The International Meridian Conference established Greenwich as the prime meridian in 1884.
Delegates from 25 nations met in Washington, D.C., in October 1884 to choose a common zero of longitude for navigation and timekeeping. They selected the meridian passing through the Airy Transit Circle at the Royal Observatory in Greenwich, London. The decision made Greenwich the reference point for measuring longitude east and west.
The conference also supported a universal day beginning at midnight and lasting 24 hours, although adoption of related time standards developed gradually. Greenwich Mean Time later became a major international reference, while Coordinated Universal Time now serves as the principal global time standard. The choice reflected Greenwich's established use by mariners and mapmakers, not a physical property that makes that longitude uniquely “zero.”