Eli5: What’s the difference between a mile and a nautical mile
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The other answers have done a good job explaining the difference.
Some other points about the nautical mile:
1) The speed related to nautical miles is nautical miles per hour. This speed is also known as knots. The term knots historically refers to the practice of a ship-based device for measuring speed with knots tied in a rope. Regardless, the nautical mile is very useful for navigation for both ships and aircraft. It allows distances to be directly correlated with degrees latitude, since 60 nautical miles is almost exactly one degree of latitude.
2) Nautical miles are roughly 6000 feet and 2000 yards. These are very convenient numbers for doing quick calculations in your head for things like navigation or target analysis. For example, a ship traveling at 5 knots will go 500 yards in 3 minutes. This is known as the 3 minute rule which says that the distance a ship will travel in yards in 3 minutes is speed in knots times 100. This isn’t the kind of calculation you do for a science project. It’s the kind of practical math that you do when you’re in a saturated or stressful environment and you need to quickly assess and process a lot of information.
3) Despite what I and others have told you, there is actually a defined size for the nautical mile. It is roughly 6076 feet. It is EXACTLY 1852 meters. The reason a defined length is needed is because even though lines of latitude are supposed to be equally spaced over the whole of the Earth, the Earth is not a perfect sphere. The variation is small, but having a defined length avoids the differences being a problem. The distance between lines of latitude is fairly close though over the whole Earth. This is a different phenomenon from lines of longitude differing in size, which has to do with how they are defined.
4) The nautical mile lives in a grey area for unit systems. It is not an official SI unit and non-US aviation is supposed to only use SI units. This means all non-US nations “should” be using km/h for aircraft speed. However knots as speed is far too practical to abandon. Therefore, knots and nautical miles are considered unofficial SI units and member countries are allowed to use them on a temporary basis. This is even more confounding when you realize that this temporary basis has no end date, so there is no real effort to make this switch. As such, almost every country uses nautical miles and knots in aviation. Notable exceptions are China and Russia, which both formally use km/h for aircraft speed (unless the aircraft was manufactured somewhere else).
5) An interesting point related to the last one is that US aircraft have almost never used statute miles per hour for speed. This means if you are reading an article or a pilot is speaking and they use the term MPH, then they are either converting or just using the knots value as-is. Since nautical miles and statute miles are relatively close in size, using the knots speed and then saying MPH is close enough for informal communications purposes, without having to get into the particulars about the differences.
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