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A nautical mile (symbol M, NM or nmi) is a unit of distance, set by international agreement as being exactly 1,852 meters (about 6,076 feet). Historically, it was defined as the distance spanned by one minute of arc along a meridian of the Earth (north-south), and developed from the sea mile and the related geographical mile. It is a non-SI unit (although accepted for use in the International System of Units by the BIPM), and is commonly used in international law and treaties, especially regarding the limits of territorial waters. The nautical mile is also generally used by navigators at sea and in the air,〔The United States Navy and the US Army Air Force officially adopted the nautical mile and the knot as their standard aeronautical unts for distance and speed on 26 July 1946. (Air & Space/Smithsonian, June/July 1987, p. 27)〕 and also in polar exploration, and remains in use worldwide because of its convenience when working with charts.〔 Most nautical charts use the Mercator projection whose scale varies by about a factor of 6 from the equator to 80° latitude, so charts covering large areas cannot use a single linear scale.〔 The nautical mile is nearly equal to a minute of latitude on a chart, so a distance measured with a chart divider can be roughly converted to nautical miles using the chart's latitude scale. ==Definition== The international nautical mile was defined by the First International Extraordinary Hydrographic Conference, Monaco (1929) as exactly 1,852 meters.〔 This is the only definition in widespread current use, and is the one accepted by the International Hydrographic Organization and by the International Bureau of Weights and Measures (BIPM). Before 1929 different countries had different definitions, and the Soviet Union, the United Kingdom and the United States did not immediately accept the international value. The Imperial and U.S. definitions of the nautical mile were based on the Clarke (1866) Spheroid: they were different approximations to the length of one minute of arc along a great circle of a sphere having the same surface area as the Clarke Spheroid.〔 The United States nautical mile was defined as 1,853.248 meters〔Aside from rounding this is the exact length of a great-circle minute on a sphere of radius 6,370,997.2406 meters, which is the sphere that has the same area as the Clarke 1866 spheroid as usually defined.〕 (6,080.20 U.S. feet, based on the definition of the foot in the Mendenhall Order of 1893): it was abandoned in favour of the international nautical mile in 1954.〔 The Imperial (UK) nautical mile, also known as the Admiralty mile, was defined in terms of the knot, such that one nautical mile was exactly 6,080 international feet (1,853.184 m):〔 it was abandoned in 1970〔 and, for legal purposes, old references to the obsolete unit are now converted to 1,853 meters exactly.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Nautical mile」の詳細全文を読む スポンサード リンク
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