How far is a nautical mile? Surprisingly, it was not until 1929 that a body of countries agreed in Monaco that the sea mile (officially, the international nautical mile) measures 1,851.8 metres.
This makes today's official nautical mile slightly shorter than the former English nautical mile (1,853 metres) but longer than an English land mile (1,609 metres). So what about before 1929?
Counting knots
Research has shown that the emergence of the nautical mile as a unit of measurement at the end of the 16th century was closely tied in with the introduction of the log-and-line used on board ships to measure the distance the ship travelled (something you definitely need to know to figure out where you are). And that a professor at Leiden University, Willebrord Snellius (1580-1626), was instrumental in this development.
One of the tools that ships of the East India Company had on board for navigation was the log-and-line. But this instrument was only introduced on East Indiamen relatively late in history.
The log-and-line seems to have originated in England around 1574. It is a round or triangular wooden plank, the ’log,’ weighted with lead on one side to make it float upright in the water, and attached to a long rope, the line.
This line had knots tied in it at regular intervals. When tossed overboard from a moving ship, the drag of the log would hold it in place, and the crew could count the number of knots that the log line drew out over a given period of time, a quarter or a half of a minute. The distance the ship travelled in a twenty-four hour period could be calculated based on the "number of knots’ the ship was travelling.
Guesswork on the middle of the ocean
But it would be decades before the log was used on Dutch ships. Dutch sailors preferred methods with which they were familiar: guesswork. That only changed when a new method of dividing the log line emerged. Since then, the "knot" as a measure of speed has represented one mile per hour. It was in 1615 that scientist Willibrord Snellius calculated one degree of latitude on the Earth at 22,800 Rhenish rods (by taking a measurement of the longitude between Alkmaar and Bergen-op-Zoom). That number divided by 15 equalled one German mile. He published his findings in Latin under the title Eratosthenes Batavus. Snellius performed this calculation again in 1622.
His findings were used in the East India Company's navigation lessons. The new calibration of the log line based on the German mile made it much easier to determine the distance travelled. Even still, it would not be until 1655 that the log-and-line and calculations in knots or nautical miles were in general use on East India Company ships.
The ‘nautical mile’ became the international standard
By 1790, the log was being used on both East India Company ships and the Dutch war fleet. Of course, there were other instruments in use at this time to determine a ship's position at sea, such as the octant and the quadrant. But the use of the log-and-line and ’dead reckoning’ remained important. In the meantime, different countries had come to adopt their own definition of the nautical mile, and there were many different ones in use.
When France introduced the metric system in 1795, the English nautical mile was fixed at just under 1,852 metres, even though the "admiralty mile" or "nautical mile" used by the English navy measured slightly over 1,853 metres. This difference may seem minor, but it adds up dramatically over great distances.
All this finally changed in 1929, when the nautical mile was officially established by international agreement at 1,851.8 metres.
The earth counted around
Today's international nautical mile is based on the following calculation: The Earth is divided into 360 degrees of longitude. At the equator, one degree of longitude is calculated at 40,000 divided by 360, or 111.11 km. One degree of longitude is subdivided into 60 minutes of longitude, so one minute of longitude is 1,851.8 m.
