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What Is a 'Nautical Mile'? (2011)
- leoc 7y agoThe metre was originally supposed to be based on a division of the Earth's circumference as well.
- roelschroeven 7y agoIndeed, 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.
- leoc 7y agoMine also. ¯\_(ツ)_/¯
- cwtpk 7y agoNobody reads tfa so it's cool.
- tinus_hn 7y agoWell, the article claims that that is the definition which is false. The definition of a meter is ‘the distance traveled by light in 1/299792458 of a second’ so a kilometer is a thousand times that. The story in the article is the original, obsolete definition.
- FabHK 7y agoStill bears highlighting, I think: From pole to equator it's 90 (deg) * 60 (minutes/deg) = 5400 nautical miles or 10000 kilometres. Pretty much the only way I can remember the conversion factor. Well, that's not true, 1.852 also consists of the country code of the USA followed by the country code of Hong Kong, so that's easy.
- codeduck 7y ago> 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.
- chrisseaton 7y agoI 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?
- taejo 7y agoLines 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.
- codeduck 7y agoThanks, I've corrected my previous comment.
- codeduck 7y agoAn arc along a line of longitude is a deviation in the latitude scale. Lines of longitude lie on great circles since they converge on the poles, but when you move a degree of longitude you are moving west or east along a line of latitude, and the only line of latitude which is a great circle is the equator. The reason you use the latitude scale is that lines of latitude are equidistant between one another - it is the same distance between two lines of latitude no matter your longitude, but it is not equidistant between two lines of longitude at different latitudes. So even if your chart spans several hundreds of miles, the latitude scale remains constant across that distance regardless of the distortions of the projection used to render the globe onto the two dimensional chart surface.
- seanalltogether 7y agoIts 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.
- codeduck 7y agoDistance 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.
- 292355744930110 7y agoThe length of people's feet vary and so does the amount of land a yoke of oxen could plough in one day.
- SiempreViernes 7y agoI guess this mostly means that Nautical Mile is a unit that traces back to the introduction of the spherical coordinate system in cartography.
- rocqua 7y agoAs a nice bonus this means your coordinate grid also gives a scale bar. This makes measuring distance in nautical miles using a chart a lot more practical.
- throw0101a 7y ago> Most imperial units seem to be based on simple laymen measurements. I would not label the nautical mile as "imperial". It was based on mathematical properties of the planet: > Historically, it was defined as one minute (1/60 of a degree) of latitude. Just like the metre originally was: > The metre was originally defined as 1⁄10,000,000 of the meridian arc from the North pole to the equator passing through Dunkirk. * https://en.wikipedia.org/wiki/Nautical_mile https://en.wikipedia.org/wiki/Nautical_mile
- Tepix 7y agoIt's the only mile that metric people approve of :-)
- gilgoomesh 7y agoNo, 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.
- athenot 7y agoYes. 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…
- throw0101a 7y ago> 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. The nautical mile was not 'customary', but based on mathematical properties. Specifically, it was defined as one arc-minute (1/60 of degree) of latitude. Degrees of latitude are measure from the centre of the Earth and not the surface. This is no more arbitrary than how the metre was defined: > The metre was originally defined as 1⁄10,000,000 of the meridian arc from the North pole to the equator passing through Dunkirk. * https://en.wikipedia.org/wiki/Nautical_mile https://en.wikipedia.org/wiki/Nautical_mile It is now been translated in relation to the metric system as 1852m.
- scooble 7y agoWhat conversions do you typically do to/from NM? As far as I'm aware, it is only used for navigation, where all related info, such as speed, position etc. relate directly to it.
- theandrewbailey 7y ago> A kilometer is also defined using the planet Earth as a standard of distance. Wrong. It used to be that way, but not anymore.
- iams 7y agoWhich is why it says: ... you would have the traditional unit for the kilometer as defined in 1791 by the French Academy of Sciences.
- theandrewbailey 7y agoWouldn't you need the 1791 Earth, too? The article doesn't mention that it's not the current definition.
- 292355744930110 7y agoThe same can be said of the nautical mile. It is no longer defined as an minute of latitude and in instead exactly 1852 meters.
- hammock 7y agoAt 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.
- signa11 7y agoi would guess that would be mean-sea-level...
- pletnes 7y agoPossibly the geoid: https://en.wikipedia.org/wiki/Geoid?wprov=sfti1 https://en.wikipedia.org/wiki/Geoid?wprov=sfti1 It’s a surface describing constant (magnitude of) the acceleration of gravity.
- ajross 7y agoThey 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.
- g051051 7y agoHah! I was in the Navy and never knew that. P.S.: Supply Corps, so never needed to know it.
- fourier_mode 7y agoI 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)
- astazangasta 7y ago24901.5 mi is the number i memorized for circumference as a wee bairn, before i knew about oblate eccentricity.
- reacweb 7y agoThe perimeter is 40,000 km. You just need to learn pi ;-)
- diveanon 7y agoI 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".