A nautical mile (nmi, naut mi or NM) is absolutely not an Imperial measurement, although many think it is. Sometimes called a geographical or sea mile, the nautical mile is an SI "tolerated" unit of distance, still heavily used in aviation & shipping.

A nautical mile is obtained by dividing the circumference of the earth into 360 degrees and then further dividing these degrees into 60 minutes. Thus:

One nautical mile equals one minute, or 1/21,600 of the circumference of the earth.

So, if you travelled 60 nmi in a straight line, you will have moved one degree on the earth's surface, which may start to hint at this unit's continuing utility. More about that below.

1 nautical mile is set by convention at an average of 1,852 meters, or 6,076.11549 feet, which makes the nautical mile 1.852 kilometers or 1.150779 statute miles. This corresponds to an idealised, average circumference.

This may seem all rather odd and arbitrary to landlubbers, but it makes good sense at sea, or in the air above clouds, both locations being rather featureless as far as landmarks go; but in both locations latitude can be measured, and so one can find one's location on a chart without taking a bearing off a fixed (earthly) point. Having a unit that bears a direct relationship to the divisions on your chart, your compass, and your sextant (or modern equivalent) makes the calculations involved much simpler.

During the years when Brittania ruled the waves, the nautical mile was known as an Admiralty mile, and set by law at 6080 feet (1853.18 meters), exactly 800 feet longer than a British statute mile. In 1929 an international conference in Monaco, taking advantage of better data, redefined the nautical mile to be exactly 1852 meters or 6076.11549 feet, the distance I quote above. The U.S. held out until 1954. Before that time the U.S. nautical mile was equal to 6080.20 feet (1853.24 meters), but now the international nautical mile has been accepted.

No commentary on the nautical mile would be complete without mention of the knot, a unit of speed. 1 knot is 1 nautical mile per hour. So if a boat's top speed is given as "30 knots" then that is 30 nautical miles per hour, approx 35 mph, or 56 km/h. You can drive old Navy types quite insane by saying "knots per hour" which is, of course, nonsense unless you are coining some brand-new-never-before-used unit of acceleration.

While on the subject of knots, some care does need to be taken when piloting speedboats on inland waterways in the USA, as in an effort to avoid confusion, speed limit signs are marked in plain old miles per hour. If you have a boat with its speedo marked in knots, and expect speed signs to be likewise marked (as they are in harbours) this could result in a hefty fine. Elsewhere in the world most waterways are marked in knots. However, in another effort to avoid confusion, many older cheaper "ski-boat" type motorboats in these locations have their speedos marked out in km/h!

Sources:
http://bipm.org/
http://www.unc.edu/~rowlett/units/
www.boatsafe.com/

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