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. The line then heads south over the Humber Estuary, passing just east of Cleethorpes before continuing down through Louth and Boston in Lincolnshire. In very simple terms, from the North Pole the Greenwich Meridian crosses ice and water until it `enters’ Britain just north of Withernsea. However, in recent years, new markers have tended to be based on the Airy line. 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. 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. One meridian marker, The Chingford Pillar, was erected in 1824 on the edge of Epping Forest on the earlier Bradley line. Several other features, plaques or markers have also been placed by individuals, societies and authorities at various other points along the route. 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. However, there are places where it can be visibly identified. It is an invisible line for there is simply no need for it to be physically marked out on the ground. 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. 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. 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. Over a period of many years, countries which had not necessarily been party to this original agreement accepted and adopted the decision. 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.
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.Īs 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. It is the positioning of this neighbouring equipment, just 5.79 metres (19ft) away, which eventually became the basis for international time. 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. 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. In 1725, Edmund Halley, the second Astronomer Royal established a second Meridian. The earliest is Flamsteed’s, named after the first Astronomer Royal, which was established in 1675. 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.Įven today, it can be confusing as there are four Meridians all passing through the Old Royal Observatory. It is the basis for timekeeping and navigation throughout the world. 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. In the same way that the Equator separates the Northern and Southern hemispheres, the Greenwich Meridian divides the East from the West.
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