconsistencies of the movement of the earth in its orbit and
rotation about its axis. Because of small, but observable movement of a centimetre
or so reference frames are suffixed by a date. In 1989 ITRF89 and ETRF89 were
identical. Since then ETRF has moved with the stable part of Europe. This is a
small movement of a few centimetres relative to ITRF.
The Airy Transit's position
Comparing the longitude of the Airy Transit in the system available
in 1936 to the
longitude determined with space techniques gives a difference.
Using Ordnance Survey Level 1 Transformation
Airy Transit, GB36 =
N 51 28 38.265 E 00 00 00.418
Airy Transit, ETRF89 = N 51 28 40.1247 W 00 00 05.3101
X 3980637.8044 Y - 102.4779 Z 4966897.8318
The difference in longitude between the two systems at the
Airy Transit is 102.478
metres. Therefore the International Reference Meridian is 102.5 metres east of the
Airy Transit at Greenwich. As the IRM is tied to the definition of time the real
reference meridian to be used for the Millennium is the IRM which, as shown above,
is about 100 metres east of the Greenwich Meridian.
 Atomic Time (TAI) is the timescale of the Bureau International
des Poids et
Mesures (BIPM) and its unit is one SI second. On 1 January 1958 there was no
difference between UT1 and TAI. Since then the differences between UT1 and TAI
have grown from 0.0 seconds to - 29.6292 seconds on 1 April 1996.
On 1 January 2000 it is expected that UT1-TAI will be - 34
seconds (with an
uncertainty of 1 second).
 TAI - UTC = 30 seconds (as of 1 January 1996).
Ordnance Survey, Britain's national mapping agency
All content Crown Copyright © 1999