If we want to define longitude in a geographic coordinate system, we need a prime meridian, a starting point where longitude, by definition, is 0º.
Unlike the Equator, the parallel at which latitude is 0º, the prime meridian is arbitrarily defined. As a consequence, it changed position many times, being placed on different parts of the globe by the several agreements stipulated throughout history.
Vincenzo Maria Coronelli (1650-1718), founder of the Academy of the Argonauts, the very first society devoted to Geography, wrote that “The ancient and modern geographers do not agree about the location of the prime meridian; among the ancients, Eratosthenes placed it at the Pillars of Hercules, Marinus of Tyre at the Fortunate Isles, Ptolemy in his Geography has followed the same opinion; but in his books on astronomy he moved it to Alexandria in Egypt. Among the moderns, Ismail Albufeda marks it in Cadiz, Alfonzo in Toledo, Pigafetta and Herrera have done the same; Copernicus places it at Freudenberg; Reinhold at Mount Royal or Königsberg; Kepler at Oranienburg; Longomontanus at Copenhagen; Lansberg at Goa; Riccioli at Bologna. The atlases of Janszoon and Blaeu at Mount Pico”. He himself situated the prime meridian in the most western part of El Hierro (Canary Islands), and we can add that Mercator initially preferred the Fuerteventura Island in the Canaries, and then El Hierro. Although not exhaustive and with some inaccuracies, as noted by Stevenson in his “Terrestrial and Celestial Globes”, this passage clearly illustrates that the problems in determining longitude started far before the attempts to measure it.
Father Vincenzo Maria Coronelli was a cosmographer, cartographer, publisher, encyclopedist, and the producer of precious atlases and globes.
During the 18th century, many countries in Europe established a prime meridian passing through their capitals to be used in national cartography. For example, the Meridian of Rome, in Italy, defined by the Jesuit and astronomer Father Angelo Secchi, the Paris Meridian in France that replaced the El Hierro Meridian chosen by Louis XIII and Cardinal Richelieu, or the famous Greenwich Meridian in England.
It was only at the International Meridian Conference of 1884 in Washington D.C., that the Greenwich Meridian was defined as the international reference of 0º longitude.
Although one would think that this put an end to the migration of the meridian, what modern satellite-navigation receivers indicate as 0º longitude is actually 102 meters away from the original Greenwich Meridian, where the International Terrestrial Reference Frame was introduced by the Bureau International de l’Heure in 1984. [Silvana Munzi]
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