Paris meridian

The Paris meridian is a meridian line running through the Paris Observatory in Paris, France—now longitude 2°20′14.03″ east. It was a long-standing rival to the Greenwich meridian as the prime meridian of the world.

In the year 1634, France ruled by Louis XIII and Cardinal Richelieu decided that the Ferro meridian should be used as the reference on maps, since this island is the most western position of the Old World. It was also thought to be exactly 20 degrees west of Paris.

A French astronomer, Abbé Jean Picard, measured the length of a degree of latitude and computed from it the size of the Earth during 1669–1670. In 1666, Louis XIV of France had authorized the building of an observatory in Paris to measure longitude. On Midsummer's Day 1667, members of the Academy of Sciences traced the future building's outline on a plot outside town near the Port Royal abbey, with Picard's meridian exactly bisecting the site north-south. French cartographers would use it as their prime meridian for more than 200 years.

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. It was later found that the actual island of El Hierro itself is in fact 20° 23' 9" west of Paris, but the Ferro meridian was still defined as 20 degrees west of Paris.

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