Prime Meridian

The Prime Meridian (Longitude = 0°) and the opposite 180th meridian (Longitude = 180°), which the International Date Line follows, divide the earth in a Eastern and Western hemisphere. It was established by Sir George Airy in 1851 and was determined as the international standard at the International Meridian Conference (Washington, D.C.) in October 1884. This map shows the Royal Greenwich Observatory in London and some important indications of the Prime Meridian.

See also: https://www.glotter.com/globetrotter/countries-crossed-by-the-prime-meridian