Greenwich was selected as the world’s prime meridian in 1884.
The International Meridian Conference met in Washington, D.C., in October 1884 to choose a common zero of longitude for maps, navigation, and international timekeeping. Delegates voted to adopt the meridian passing through the Airy Transit Circle at the Royal Observatory in Greenwich, London. This line became 0 degrees longitude.
Greenwich was chosen partly because British nautical charts and navigation practices were already widely used. The conference also recommended a universal day beginning at midnight and a system of longitude measured east and west from Greenwich. The agreement did not instantly force every country to change its maps, but Greenwich became the dominant international reference.
The modern prime meridian used by satellite-based geodesy lies about 102 metres east of the historic Greenwich meridian because Earth is represented with newer reference systems. The historic line at the observatory remains the famous visitor landmark, while the internationally used coordinate reference is tied to the International Terrestrial Reference Frame.