London is traditionally recognized as the city containing the Greenwich prime meridian.
The historic prime meridian passes through the Royal Observatory in Greenwich, an area of southeast London. It was adopted internationally as the reference for zero degrees longitude at the International Meridian Conference in Washington, D.C., in 1884.
Longitude is measured east or west from this reference line up to 180 degrees. The meridian also became associated with Greenwich Mean Time, a standard used to coordinate clocks and navigation before the wider adoption of Coordinated Universal Time.
Modern satellite-based geodesy places the internationally used zero-longitude reference about 102 metres east of the observatory’s historic line. This difference exists because modern Earth-centred coordinate systems and older astronomical measurements use different reference frameworks. The historic line remains a major visitor attraction at the observatory.