What is a Nautical Mile?

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  • Written By: Mary McMahon
  • Edited By: Bronwyn Harris
  • Last Modified Date: 07 October 2019
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A nautical mile is a unit of measurement used in aeronautics and maritime navigation. Some people also prefer to refer to a nautical mile as a sea mile, maritime unit, or sea unit, in a reference to the maritime usage of the measurement. When speed is given in nautical miles, the correct term is “knots,” as in “the ship traveled at 23 knots,” indicating that the ship was moving at a rate of 23 nautical miles per hour.

Technically, a nautical mile is equal to the length of one minute of arc in a great circle. If this statement has caused you to blink furiously in confusion, think about the Earth as a large orange. If you cut the orange in half along the middle, or equator, you have bisected the orange along one of its “great circles.” There are numerous other great circles, as a great circle is any line around a sphere which can be traced to cut the sphere precisely in half. There are 360 degrees in a great circle, and each degree can be further broken down into 60 minutes, sometimes called minutes of arc. Therefore, the Earth measures 21,600 nautical miles around the equator.


After much international debate, it has been agreed that a nautical mile is equal to 6,076 feet (1,852 meters). International agreement on the measurement of a nautical mile is important, since many treaties and agreements include nautical miles as a unit of measurement. Furthermore, it ensures smooth navigation between various nations, which is especially important for major trading partners. As a result, a nautical mile is recognized within the framework known as the International System of Units (SI), which is a system of measurements which have been clearly defined and agreed upon by most countries in the world.

Usage of nautical miles varies around the world. Since a nautical mile is not technically an SI measurement, some nations prefer to use distances which are recognized under this system, which prefers metric measurements like meters and kilometers to measurements such as inches and feet. In some regions, the nautical mile is heavily used and understood by seafarers, while other areas, distances may be expressed in other measurements, especially when an international team is cooperating on a project.

For anyone who is dying to know about the origin of “knot” as a term for a unit of speed in maritime navigation, the word is closely linked to the technique sailors used to employ to measure their speed. A weighted line would be thrown out from a ship by one sailor while another held a timer. Knots in the line were placed at precise lengths, so that the sailor could count off the knots to figure out how far the ship had traveled in a set amount of time, typically 30 seconds. This measurement could be extrapolated into an estimate of the overall speed of the ship, and while more precise navigational tools are used now, “knots” for speed continues to be used.


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Post 5

so what's the origin of the nautical mile?

Post 3

If a nautical mile is squared is it a nautical square mile, or a square nautical mile?

Post 2

LOL. We're both right now. The original article read "21,600 miles", not "21,600 nautical miles". Thanks for making the change.

Post 1

Gee I always thought the diameter of Earth was around 24,900 miles not 21,600 as in paragraph two. I think you used 5280 feet (mile) instead of 6076 (nautical mile) in your calculation.

Moderator's reply: We're both right! The earth is around 24,900 miles around the equator — 24,857 miles, if you want to be exact. But it's 21,600 nautical miles.

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