The story of Ferro Meridian

For centuries, many maps measured the world from this island. Already in the 2nd century A.D., Ptolemy considered placing the zero meridian at the western edge of the known world, so that maps had only eastern longitudes.

In 1634 France, ruled by Louis XIII and Cardinal Richelieu, decided that the meridian of Ferro, as El Hierro was called, should be the reference on maps, since the island was considered the westernmost point of the Old World. It was thought to lie exactly 20 degrees west of the Paris meridian, and many old maps carry Paris degrees along the top and Ferro degrees, offset by 20, along the bottom.

Later it was found that the island itself actually lies a little further, 20° 23′ 9″ west of Paris, but the Ferro meridian was still defined as 20 degrees. In the 1920s, Austria, Germany and Czechoslovakia adopted a value of 17° 40′ west of Greenwich for their survey networks.