The position of the prime meridian for the determination of longitude has often changed over time, keeping pace with the changing of geographical and nautical knowledge, and national interests: when the known world was bounded on the west by the "Pillars of Hercules", there was placed the prime meridian, later moved back to the "Fortunate Isles", i.e. the Canaries.
With the advance of navigation in the Atlantic, discovering that there was no western border, astronomers placed the prime meridian at different locations: Copernicus placed it at Freudenburg, and Kepler at Uraniborg. Mercator initially preferred Fort Ventura Island in the Canary Islands and then the Hierro Island, while the next century GB Riccioli placed it in Mallorca.
To settle uncertainties, Louis XIII, by a decree of 1 July 1634, established it at the Hierro Island. However, a century later maritime nations again adopted different references: the British chose the meridian of London, the French Paris, the Dutch the Peak of Tenerife - considered the highest mountain in the World - and the Spanish the Azores or the Peak of Tenerife.
By mid-19th century most European countries had adopted the meridian passing through the National Astronomical Observatory, the British and the countries under British influence had adopted Greenwich, while the pre-unitary states of Italy and the United States had chosen the meridian of Paris.
At the end of the century, international communications having increased, geographers strove hard to reach the unification of longitude and time: in 1881, at the 3rd International Geographical Congress, the proposal was put forward to adopt Greenwich meridian and Mean Time, but the issue involved questions of prestige and prominence among states, and was not resolved for another few years.
It was re-examined in 1883 at the International Geodetic Conference in Rome, while the United States decided to adopt the meridian of Hierro and Germany opted for the meridian of Central Europe, 15 ° E from Greenwich, in 1893 adopted by Italy too.
By 1884, at the International Geographical Congress in Washington, the proposal gained the assent of almost all nations present, and was finally ratified in 1912 at the First International Hour Conference.
On February 23, 2007 in the Vatican Gardens a plaque was inaugurated, indicating the route of the Prime Meridian of Italy.
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