Every place on Earth has a longitude, and every longitude is measured from a line that runs through a park in south-east London. That line is arbitrary: nothing in nature makes Greenwich special. How it became the zero of the world map is a story of ships, scientists and diplomatic haggling, and it has a surprising twist, because the line visitors stand on is not quite where modern satellites put zero.
Latitude has a natural starting point, the equator. Longitude does not. Any north-south line can be called zero, and for centuries every country chose its own. Maps and ships' charts used Paris, Rome, Cádiz, Washington and other capitals as their reference, which made it hard to compare positions or navigate with a chart from another nation.
The problem became urgent in the nineteenth century. Fast steamships, railways and the telegraph meant that people in different countries needed to agree on where they were and what time it was. By the 1880s, a majority of the world's shipping already used charts based on the Greenwich meridian, because British charts were so widely used.
Greenwich had two advantages. The Royal Observatory there had been producing the astronomical tables that sailors used to find longitude since the eighteenth century. And in 1851 the Astronomer Royal, George Airy, built a large new telescope, the Airy Transit Circle, whose position defined a meridian that astronomers and navigators already relied on. It was a case of choosing the line people were already using.
In October 1884 delegates from 25 nations met in Washington, D.C., for the International Meridian Conference. After debate, they voted to adopt the meridian through the Greenwich transit instrument as the world's prime meridian. The vote was 22 in favour, one against (the Dominican Republic, then called San Domingo) and two abstentions: France and Brazil.
France had its own meridian through the Paris Observatory and held out. Paris time stayed in official use in France for years, and the country did not formally switch to Greenwich-based time until 1911. You can read how this played out in our page for Paris.
The conference also recommended a universal day, but it did not create the time zones we use today. It left that to individual countries, which adopted them over the following decades, as we describe in our guide to how the world agreed on the time.
The prime meridian runs from the Arctic to Antarctica. Over land it crosses eight countries: the United Kingdom, France, Spain, Algeria, Mali, Burkina Faso, Togo and Ghana. In the UK it passes through Greenwich and across the east coast, and in France through the Normandy coast and the south-west.
Because of the 1884 decision, the mean solar time at Greenwich became the world's time reference: Greenwich Mean Time, or GMT. Each 15 degrees of longitude east or west corresponds to one hour of difference, which is the basis of the time zone system.
Today the reference is Coordinated Universal Time (UTC), which is based on atomic clocks rather than on the Sun, as we explain in our article on how atomic clocks work. GMT survives mainly as the name of the winter time zone in Britain, as on our London page, and as the permanent time in Iceland.
In Greenwich Park, visitors stand with one foot on each side of a brass strip marking the prime meridian at the Royal Observatory. But if you carry a GPS receiver along that strip, it will not read zero degrees. It shows about 5.3 seconds of arc west of zero, which is roughly 102 metres.
The reason is that the two definitions are not the same. The old meridian was fixed by the Airy Transit Circle, whose instruments were levelled using the local pull of gravity, so the line is affected by small gravity variations near Greenwich. Modern positioning uses a reference frame centred on the Earth's centre of mass, maintained by the International Earth Rotation and Reference Systems Service. Its zero longitude, called the IERS Reference Meridian, lies about 100 metres east of the brass strip.
For most purposes the difference does not matter. For anyone making an accurate map, it does, and satellite navigation systems use the modern line.
On the opposite side of the globe, at 180 degrees, is the antimeridian. The International Date Line roughly follows it, with detours to keep countries on one side. The two lines together split the world into an eastern and a western half, and the date changes where they meet the Pacific.