Greenwich, meridians and maps

A popular day trip for visitors to London is a riverboat excursion down the Thames to Greenwich. Here you can climb the hill to the old Royal Observatory and have your photograph taken with one foot either side of the brass strip marking the Greenwich meridian.

The Greenwich meridian was internationally recognised as the prime meridian 125 years ago next Tuesday. On 13 October 1884 delegates from 25 nations met in Washington, DC, to agree a reference line from which to calculate all measurements of longitude. After much argument, the conference adopted the Greenwich meridian.

The main reason for the choice was that 72 per cent of the world’s shipping already used British charts created by pioneering 18th century navigators such as James Cook. These charts were drawn with reference to the meridian at Greenwich, and navigators were already finding their longitude with chronometers set to Greenwich Mean Time.

Greenwich was adopted by 22 votes to one, with only San Domingo dissenting, and France and Brazil abstaining. France had fought for the Paris meridian, and the Paris-Greenwich rivalry is a plot element in Jules Verne’s ‘Twenty thousand leagues under the sea’, published just before the international conference.

Despite the international agreement, France stuck with the Paris meridian until 1911 for timekeeping purposes and 1914 for navigation. To this day, some French maps still show the Paris meridian.

The meridian adopted internationally in 1884 had been established only in 1851 by Britain’s Astronomer Royal, Sir George Airy. But it was actually the fourth Greenwich meridian to be designated since Charles II ordered the building of the Royal Observatory in 1675. Quirkily, maps produced by the Ordnance Survey still use an older meridian.

When the first OS map was published in 1801, Britain’s official prime meridian was one established by the third Astronomer Royal, James Bradley. Although Airy’s new prime meridian superseded it 50 years later, the Ordnance Survey has continued to use Bradley’s meridian, which at Greenwich lies about 6m west of Airy’s.