
Why is speed at sea measured in knots?  - nmorell
https://alum.mit.edu/pages/sliceofmit/2014/04/09/why-is-speed-at-sea-measured-in-knots/
======
rdl
Not mentioning that the nautical mile is essentially 1 minute of arc on any
meridian, which is the key fact. Once you have the distance of nautical mile,
then it's reasonable to measure your speed with respect to something fixed
relative to the water around you.

("Why is a nautical mile 1 MOA and a statute mile something else" is the more
interesting question, IMO)

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mikeash
I can't read the article, so forgive anything that's off base here....

Omitting that seems ludicrous. Without that, you're really answering the
question, "why are they _called_ 'knots'". Which is interesting, but
completely different from the question of why you _use_ knots.

~~~
dfc
[http://web.archive.org/web/20140409162921/https://alum.mit.e...](http://web.archive.org/web/20140409162921/https://alum.mit.edu/pages/sliceofmit/2014/04/09/why-
is-speed-at-sea-measured-in-knots/)

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bladedtoys
Actually I'm very surprised (read skeptical) of the precision and firmness of
the claims in this article.

For example "...the nautical mile – 1.852 kilometers – was introduced in the
15th century..."

For one thing, there was effectively no international standardization on other
units in the 15th century. Indeed different countries did not even agree on
the current calendar date. So the claim that the knot was precisely
international defined in the 15th seems incredible.

Also, the diameter of the earth was quite controversial in the 15th century
(Columbus notoriously used a very low-ball figure to raise funds for his
westward expedition to China). So it seems unlikely a precise figure for the
length of a minute of arch could be agreed on and less likely that it would be
accurate.

~~~
mjlee
I think the author has become a little confused by what nautical mile means
today and has meant in the past.

Originally (maybe as far back as the 15th Century) a nautical mile was just 1
minute of arc of the Earth's circumference no matter where you were on the
Earth.

A nautical mile at the pole would be quite short, a nautical mile at the
equator would be quite long.

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ErsatzVerkehr
> A nautical mile at the pole would be quite short, a nautical mile at the
> equator would be quite long.

No, this is emphatically not the case. You're confusing a minute of curvature
-- a minute along a great circle path -- with a minute of latitude.

~~~
mjlee
A minute of latitude was the originally definition of the nautical mile. I'm
not exactly sure when it changed to be defined as the mean nautical mile.

I'm not talking about how it is today, I'm talking about the inconsistencies
in this article.

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bentcorner
Learning today that the word "log" (as in event log or /var/log/messages)
_actually has roots in a physical log_ is mind blowing. I feel like when I was
looking this up that there's a gigantic april fools prank being played on me.

~~~
Retric
Thanks for pointing that out, I never made the connection. Anyway, I just
found this out today and thought I would pass it on.

"The GPS satellites also have NUDET (Nuclear Detonation) sensors on them to
detect nuclear detonations almost anywhere on earth." Which is reasonable as
it's run by the airforce and they want global coverage, but I can't help but
wonder which was the original goal nuclear detonation detection or navigation.

~~~
ISL
It's easier and more straightforward to do nuclear detonation sensing than
maintain a properly-synchronized network of orbiting atomic clocks.

If the NUDET sensor is a standard package, military-sponsored satellites may
have them bolted on as a matter of course.

~~~
dfc
Each bird has redundant cesium clocks. It would be interesting to know if they
also include redundant NUDET sensors.

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arethuza
Growing up in a fishing port we had the chance to study Navigation and
Seamanship at high school - the school building even had a bridge and radar,
even though there wasn't much chance of it going anywhere.

I was deeply unhappy when I had to choose French rather than Navigation and
Seamanship on the (mistaken!) belief that I needed it to get into University.

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homarus
There is a very practical explanation for why a nautical mile equals one
minute of arc on a great circle around the earth.

When doing celestial navigation (using a sextant) you are measuring the
elevation of the sun, moon or stars above the horizon and then, combined with
azimuth (compass) angles, you perform a series of trigonometry problems and
chart (map) measurements to ascertain your location.

This work is made simpler by having a common measure underpinning arc
(minute), distance (nautical mile) and speed (knot = 1 nm/hr).

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psuter
For those who have the chance, the Air and Space museum in DC currently has a
very good exhibition on the history of navigation techniques [1].

[1] [http://timeandnavigation.si.edu/](http://timeandnavigation.si.edu/)

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milliams
Seems down to me. Does anyone have a cached version?

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Pyrodogg
Even the full cached version hangs. Text-only works though.

[http://webcache.googleusercontent.com/search?q=cache:bGSHA2K...](http://webcache.googleusercontent.com/search?q=cache:bGSHA2KAQaEJ:https://alum.mit.edu/pages/sliceofmit/2014/04/09/why-
is-speed-at-sea-measured-in-knots/&hl=en&gl=us&strip=1)

~~~
JuiceSSH
I think this is the same content, different MIT URL:

[http://engineering.mit.edu/ask/why-speed-sea-measured-
knots](http://engineering.mit.edu/ask/why-speed-sea-measured-knots)

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ginko
This gives me a 403 Forbidden error message.

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seszett
It looks like they are checking the referer and serving 403 to visitors from
HN.

Just press enter in the address bar once you have the 403, the same request
will be done without a referer and it will load fine.

~~~
Revex
Thank you for this. Any idea why they would 403 visitors from HN?

~~~
lnanek2
Pretty damn silly, many web sites would kill for the traffic of getting on the
front page of HN. Maybe they mistook it for a DOS attack.

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bjohnso5
Check out Black Sails, there's a scene in one of the later episodes of the
season that portrays the measurement of speed using the chip board and knotted
rope. The term "knot" immediately clicked for me when I saw that.

~~~
thrill
OT to the article, but the video at
[https://www.youtube.com/watch?v=On_Lh1wiSjk](https://www.youtube.com/watch?v=On_Lh1wiSjk)
is a nice examination of the instrument used for the unique sounds in the
opening credits of Black Sails.

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hoektoe
Did I miss something, why 14.4m between knots? Or why set 1.852 kilometers as
the nautical mile?

~~~
jmj42
From Wikipedia: "The nautical mile is a unit of length that is approximately
one minute of arc measured along any meridian."

~~~
deelowe
Still doesn't answer "why." More specifically, why is it more advantageous to
measure it this way versus some other way? Why do airplanes measure their
speed/distance in knots? Why not cars, trucks, runners, and physicists?

~~~
andyjohnson0
Ships and airplanes traditionally navigate using latitude/longitude
measurements. Since that coordinate system is based on degrees/minutes/seconds
of arc, a length unit that uses the same system is more natural.

~~~
deelowe
;-) I kind of knew this already, but thanks for confirming. Point being,
there's not a good reason "why" today. Truth is that it's convention at this
point. People used to use maps and various tools to do navigation by sea and
air. The methods they developed assumed certain things (traveling in an arc,
usage of maps with lat/long lines, etc..), which meant the knot was more
preferable.

~~~
escherplex
Valid points all but we're using 20/20 hindsight to render judgments on
received conventions during a time in which the more logical metric system
exists. A quick glance at 'Knot (unit)' in Wikipedia supplies a
'rationalization' for the use of knots involving old Mercator projection
navigation maps. But all of these 'knots' and 'mile' measures are based on the
English 'foot'; where did the old 'foot' standard come from? Some 10/11
convention imposed by an English King Henry III on an older system. And by
extension, why retain 24 hours in a solar day or the Babylonian 360 degrees in
a circle? Or more recently, why the Cartesian convention of ordinate-vertical
and abscissa-horizontal on graphs rather than the other way around (sundial-
clockwise would be positive; back-Kronecker [0 1,1 0] to convert)? After a
while these mathematical musings start to resemble idle etymological ramblings
in linguistics: eg, 'extreme' conflates 'out of' and 'sewer'; but then why
those particular phonemes for the sense of 'out of' or 'sewer'? As my brother
the research psychiatrist frequently retorts "You're trying to be logical.
People aren't logical"

~~~
bsder
24 and 360 are really good unit bases for something you have to divide by
hand. 24 gets you 2/3/4/6/8/12 while 360 gets you
2/3/4/5/6/8/10/12/15/18/20/24/30/36/45/60/72/90/120/180

And, we _don 't_ always adhere to cartesian conventions. Right-hand and left-
hand rules abound in engineering. And even computer graphics regularly flips
the axis directions depending upon usage (perspective transforms, for
example).

And, decimal degrees isn't that uncommon either.

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cientifico
Is this article block to Germany?

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lsaferite
They are doing referrer blocking. Try pasting the URL in your browser directly
instead of clicking the link.

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vesinisa
The measurements of the chip log were recorded using a traverse board:
[http://en.wikipedia.org/wiki/Traverse_board](http://en.wikipedia.org/wiki/Traverse_board)

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jqm
I do knot know because I couldn't access the page.

