Much more information is contained in the booklet `The Greenwich Meridian' by
 Stuart Malin and Carole Stott, first published by Ordnance Survey in 1984. Although
 currently out of print, the publication is held in many libraries.

 Background

 In the same way that the Equator separates the Northern and Southern
 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
 decision.

 So since 1884, the Airy line has been The Greenwich Meridian, although for
 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 line (because
 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
 be plotted.

 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
 indicated.

 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
 through Britain.

 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.

 Other publications

 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
 reached.
                     Ordnance Survey, Britain's national mapping agency
                          All content Crown Copyright © 1999

                                            ***************

 The Greenwich Meridian in the Space Age

 Introduction

 This paper has been prepared by Carl Calvert, Geodetic Advisor, Ordnance Survey
 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

 Time itself has a scale and an origin. For most of us the scale is Atomic Time
 (TAI)[1] and Universal Time (UT) is a defined time, synonymous with Greenwich
 Mean Time (GMT) which it has replaced - but Greenwich has not been forgotten.

 Sidereal time

 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.

 Radio time

 Civilian time (Broadcast) as recommended by the International Radio Consultative
 Committee (CCIR) is Universal Time Coordinated (UTC). This is the time that we
 hear on radio transmissions.

 Atomic time

 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[2].

 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 earth.

 Longitude

 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.

 [1] 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).

 [2] TAI - UTC = 30 seconds (as of 1 January 1996).

                     Ordnance Survey, Britain's national mapping agency
                          All content Crown Copyright © 1999