11/10/2023 0 Comments Kilometers to nautical milesBy the mid-19th century, France had defined a nautical mile via the original 1791 definition of the metre, one ten-millionth of a quarter meridian. France and other metric countries state that in principle a nautical mile is an arcminute of a meridian at a latitude of 45°, but that is a modern justification for a more mundane calculation that was developed a century earlier. Since the Earth is not a perfect sphere but is an oblate spheroid with slightly flattened poles, a minute of latitude is not constant, but about 1,861 metres at the poles and 1,843 metres at the Equator. In 1637, Robert Norwood proposed a new measurement of 6120 feet for an arcminute of latitude, which was within 44 feet of the currently accepted value for a nautical mile. The ratio of 60 miles, or 20 leagues, to a degree of latitude remained fixed while the length of the mile was revised with better estimates of the earth’s circumference. Both Gunter and Oughtred put forward the notion of dividing a degree into 100 parts, but their proposal was generally ignored by navigators. In 1633, William Oughtred suggested 349,800 feet to a degree (5830 feet per arcminute). In 1624 Edmund Gunter suggested 352,000 feet to a degree (5866 2/3 feet per arcminute). In the early seventeenth century, English geographers started to acknowledge the discrepancy between the angular measurement of a degree of latitude and the linear measurement of miles. However, these referred to the old English mile of 5000 feet and league of 15,000 feet, relying upon Ptolemy's underestimate of the Earth's circumference. In 1574, William Bourne (mathematician) stated in A Regiment for the Sea the "rule to raise a degree" practiced by navigators: "But as I take it, we in England should allowe 60 myles to one degrée: that is, after 3 miles to one of our Englishe leagues, wherefore 20 of oure English leagues shoulde answere to one degrée." Likewise, Robert Hues wrote in 1594 that the distance along a great circle was 60 miles per degree. Eventually, the ratio of 60 miles to a degree appeared in English in a 1555 translation of Pietro Martire d'Anghiera's Decades: "He assigned likewise to every degree three score miles." īy the late 16th century English geographers and navigators knew that the ratio of distances at sea to degrees was constant along any great circle (such as the equator, or any meridian), assuming that Earth was a sphere. Whether a correction or convenience, the reason for the change from 62-1/2 to 60 miles to a degree is not explained. et latitud sub equinortiali formet stadia 500 que fanut miliaria 62-1/2" (one degree longitude and latitude under the equator forms 500 stadia, which make 62-1/2 miles). Notably, an earlier manuscript map by Nicolaus Germanus in a previous edition of Geography (Ptolemy) stated "unul gradul log. The earliest reference of 60 miles to a degree is a map by Nicolaus Germanus in a 1482 edition of Geography (Ptolemy) indicating one degree of longitude at the Equator contained "milaria 60". Navigation at sea was done by eye until around 1500 when navigational instruments were developed and cartographers began using a coordinate system with parallels of latitude and meridians of longitude. The word mile is from the Latin phrase for a thousand paces: mille passus. History Visual comparison of a kilometre, statute mile and nautical mile It conflicts with the SI symbol for nanometre. Government Coast Pilots and Sailing Directions. nm is a non-standard abbreviation used in many maritime applications and texts, including U.S.M is used as the abbreviation for the nautical mile by the International Hydrographic Organization.nmi is used by the Institute of Electrical and Electronics Engineers and the United States Government Publishing Office.NM is used by the International Civil Aviation Organization.There is no single internationally agreed symbol, with several symbols in use. Unit symbol Historical definition – 1 nautical mile The derived unit of speed is the knot, one nautical mile per hour. Today the international nautical mile is defined as 1,852 metres (about 6,076 ft 1.151 mi). Historically, it was defined as the meridian arc length corresponding to one minute ( 1 / 60 of a degree) of latitude at the equator, such that Earth's polar circumference is very near to 21,600 nautical miles (that is 60 minutes × 360 degrees). Nautical mileĪ nautical mile is a unit of length used in air, marine, and space navigation, and for the definition of territorial waters. unit based on the Equator, see geographical mile. For distance measurement on land, see mile. This article is about distance measurement at sea.
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