The Prime Meridian

 Visitors stand astride the Meridian Line with a foot in each hemisphere. Photograph by Cliff Wilkinson

The Prime Meridian is the line agreed in 1884 as marking 0° longitude. It runs through the park, with markers in place in the Royal Observatory courtyard and outside the northern wall. 

It might be an arbitrary, largely invisible line, but the Prime Meridian is one of Greenwich Park’s most important claims to fame. It signifies 0° longitude, divides the world into western and eastern hemispheres and regulates time zones across the planet. 

Its location in Greenwich was decided at a conference in Washington D.C. in October 1884, when all but three of the 41 delegates voted for Greenwich to become the Prime Meridian. London as the primary choice came down to two reasons: the United States had used the Greenwich Meridian as the basis for its time zones when these were put in place the year before, and London’s position as the global capital of maritime trade meant most ships were using Greenwich Meridian-based charts during voyages. 

The Prime Meridian used a line established by Astronomer Royal Sir George Biddell Airy, who, in 1851, defined 0° longitude using observations of transit circles. Finally, some 33 years later, the rest of the world agreed. 

These days, it’s possible to straddle two hemispheres in the Royal Observatory’s courtyard and explore the story of the Prime Meridian in the galleries there. Another option is to pass through the iron gate just outside the Observatory and find a wall plaque and metal line in the ground showing the line’s location: look towards the sundial near the boating pond and follow the meridian line. 

After some new calculations were taken in 1988, the Prime Meridian moved 102.5 metres to the east of the historic ‘Prime Meridian of the World’ once at the latitude of the Airy Transit Circle.