Nautical mile and KNOT

A nautical mile is a unit of length. It is not an SI unit. The nautical mile is used around the world for aviation purposes.

Definition

The international standard definition is: 1 nautical mile = 1852 metres exactly.

Unit symbol

There is no official international standard symbol for the unit nautical mile. The symbols NM, nm and nmi are commonly used in some areas.

Conversions to other units

1 nautical mile converts to:


[*] 6076.1155 feet
[*]1.852 km (exact)

History

The nautical mile was historically defined as a minute of arc along a great circle of the Earth. It can therefore be used for approximate measures on a meridian as change of latitude on a nautical chart. However, like all planets, the Earth is not a perfect sphere. It bulges at the equator like a spinning top, so the length of one minute of arc on the Earth's surface varies from 1862 m at the poles to 1843 m at the Equator. The Earth's surface also has bumps and hollows like a potato. Thus, there is no fixed relationship between angle and arc length along the Earth's surface; one minute of arc can vary in length by tens of metres.
International agreement was achieved in 1929, when the International Extraordinary Hydrographic Conference, Monaco adopted a definition of 1 international nautical mile = 1,852 metres, very close to the average length of one minute of latitude i.e. one minute of arc along a line of longitude (a meridian).

Associated units

The derived unit of speed is the KNOT, defined as one nautical mile per hour.
The term "knot" derived from the practice of using a knotted rope as a method of gauging speed of a ship. The rope would be thrown into the water and the rope trailed behind the ship. The number of knots that passed off the ship and into the water in a given time would determine the speed in "knots".