London is home to the historic Greenwich Prime Meridian.
The meridian passes through the Royal Observatory in Greenwich, an area of southeast London. At the 1884 International Meridian Conference in Washington, delegates selected the Greenwich meridian as the international reference for zero degrees longitude. This choice reflected the widespread use of British nautical charts and the importance of the British maritime fleet.
Longitude is measured east or west from the prime meridian, while latitude is measured north or south from the Equator. The meridian also became the basis for the original Greenwich Mean Time system, although modern global timekeeping uses Coordinated Universal Time and highly precise atomic standards.
The physical line marked at the observatory is historically important, but modern satellite-based reference systems place the zero-longitude reference slightly east of it. This difference comes from changes in geodetic models and the movement of Earth’s crust, not from a change in London’s historical role.