nautical mile

nautical mile, the unit of distance used at sea, practically speaking a minute of latitude. The publication in London in 1637 of The Seaman's Practise by the English surveyor Richard Norwood, who had measured an arc of the meridian, effectively marked the distinction between the land mile and the sea mile. ‘There are’, he wrote in his preface, ‘a greater number of feet contained in a degree than the common opinion that a thousand paces (of 5 feet) make a mile.’ If the earth were a perfect sphere, a nautical mile would, of course, measure an arc length of one minute at all places and in all directions; but it is an oblate spheroid so that the nautical mile varies slightly with the latitude, and is shortest at the equator and longest at the poles. Norwood proposed a standard length of 6,120 feet (1,867 m) which he later changed to 6,000 feet (1,830 m) on the grounds that ‘every man desires to have his reckoning something before the ship so that he fall not in with a place unexpected’. In other words the short mile was safer.

By 1730 the nautical mile as a minute of a great circle on the earth was well established. The question then arose as to which figure of the earth should be adopted. The difficulty was that although there may have been agreement about the figure of the earth there remained the problem that different countries and authorities adopted their own values for the land mile, the foot, and the metre. Finally in 1924 the International Hydrographic Bureau proposed a figure for an International Spheroid which was generally accepted and in 1929 the Bureau recommended a standard length of 1,852 metres (6,076 English ft) for the nautical mile which is now the accepted figure. Errors arising from the use of the standard nautical mile rather than its value at the latitude in which the ship lies are of no navigational significance. Thus for the seaman the nautical mile remains a minute of latitude. Moody, A. , ‘Early Units of Measurement and the Nautical Mile’, Journ. Inst. Navigation, 5 (1952), 262.

Mike Richey