Days of Yore
Embryonic odometer: a knotted line, a log
AMSTERDAM – How long is a nautical mile? It was not until 1929 when an international conference in Monaco answered that question: 1851.8 meters. Which makes it a tad shorter than the old English nautical mile (1,853 meters) and a tad longer than an English land mile (1,609 meters). The notion of nautical miles stems from the introduction of a log-and-line navigation system in the 16th century.
Seafarers’ embryonic odometer seems to have originated in England ca. 1574. A slab of wood, a log, was weighted on one side to make it float upright and attached to a rope with knots at even intervals. When tossed overboard, the crew counted the knots passing through their hands over 15 or 30 seconds. A ship’s speed is still reflected in knots /hr. 1 knot = 1 nautical mile. The log-and-line system was long ignored by Dutch crews who stuck to what they knew best: guesswork. Only by 1655 did log-and-line calculations come into in general use on Dutch East India Company ships. Still chaos reigned. When Napoleonic France opted for the metric system in 1795, a nautical mile was fixed at just under 1852 meters. But the “admiralty mile” of the English navy was judged to be just over 1853 meters.
Today’s nautical mile is based on this calculation: the earth has 360 degrees of longitude. At the equator, 1 degree of longitude is 40,000 (earth’s circumference in kms) : 360 = 111.11 km. One degree of longitude is divided into 60 minutes of 1,851.8 m each, i.e. the length of nautical mile.
photo: Eddo Hartmann / www.hetscheepvaartmuseum.nl