En rouge le premier méridien de Mercator, en vert le méridien de Greenwich. © UNIL

The idea of a prime or zero meridian, serving as a reference for the other longitude lines on Earth, reflected geophysical considerations. Based on his investigation of magnetism, Gerardus Mercator set his prime meridian on Fuerteventura in the Canary Islands, 14.1° to the west of the Greenwich meridian used universally nowadays.