Which line of longitude was chosen as the prime meridian in 1884?

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

Delegates at the International Meridian Conference in Washington, D.C., selected the meridian passing through the centre of the transit instrument at the Royal Observatory in Greenwich, England. It became the reference for measuring longitude east and west, with longitude values extending to 180 degrees in either direction.

The decision reflected practical realities of 19th-century navigation. By the conference, most nautical charts and many ships already used Greenwich as their reference, largely because Britain’s maritime influence and the widespread use of British Admiralty charts had made it common internationally.

The modern International Reference Meridian, used by satellite-based systems such as GPS, does not pass exactly through the historic Greenwich line. It lies about 102 metres east of it because modern geodesy uses a global reference frame. Nevertheless, Greenwich remains the conventional prime meridian in geography, timekeeping history, and public understanding.