The equator is set by the planet’s shape, but zero longitude has to be chosen. In the second century CE Ptolemy put it at the Fortunate Isles, the western edge of the known world, so every coordinate stayed positive, and the Indian Surya Siddhanta ran its meridian through Ujjain. In 1493 Christopher Columbus found a line in the Atlantic where his compass pointed to true north, and the hope of a magnetic zero shaped the 1494 Treaty of Tordesillas and Gerardus Mercator’s 1541 globe, until navigators learned magnetic lines curve and drift. Nations then drew their own meridians through Paris, Berlin, Copenhagen, Rome, and Washington. Astronomer Royal Nevil Maskelyne’s Nautical Almanac, which precomputed lunar distances from Greenwich, became the standard at sea, and at the 1884 International Meridian Conference 22 of 25 nations chose Greenwich while France abstained and used Paris until 1911.

George Airy’s 1851 Transit Circle defined that line, but it was aligned by a plumb line pulled slightly sideways by local terrain. Satellite laser ranging and very long baseline interferometry, which listens to distant quasars, located the Earth’s center of mass, and by 1984 the resulting IERS Reference Meridian sat 5.3 arc seconds, about 102 meters, east of the brass line where tourists pose. Other worlds need their own zeros: the crater Airy-0 on Mars, the crater Hun Kal on Mercury, the planet facing side of tidally locked moons, and on Jupiter the rotation of its magnetic field in System III coordinates.

• French translators of Maskelyne’s tables crossed out Greenwich and wrote Paris while keeping his math.
• French delegate Pierre Janssen argued for a neutral meridian such as the Bering Strait or the Azores.
• The Greenwich observatory drifts about 2.5 centimeters a year toward the satellite defined line on the Eurasian plate.
• Hun Kal sits where Mercury’s sun stops and briefly reverses at noon, the planet’s hottest longitude.
• Today’s reference meridian passes through the UK, France, Spain, Algeria, Mali, Burkina Faso, Togo, and Ghana.