Which meridian was internationally chosen as the prime meridian in 1884?

The Greenwich meridian was internationally chosen as the prime meridian in 1884.

Delegates at the International Meridian Conference in Washington, D.C., voted to adopt the meridian passing through the transit instrument at the Royal Observatory in Greenwich, England, as the starting line for measuring longitude. The conference met in October 1884.

The decision reflected Greenwich’s widespread use in nautical charts and timekeeping, especially by British shipping. The adopted meridian defined zero degrees longitude and provided a common reference for global maps, navigation, and time-zone calculations. The conference also supported a universal day beginning at midnight.

Modern geodesy uses the International Reference Meridian, which lies about 102 metres east of the historic Greenwich line because satellite-based coordinate systems model Earth differently. Nevertheless, “Greenwich meridian” remains the conventional historical answer, and Greenwich continues to symbolize zero longitude.