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| Meridians | |
|---|---|
| Name | Meridians |
| Type | Geographic coordinate |
| Region | Global |
Meridians are imaginary longitudinal lines that connect the geographic poles and serve as references for position, time, navigation, and cartography. They underpin global systems of measurement and governance by linking places such as Greenwich, Paris, Washington, D.C., Beijing, and Tokyo to standards in mapping, astronomy, and international agreements. Their adoption involves institutions like the Royal Observatory, Greenwich, the International Meridian Conference, the Geodetic Survey of India, and the U.S. Naval Observatory.
The term derives from Latin meridies as used by figures such as Ptolemy and commentators from the Renaissance like Gerardus Mercator and Eratosthenes, and was formalized in works by James Bradley and Jean Picard. Definitions adopted by bodies like the International Astronomical Union and the International Hydrographic Organization describe meridians as great circles on the Earth that pass through the North Pole and South Pole, analogous in celestial practice to lines used by Nicolaus Copernicus, Tycho Brahe, and Johannes Kepler in star catalogs. Legal codifications in treaties such as those negotiated by delegations from Great Britain, United States, France, and Spain at conferences influenced the term's usage in cartographic law.
Geographic meridians are measured as angular distances east or west of a chosen prime reference, a convention implemented in mapping projects by institutions like the Ordnance Survey, the National Geographic Society, and the Institut Géographique National. Cartographers such as Alexander von Humboldt and John Walker used meridians to produce globes and maps for explorers including James Cook and Ferdinand Magellan, while modern geodesists at NASA, the U.S. Geological Survey, and the European Space Agency use satellite networks like GPS (Global Positioning System), GLONASS, and Galileo (satellite navigation) to determine longitude with reference frames maintained by the International Earth Rotation and Reference Systems Service.
The choice of a prime meridian established time standards that enabled synchronization of railroads and telegraphs in the 19th century, influencing administrations such as the British Empire, the United States Congress, and the French Third Republic. The 1884 International Meridian Conference in Washington, D.C. codified the Greenwich meridian for international navigation and for adoption by systems like Railroad time in the United States and the later Coordinated Universal Time managed with input from the International Bureau of Weights and Measures and the International Telecommunication Union. Time zones implemented by governments including those of Russia, China, and India reflect political decisions about the prime meridian's implications.
Meridians evolved from ancient geographic grids in works by Ptolemy and measurement campaigns by Eratosthenes to medieval Islamic scholars such as Al-Battani and Ibn al-Shatir, then through Renaissance mapmakers like Gerardus Mercator and Abraham Ortelius. The rise of national surveys—Great Trigonometrical Survey, Cassini family, and the Triangulation of France—produced meridian arcs measured by scientists including Jean Picard, Pierre-Simon Laplace, and Alexis Clairaut. Instruments from makers like John Hadley and observatories including the Paris Observatory and the Royal Observatory, Greenwich enabled improved longitude estimates used in seminal atlases by Aaron Arrowsmith and Thomas Jefferson's survey commissions.
Practical navigation depended on accurate determination of longitude via chronometers by innovators such as John Harrison and marine chronometer makers including Thomas Mudge and institutions like the Board of Longitude. Coastal mapping and cadastral surveying carried out by agencies including the Hydrographic Office (United Kingdom), the U.S. Coast and Geodetic Survey, and the Survey of India used meridians for triangulation, boundary demarcation, and tide tables produced from data by explorers like Matthew Flinders and naval officers such as James Rennell. Modern surveying integrates meridians with technologies developed by Warren M. Carey and satellite geodesy programs including TOPEX/Poseidon and GRACE.
Meridians have been symbols and tools in imperial expansion by powers like the British Empire, Spanish Empire, and Dutch East India Company, and they have been implicated in territorial disputes involving actors such as Brazil, Argentina, Chile, and France (overseas territories). Choices of prime meridians influenced national identity and administration in capitals like Paris, London, and Washington, D.C., and informed cartographic propaganda in works by national academies including the Académie des Sciences and the Royal Society. Debates over standardization intersected with international law administered by forums like the League of Nations and later the United Nations.
Astronomers at institutions such as the Harvard College Observatory, the Royal Observatory, Greenwich, and the Uppsala Astronomical Observatory used meridians for transit observations that determined sidereal time and planetary ephemerides compiled by Simon Newcomb and Urbain Le Verrier. The adoption of celestial reference frames like the International Celestial Reference Frame ties terrestrial meridians to quasars cataloged by surveys such as the Very Long Baseline Array and agencies including CERN and ESO. Geophysical studies of Earth rotation, precession, and polar motion conducted by the International Earth Rotation Service and researchers like Walter Munk link meridian definitions to models used by IPCC-affiliated climate science and oceanography projects managed by organizations including the National Oceanic and Atmospheric Administration.