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The Wiktionary defines longitude as 1) an angular distance measured west or east of the prime meridian or as 2) any imaginary line perpendicular to the equator and part of a great circle passing through the North Pole and South Pole. [1]

By determining longitude and latitude, a person can determine precisely where they are on the surface of the globe.

Historically it was much easier to determine latitude than it was to determine longitude. This difficulty in measuring longitude is caused by the rotation of the earth. The longitude problem was eventually solved by the invention of an accurate chronometer in the late 1700's by John Harrison.[2]

After the invention of the chronometer it was possible for a navigator to determine “Longitude by Chronometer” which is measured by determining the difference between the time at the ship's location and the time at a known reference line of longitude as determined by the chronometer. [3]
After the invention of the chronometer different map makers, navies and navigators used different reference lines of longitude. The Royal Navy used the line of longitude passing through Greenwich. In 1884, the International Meridian Conference adopted the Greenwich meridian as the universal prime meridian or zero point of longitude. [4]


  1. http://en.wiktionary.org/wiki/longitude
  2. Sobel, Dava. Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time. Forth Estate London, 1995 ISBN 1-85702-571-7
  3. Smyth, W. H. (William Henry), 1788-1865 Admiral. The Sailor's Word~Book. Blackie and Son, Paternoster Row, 1867 Reprinted by Algrove Publishing Limited Almonte, ON Canada 2004. ISBN 1-897030-05-3
  4. http://en.wikipedia.org/wiki/Longitude#History
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