
What Is a 'Nautical Mile'? (2011) - devaroop
https://science.howstuffworks.com/innovation/science-questions/question79.htm
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codeduck
> A minute of arc on the planet Earth is 1 nautical mile.

Only if that minute of arc lies on a great circle route.

As an interesting addendum, when measuring distance made good on a nautical
chart, you will use a divider to measure the straight line distance, then lay
this onto the latitude scale to convert it to nautical miles.

If you were to use the longitude scale, you would be off by approximately the
cosine of your latitude, because the lines of latitude( _) (apart from the
equator) are not great circles.

(_) corrected longitude to latitude here - longitude is obviously great
circles, latitude apart from the equator are not.

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chrisseaton
I can believe that it's true that lines of longitude are not great circles,
but I can't picture why in my head. Isn't a great circle formed by extending
the shortest distance between two points on the surface of a sphere?

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taejo
Lines of longitude are great circles. Grandparent must mean latitude: the
equator is a line of latitude, and the only one that is a great circle.

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codeduck
Thanks, I've corrected my previous comment.

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seanalltogether
Its interesting that the nautical mile is based on a more abstract number that
would define when looking at maps and charts, rather then something more
immediate and physical like the distance to the horizon when out on the ocean.
Most imperial units seem to be based on simple laymen measurements.

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codeduck
Distance to the horizon is variable. Sea state, height of deck over water,
visibility etc will all affect it. Whereas, with a sextant and a decent
maritime chronometer it is very easy (*) to determine longitude and latitude.

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292355744930110
The length of people's feet vary and so does the amount of land a yoke of oxen
could plough in one day.

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leoc
The metre was originally supposed to be based on a division of the Earth's
circumference as well.

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roelschroeven
Indeed, IIRC the intention was to define the meter so that the distance
between the North Pole and the equator at the meridian through Paris would be
exactly 10000 km, making the full circumference along that meridian and the
one opposite it 40000 km.

EDIT: that information is already in the article, making my comment redundant
and unnecessary.

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leoc
Mine also. ¯\\_(ツ)_/¯

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hammock
At what elevation are they measuring these arcs / circumference of the earth?
At sea level? Feel like that needs to be explicit if we are trying to be
technical. Because you can't "travel around the Earth at the equator" in
"21,600 nautical miles, 24,857 miles or 40,003 kilometers" without going
underneath a mountain or two.

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ajross
They aren't. It was based on an early spherical model, but in the modern world
a nautical mile is defined as exactly 1852 meters, period. The notion of the
arc minute is historical context in the same way that a "mile" was originally
a thousand (mille) paces.

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g051051
Hah! I was in the Navy and never knew that.

P.S.: Supply Corps, so never needed to know it.

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theandrewbailey
> A kilometer is also defined using the planet Earth as a standard of
> distance.

Wrong. It used to be that way, but not anymore.

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iams
Which is why it says: ... you would have the traditional unit for the
kilometer as defined in 1791 by the French Academy of Sciences.

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theandrewbailey
Wouldn't you need the 1791 Earth, too?

The article doesn't mention that it's not the current definition.

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diveanon
I teach this as part of the science of diving course and there is always an
audible groan at the mention of the word "mile".

By the time I am done explaining the basis it is always funny how the
classrooms attitude changes to "that makes a lot of sense".

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fourier_mode
I would never get used to how to remember the diameter of the earth. Now all I
need to remember is how many km's are there in a nautical mile, get the length
of the equator and divide it by pi. i.e. (360 x 60 x 1.85/3.14)

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reacweb
The perimeter is 40,000 km. You just need to learn pi ;-)

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Tepix
It's the only mile that metric people approve of :-)

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gilgoomesh
No, the nautical mile is hated by proponents of the metric system. As with all
customary units, it favors a specific use-case at the cost of errors and
conversions, has limited accuracy and doesn't handle orders of magnitude.

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athenot
Yes. This is the most annoying side-effect: industry-specific systems of
measuring.

As I'm typing this, I'm looking at the power extension cord for my computer,
which has a thickness 1.10 Micro Nautical Mile, ie. AWG 12, ie. 2.05mm…

