Greenwich is the London district associated with the traditional reference line for the prime meridian.
The Royal Observatory, Greenwich, was founded in 1675 to improve astronomical observations and navigation. Its meridian became the internationally recognized zero of longitude after the International Meridian Conference in Washington, D.C., in 1884.
The reference line traditionally passes through the Airy Transit Circle at the observatory. Modern satellite-based geodesy places the internationally defined IERS Reference Meridian about 102 metres east of that historic line, because Earth’s shape and coordinate systems are measured more precisely today.
Greenwich Mean Time was developed from observations made at the Royal Observatory and became central to global timekeeping. Coordinated Universal Time, or UTC, is now the principal international time standard, while GMT remains a familiar civil-time term. The observatory is part of the Royal Museums Greenwich complex and is also known for the Harrison marine chronometers.