Why is speed at sea measured in knots? | Hacker News Reader

Why is speed at sea measured in knots?

alum.mit.edu

52 pointsnmorell65 comments

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)

OR stated another way... "the average length of one minute of one degree of arc on a great circle".

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.

http://web.archive.org/web/20140409162921/https://alum.mit.e...

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

Lets handwave a conspiracy theory:

The Sumerian Ku was 518.5mm. A SAR-kus (60*60 kus) 1866meter is nearly same as a sea mile by 0.7%. One ku per second equals one sea mile per hour, equals one knot. So ancients did already know the size of earth by one and a half digits in 60er system (1/120). This required a mathemagical wisdom, like knowing how to compute square roots and pi to several 60system digits behind the comma, that was lost after 333bc.

Weights and measurements got mostly smaller and seldom bigger over time. The Sumerian mine lost 10% to become the lbs. Kings did declare the weights and length measurements at their time. Make the rod a bit shorter, if you have to hand out parcels after an invasion, and you have a good loot left for yourself. Use the bigger weights, after asking for ransom of a city, and even throw your sword into the balance, with the words VAE VICTIS! But making the weights bigger was a seldom exception. More common was the gain in declaring them smaller.

This article seems to answer "Why is it called a knot?" instead.

You're correct that the nautical mile is one minute of arc across a great circle and as such is a meaningful unit to a navigator.

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.

It certainly wouldn't have been precisely internationally defined in terms of a unit of measure that didn't exist until about four centuries later.

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.

> 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.

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.

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.

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.

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.

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

I think it was better icbm guidance since inertial guidance tends to drift quite a bit on long flights.

Fun fact, ICBMs do astronav! They have on-board cameras and computers to take a star sight and fix their location.

In the war they're designed to operate in GPS would have been disabled long ago.

The sr71 did something similar, too, but i think they just used an astrolabe instead of taking a picture.

But both still use an internal gyro in between pictures, don't they?

Well, I've always heard that the primary motivation was navigation, but funny enough, it was specifically navigation for submarines armed with nuclear ballistic missiles. Precise knowledge of the launch location was critical to launching such a missile, and when you're out in the ocean there's not a lot of options.

And why were submarine-launched ballistic missiles so important? Well, to ensure mutually assured destruction, you need to make sure that the first nuclear strike doesn't disable the victim's ability to launch a second strike. You can easily nuke missile silos and airports, but not submarines which are constantly moving. http://en.wikipedia.org/wiki/Nuclear_triad

> actually has roots

Heh, very good!

Could you elaborate on log's roots in physical log? (A link is fine)

A logbook is the book in which the results from the log were recorded.

https://en.wikipedia.org/wiki/Logbook

Better link: https://en.wikipedia.org/wiki/Chip_log

An interestingly large number of words and phrases originate from nautical vocabulary. That vocabulary in itself is so vast, it sometimes feels like a different language.

Here's a partial list of nautical words in common use today: http://www.dailywritingtips.com/50-nautical-terms-in-general...

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.

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).

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/

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

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

http://webcache.googleusercontent.com/search?q=cache:bGSHA2K...

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

http://engineering.mit.edu/ask/why-speed-sea-measured-knots

http://archive.is/L86K7

It looks like visits to the linked URL are blocked when the referrer isn't an MIT page. If you visit the authors' home page you can view the original article:

https://alum.mit.edu/pages/sliceofmit/author/hoagland/

EDIT: Duh. You can't click that link because it's blocked by referrer again. So just copy/paste that URL into your browser and you'll be all set. In fact, you can do the same with the original link: https://alum.mit.edu/pages/sliceofmit/2014/04/09/why-is-spee...

This gives me a 403 Forbidden error message.

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.

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

Server load.

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.

http://engineering.mit.edu/ask/why-speed-sea-measured-knots

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.

OT to the article, but the video at 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.

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

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

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?

It's just a convenient way to define a unit of length. Look up the original definition of the metre and you'll see that it's basically the same except for some constants.

The metre was originally defined as 1/10,000 of the distance from the equator to the north pole through Paris. There's a corresponding measure of angle (gradian -- 1/400 of a circle) which has never become widely adopted (although my old casio calculator supported it)

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.

;-) 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.

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"

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.

There's still an expectation that a skilled navigator can work out position and movement by hand, using charts, speed and compass headings ("dead reckoning") and have those results reasonably match/confirm the information reported by more modern systems. Using knots makes this significantly easier than it would be if speed and distance were reported using statute miles or another unit that's not easily converted into change in Lat/Long.

You don't travel on along a great circle when you driving or running. But when you are flying and sailing long distances, you do.

The nautical mile is the length of a minute of arc along a great circle on the surface of Earth. I think the 14.4m figure is wrong; that should be 15.4m, which is the distance covered in 30 seconds by a vessel going at 1 nautical mile per hour.

I'm randomly reading some of the links here. The Wikipedia article on the Chip log associates the 14.4 meters with a 28 second sandglass.

It was originally 7 fathoms. Later the value was refined a bit because of newer measurements and updated definition of the nautical mile. Why 7, I don't know. Wikipedia seems to indicate that the definition of a knot was derived from that of a nautical mile, but I wonder if it wasn't the other way around originally. https://en.wikipedia.org/wiki/Chip_log#Origins

Because the article insists on using metric when using other units would have made for a much better explanation since it would show the evolution and not be confusing like 14.4m.

rdl and other explain the reason for a nautical mile above: https://news.ycombinator.com/item?id=7559252

There are approximately 40000 km in a great circle passing through both poles and Paris.

40000 km / 360 degree * 1 degree / 60 arcminute = 1.851... km / polar arcminute

if 1 knot is 1 arcminute / hour, then to get the distance in meters per knot, you use the following factors:

1.851... km / hour * 1000 m / 1 km * 1 hour / 60 minutes * 1 minute / 60 seconds * 30 seconds / knot = 15.432 m / knot

Is this article block to Germany?

Blocked in Romania.

Blocked in Brazil too. :(

Out of curiosity, is it just the OP link, or is the entire alum.mit.edu domain blocked?

Blocked in Canada

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

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

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

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