A line to measure the world
Before 1884, the world lacked a universal prime meridian, the designated 0° longitude for maps and timekeeping. Nations used their own, with Paris serving as the prime meridian for French cartographers for over 200 years. This line was not arbitrary. On the summer solstice of 1667, members of the French Academy of Sciences laid out the north-south line that would precisely bisect the newly commissioned Paris Observatory. This building, still in use today, became the anchor for all French maps and geodetic science.
The meridian's location is now fixed at 2°20′14.025″ East of Greenwich. Inside the observatory, a brass line set into the floor of the Cassini Room traces the meridian's path, an instrument used by astronomers Jacques Cassini and his father Jean-Dominique Cassini to measure the sun's position with high precision. For more than two centuries, this line was the starting point for measuring the world, at least from a French perspective.
From meridian to meter
The Paris Meridian's most significant contribution to science was its role in defining the meter. During the French Revolution, the National Assembly sought to replace the chaotic, feudal system of measurements with a logical, universal standard. A commission from the French Academy of Sciences proposed a radical new unit of length: the meter, defined as one ten-millionth of the distance from the North Pole to the Equator, measured along the Paris Meridian.
To calculate this, an ambitious survey was launched from 1792 to 1798. Astronomers Jean-Baptiste Joseph Delambre and Pierre Méchain were tasked with measuring the arc of the meridian from Dunkirk on the English Channel to Barcelona, Spain. The seven-year mission was fraught with difficulty, including technical challenges and political turmoil that saw the astronomers imprisoned multiple times. Their data, however, allowed for the creation of a platinum bar, the "Mètre des Archives," which became the official standard for the new unit of length in 1799.
Rivalry, resistance, and remembrance
The global standard for longitude was settled at the 1884 International Meridian Conference in Washington, D.C. By a vote of 22 to 1, the world adopted the meridian passing through Greenwich, England, as the new prime meridian. France abstained from the vote and continued to use the Paris Meridian for its maps and timekeeping until 1911.
In 1994, a tribute to the Paris Meridian and the astronomer François Arago was created. Dutch artist Jan Dibbets designed an installation of 135 bronze medallions set into the city's streets, sidewalks, and even through buildings like the Louvre. These 12-centimeter discs trace the 9.2-kilometer path of the old meridian through Paris. Each medallion is marked with the name ARAGO and pointers for North (N) and South (S), records the scientific history beneath the city's streets.