What line of longitude is traditionally defined as 0 degrees?

The Prime Meridian is the line of longitude traditionally defined as 0 degrees.

The modern internationally recognized reference meridian passes through the Royal Observatory in Greenwich, London. Longitude is measured east or west from this line, while latitude is measured north or south from the Equator. The Prime Meridian therefore provides the starting point for global longitude coordinates and time-zone calculations.

The International Meridian Conference in Washington, D.C., selected the Greenwich meridian as the world’s prime meridian in 1884. The choice reflected the widespread use of British nautical charts and the importance of Greenwich in navigation.

The astronomical reference used by modern satellite systems differs slightly from the historic line marked at Greenwich, by roughly 100 metres. Nevertheless, maps and everyday geographic explanations continue to identify Greenwich as the Prime Meridian.