The prime meridian is the north-south line of longitude defined as 0 degrees, the starting point for measuring longitude east and west and the line that divides Earth into the Eastern and Western Hemispheres. Unlike the equator, its position is a human choice: in 1884 delegates at the International Meridian Conference in Washington, D.C., voted to adopt the meridian through the Airy Transit Circle telescope at the Royal Observatory in Greenwich, London, largely because nearly two-thirds of the world’s ships already used charts based on it.
Today the official zero line is the IERS Reference Meridian (also called the International Reference Meridian), which runs about 102.5 meters east of the historic Greenwich line. That is why a phone using satellite navigation does not read exactly 0 degrees when you stand on the marked line in the observatory courtyard. From pole to pole, the meridian passes through the UK, France, Spain, Algeria, Mali, Burkina Faso, Togo, Ghana and Antarctica.
Source: Royal Museums Greenwich, “What is the Prime Meridian, and why is it in Greenwich?”.
Image: Tourists at the Prime Meridian line, Royal Observatory, Greenwich, photo by Daniel Case, CC BY-SA 3.0, via Wikimedia Commons.