greenwich020 Ladies and Gentlemen, here where you stand lies the Greenwich Meridian. The Prime Meridian (as it's sometimes called) is an imaginary line running north-south through Greenwich. In 1884 the line was named as the world's Longitude Zero by the International Meridian Conference. Every position on earth is defined by its longitude (its distance east or west from Greenwich) and its latitude (the distance north or south of the Equator). Both latitude and longitude are measured in segments of a circle: degrees, minutes and seconds.