Knot (unit)
Modern use

Although the unit knot does not fit within the SI system, its retention for nautical and aviation use is important because the length of a nautical mile, upon which the knot is based, is closely related to the longitude / latitude geographic coordinate system. As a result, nautical miles and knots are convenient units to use when navigating an aircraft or ship.

On a standard nautical chart using Mercator projection, the horizontal (East–West) scale varies with latitude. On a chart of the North Atlantic, the scale varies by a factor of two from Florida to Greenland. A single graphic scale, of the sort on many maps, would therefore be useless on such a chart. Since the length of a nautical mile, for practical purposes, is equivalent to about a minute of latitude, a distance in nautical miles on a chart can easily be measured by using dividers and the latitude scales on the sides of the chart. Recent British Admiralty charts have a latitude scale down the middle to make this even easier.

Speed is sometimes incorrectly expressed as "knots per hour", which would mean "nautical miles per hour per hour" and thus would refer to acceleration.
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