eli5, Why isn’t there a nautical kilometer?

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eli5, Why isn’t there a nautical kilometer?

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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.

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