Which Greenwich-based line was adopted as the world’s prime meridian in 1884?

The Greenwich Meridian was adopted as the world’s prime meridian in 1884. It was defined as the line passing through the Airy Transit Circle at the Royal Observatory in Greenwich, London, and assigned longitude 0 degrees.

Delegates from 25 nations attended the International Meridian Conference in Washington, D.C., in October 1884. The conference selected Greenwich because British nautical charts and navigation systems were already widely used, and much of the world’s shipping depended on them. The decision made Greenwich the reference point for global longitude and standardized timekeeping.

Modern satellite-based geodesy defines the International Reference Meridian, which lies about 102 metres east of the historic Greenwich line at the observatory. Nevertheless, Greenwich remains the familiar cultural and historical reference for the prime meridian. The Paris Meridian and Ferro Meridian were earlier alternatives, but neither became the globally adopted standard.