Longitude needs a starting line. The prime meridian is the arbitrarily chosen zero-degree line of longitude that runs from the North Pole through Greenwich, England, to the South Pole. All other meridians are measured east or west from it.

The Greenwich meridian was selected at the International Meridian Conference of 1884, largely because of the dominance of British shipping and charts at the time. It also defines Coordinated Universal Time (UTC). The line itself is marked at the Royal Observatory in Greenwich and can be visited by the public.

Other meridians have been used historically, and some countries still employ local prime meridians for national mapping. For global purposes, however, the Greenwich meridian remains the universal reference. Standing on the brass strip at Greenwich, a visitor straddles the Eastern and Western Hemispheres.

• Zero-degree line of longitude
• Passes through Greenwich, England
• Reference for global longitude and time
• Established by international agreement in 1884