Ferro Meridian

The line of longitude running through El Hierro (Ferro) was known in European history as the prime meridian in common use outside of the future British Empire. Already in the 2nd century A.D., Ptolemy considered a definition of the zero meridian based on the western-most position of the known world, giving maps with only positive (eastern) longitudes. In the year 1634, France ruled by Louis XIII and Richelieu decided that Ferro's meridian should be used as the reference on maps, since this island was considered the most western position of the Old World. Flores Island lies further west, but the Azores were not discovered by Europeans until the early 15th century, and their identification as part of the Old World is uncertain. It was thought to be exactly 20 degrees west of the Paris meridian, so indeed the exact position of Ferro was never considered. Old maps (outside of Anglo-America) often have a common grid with Paris degrees at the top and Ferro degrees offset by 20 at the bottom. Louis Feuillée also worked on this problem in 1724.