eli5, Why isn’t there a nautical kilometer?
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because kilometers are a standardized and universal unit, while a mile is not, throughout history several definitions of what a mile was came up, which is why to this day we have to specify nautical miles because they are different from regular miles
The reason they use nautical miles in sailing and aviation is because it’s defined as 1 minute of latitude at the equator and that probably simplified planning routes back in the day
of course with computers nowadays it would take a simple software update to change the units to metric but you’d have to re-train all the pilots, captains and update all maps and navigation charts for very little gain
I think by the time they invented kilometers as a concept they wanted to standardise things in general so they wouldnt want another measurement for no reason
A kilometer is a thousand meters, it’s what the word means. If you change the number of meters or the measurement of a meter than it can no longer be a kilometer as it won’t be a thousand meters or it won’t be meters at all.
Kilometer is a measure of the so-called “metric” system. It is the system that is superior to all other systems and does not involve body parts like “feet” and such. The need for a nautical kilometer is not there because a kilometer is like the name suggests: one thousand meters. It is properly defined just by its name. There is a larger amount of people on the planet that do not feel comfortable with measuring distance with units that actually make sense. Instead they rely on a system where strange ratios are used. That is why you have a nautical mile but not a nautical kilometer.
There sorta is, we just call it the kilometer.
Every five-year-old knows that a nautical mile is 1 minute of latitude, or 1/(360*60) of the circumference of the Earth.
When the metric system was laid out, the kilometer was deliberately sized to be 1/40000 of the circumference of the Earth, to be a nice round number.
Of course, the circumference of the Earth is different depending on which way around you go (equatorial circumference is not the same as polar circumference), and we’ve gotten more precise about measuring it since the kilometer was created, so we now base the kilometer on something that is more constant and more precisely known. So I just googled “circumference of Earth” and was told 40075 km. Not exact, but not bad.
The point is, the kilometer is sorta based on the size of the Earth, just like the nautical mile is.
There is no need for one. A kilometer is defined as 1,000 meters.
There isn’t a true need for nautical miles versus land miles, either. It is just that the statue (land) mile comes from a traditional British definition of 5280 feet, the nautical mile comes from a traditional marine sailing definition of one minute of latitude, and despite having the same name these two different length units have different histories going back centuries in Europe. We *added* the word “nautical” to avoid confusion.
There are also Roman miles (1000 paces), Dutch miles, German miles, Russian miles, etc. These are essentially obsolete historical units now though, since those countries moved to metric.
Everyone *used* to have their own miles. There were Swedish miles, English miles, Italian miles… and there was a lot of difference between them. So if your charts told you to sail south for 200 miles, which miles were you using? Were you supposed to sail 200 that could be 420km (“old” English), 1234km (Welsh), 1507km (Prussian)… On land, you at least have landmarks to take a guess, but at sea? It’s important to know which mile you mean. It became easier to just use a specific mile when talking about the sea (the *nautical* mile) and set the to a value that happens to be useful at sea (one that makes calculating distances from a map easy).
Nowadays, *most* of the miles have fallen into disuse. Most countries no longer use their miles… Except for a small handful of countries which use very similar miles (a mile in the UK and a mile in the US are ever so slightly different), and specialised uses like ships and planes. Because one of these, the 1609-ish meter mile, is vastly more common than the other, we just call one a mile and we give the other a specialised name.
Why isn’t there a nautical kilometre? Because kilometre *has always been* unambiguous. A kilometre has always referred to a specified amount of distance. A mile, meanwhile, used to refer to all manner of distances.
Before the advent of the metric system there were many different units that were named mile. Just like they was a different pound in every big city and different measures for all the other stuff.
Since almost everyone switched to the metric system all the different miles died out.
The one used in the US and the nautical mile are the only major survivors.
The nautical mile and its derived units like knots (nautical mile per hour) are based on the circumference of the earth and minutes of latitude.
The Earth is round and has 360 degrees and each degree has 60 acr-minutes and each acr-minute has 60 arc-seconds. (Just like the hour for time). A nautical mile was exactly one arc-minute of the earth’s circumference.
This was useful since maps had lines one them that were acr-minutes apart (at the equator for longitudinal lines and everywhere for latitudinal lines).
You could look at a map and see roughly how many nautical miles some place was from another place and how many hours it would take to get there if you knew how many knots you were going.
The meter was also originally based on the circumference of the earth. But instead of being 360 x 60 = 21,600 nautical miles around the earth. The meter was originally defined as there being 10,000 km from equator to pole or 40,000 around the planet.
Due to being able to measure the earth slightly better today and the planet not being perfectly spherical it is actually that the 21600 nautical miles are 40,003.2 km and the earth circumference is 40,075.017 km around the equator and 40,007.863 km around the poles.
But generally if you divide the earth based on 10s you get kilometer and if you dived the earth based on 60s you get nautical miles.
American miles and other miles were based on vague ideas of how far you could get with 1000 paces, but everyone seems to have had differently long steps.
The reason for the existence of the nautical mile is that back in the day there were many different definitions and measurements of “mile” and no standardization, so sailors decided to just come up with their own version (based on 1 minute or 1/60 degree of latitude at the equator) and stick to it. A kilometer on the other hand is always a kilometer. There is no need for variations.
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