NavList:

We do not use nautical miles and knots because they sound “salty” or because it makes us sound like pirates of 'yore. We use them because they are very practical units based upon great circles.

I presume that regular posters here already know that 1 arc-minute along any great circle track on Earth is 1 nautical mile. But some “passers-by” or casual lurkers may not have paused to ponder that this is true of ANY great circle track between any two places ANYWHERE on Earth, not just lines of longitude or the equator.

Great circle tracks also represent the shortest travel-distance between two points on Earth, and along any “shortest possible great circle track on Earth” 1 arc-minute is still 1 nautical mile.

No voyager wants to travel extra distance unnecessarily, and therefore will seek to go “great circle” if they can. Long distance voyagers such as mariners and aviators will continue to travel “great circle” into the foreseeable future, so the handy convention of 1 arc-minute = 1 nautical mile along any great circle track, which has been in use for the past four centuries, will continue into the foreseeable future. Knots and nautical miles are not going away any time soon.

Moreover for star-struck celestial navigators and astronomers great circles are essential for relating coordinates and distances on the celestial sphere to those here on Earth. The cleverly contrived relationship between great circles on the Earth and on the celestial sphere allows us to take measurements along great circles on the celestial sphere and directly convert them into distances in nautical miles along corresponding great circles on the Earth, provided we measure in degrees and convert that into arc-minutes which ARE nautical miles on Earth.

Furthermore the sexagesimal (60's) base of arc-minutes and time has many more arithmetic factors than do 10's or 100's greatly facilitating mental math once you get used to it. Also there are several clever “rules of thumb” that work with nautical miles and knots. For example 1 knot is (approximately) 100 feet per minute. Or 1/10 of an arc-minute of latitude is 1/10 of a nautical mile even if you read the latitude off your GPS. That is one reason why navigators prefer the format of “degrees minutes and tenths,” not “degrees minutes and seconds.” (For longitude you have to diminish the distance of 1/10 of an arc-minute of longitude = 1/10 of a nautical mile by the factor of the cosine of your latitude. For some that is too much of a nuisance, but if back in your schooling you had to memorize a few cosines of angles you will be able to approximate it pretty quickly in your head.) But these attributes are mostly incidental to the 1 arc-minute = 1 nautical mile relationship of great circles on Earth.

BTW: As Frank posted kilometers were originally conceived by the French in a similar way to nautical miles as part of the metric system. They wanted to “decimalize” everything. So a full circle became 400 gradians instead of 360 degrees. Then along a line of longitude (a great circle on Earth) 100 gradians was going to be 10,000 kilometers. By this standard 1 gradian along ANY great circle track would have been100 kilometers. Gradians are often called “grads” so 1 grad was 100 kilometers, and 1 kilometer would have been 1 “centigrad.” But “centigrad” sounds too much like “centigrade,” the temperature scale, so that was changed to Celsius and centigrad is not commonly used. Instead it is “1 centesimal minute of arc.”

The metric system in use today has drifted away somewhat from this original concept, including abandoning some of their revised notions for the measurement of time that never really took hold, but that is a discussion of another sort.