What is the difference between Nautical Mile and Knot?

Measurement of speed and distance is very crucial for navigation at sea. In old days, navigation was done by eyes only and now it’s automated with advanced technologies. Have you ever think about the distance and speed measuring units? Let’s see the distance and speed measuring units in a simple way:

Nautical mile

It’s a unit for measuring distance at sea.
Generally denoted by NM or nm or nmi.
1 nm = 1852 m = 1.852 km.
It is not recognized as SI unit.
It is different from mile or statue mile or simple mile. Generally, NM is 1.1508 times longer than mile.

Concept

Imagine our spherical earth and cut it into two half i.e. two hemisphere. We know that every point makes 360 degree angle. Now locate the centre of the hemispherical part of earth and divide into 360 part equally i.e. each sections are 1 degree. Each degree is equal to 60 minutes (1 degree = 60 minutes). Now, this 1 minute of the latitude is nothing but 1 NM. Simply, one nautical mile equals to one minute of latitude. Sometimes it is interpreted as one arc minute of any meridian on the globe.

History behind nautical mile

Navigation is one of oldest practice for sailing the ships. For safe navigation, many instruments and concepts of navigation were developed with the help of astronomy, geometry, celestial navigation etc. Before 1929, every country had different standard of nautical mile.
Initially there was no standard of measuring distance at sea. In 1929, a conference was organized by Extraordinary Hydrographic Conference in Monaco and concluded a standard unit and universal data for maritime operations.

Knot

  • It is a unit for measuring speed at sea.
  • It is also used in aviation, international laws and treaty regarding sea as a unit of speed.
  • It is nothing but the nautical mile per hour (i.e. 1 knot = 1 NM/hr).
  • Knot is in use since 17th century and in those days, common log was used for speed.

Nautical chart

  • For safe and easy navigation at sea, sailors developed a chart which is famously called as nautical chart.
  • It is a graphic representation of maritime area with advanced navigation information.
  • Mercator projection of map is used for developing the nautical chart. In this projection longitudes and latitudes crosses each other at right angle and makes rectangular grid which is useful in navigation.

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