page_title: Knot (unit)
section_title: Modern use
hierarchical_section_title: Knot (unit) / Modern use
caption_attribution_description: English: The graphical scale from a DMA (now NGA) world map using the Mercator projection. The original map, in six pieces, measures 116 x 86 inches (2.9 x 2.2m). Simplified and reduced size on second try.
context_page_description: The knot is a unit of speed equal to one nautical mile per hour, exactly 1.852 km/h. The ISO standard symbol for the knot is kn. The same symbol is preferred by the Institute of Electrical and Electronics Engineers; kt is also common, especially in aviation, where it is the form recommended by the International Civil Aviation Organization. The knot is a non-SI unit. The knot is used in meteorology, and in maritime and air navigation. A vessel travelling at 1 knot along a meridian travels approximately one minute of geographic latitude in one hour.
  Etymologically, the term derives from counting the number of knots in the line that unspooled from the reel of a chip log in a specific time.
context_section_description: 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.
  Standard nautical charts are on the Mercator projection and 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.
caption: Graphic scale from a Mercator projection world map, showing the change with latitude
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