Talk:Nautical mile

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[edit] Wikipedia abbeviation for nuautical mile

According to WP:UNITS, the preferred wikipedia abbreviation for nautical mile is nmi. This is to avoid confusion with nanometer, although one would think the context would be enough. I think this probably should be in the article, but am putting it here, as someone questioning it is likely to ask here. I only hesitate to put it in the article as it seems like wikipedia self references are to be avoided in mainspace. --J Clear (talk) 17:59, 2 February 2008 (UTC)

[edit] Clarification

Having spent several years in the Navy, I have made some adjustments based on the following:

Nautical Miles for navigation are measured at exactly 6,000 feet (2,000 yards). A cable is 1/10 of a mile, or 200 yards. A cable also happens to be exactly 100 fathoms. While this measurement differs slightly (about 76 feet per mile) from the internaitonal standard, it is used by most navy and merchant vessels because of the much simpler mathematics involved.Mattwilkins 16:15, 18 October 2005 (UTC)

Can you add a good authoritative a reference for this? A link to some online Navy standards handbook, for example? In which country's Navy was that? Is this really an official definition, or just a crude approximation for rule-of-thumb calculations in countries that still use feet and yards? Markus Kuhn 20:28, 19 October 2005 (UTC)
The Canadian Navy. The Bride Watchkeeper's Exam uses 2000 yards to the Nautical Mile. Mattwilkins 08:11, 15 November 2005 (UTC)
Simpler only if you're using feet, fathoms and cables. Don't you mean "most U.S. navy and merchant vessels"? Jimp 2Nov05
Well all vessels use these measurements, as the metric system is very difficult to use to any effect.
How exactly is the logical and coherent International System of Units (SI) "very difficult" to use on the sea? Or the air for that matter? Samy23 22:11, 14 January 2007 (UTC)
I take it you are not a navigator. When you are working with nautical charts, it is usually easier to measure a distance using the latitude markings on either edge of the chart than to find the scale, which is frequently folded out of sight, especially on smaller craft. Also you are frequently dealing with converting degrees/minutes/seconds to distances, where the nautical mile is equivalent to 1 minute of arc (of latitude). For similar reasons, the knot is still used as the unit of velocity in the sea or air. In a way the nautical mile is similar to the hectare, both have a niche use where they are more efficient to use than SI units. --J Clear (talk) 17:49, 2 February 2008 (UTC)
This is a weird sentence "Also in maritime navigation, nautical miles can be divided into 10 cables, although the present day definition of the cable uses a much more precise method.". "A tenth of a nautical mile" is perfectly precise, especially if nautical miles are defined with respect to meters which are defined with respect to the speed of light.—Preceding unsigned comment added by Schmmd (talk • contribs)

[edit] Bulging top

"It bulges at the equator like a spinning top," says the article. Do spinning tops bulge at the equator? Jimp 2Nov05

They would. If they were made up of a sufficiently plastic material.--zumanon 14:13, 25 January 2007 (UTC)

[edit] Minute of arc at equator and at poles

In an edit summary, Ericg said "rv - if you think about it, bulging at the equator means the north-south distance is shorter at the equator, not longer."

The problem is, his thought experiment would lead to the opposite conclusion; when it is farther away, the same angle subtends a greater distance.

The problem is, we don't normally measure geocentric latitude, so we don't have our angles located at the same origin. Instead, we normally use geodetic latitude; see the article, it's too complicated to summarize here. If we used geocentric latitude, a minute of arc would be greater at the equator than at the poles.

But with the geocentric latitude we do use, a minute of arc is greater at the poles than at the equator. The numbers aren't exactly the same, however, and I haven't checked yet to see which kind of latitude the numbers used in the article correspond to. There are also a few other ways that could possibly be used to measure latitude (which is what you measure as you travel along a meridian of longitude). Gene Nygaard 00:17, 13 December 2005 (UTC)

Geodesy#Units and measures on the ellipsoid states: "A nautical mile is one minute of astronomical latitude. The radius of curvature of the ellipsoid varies with latitude, being the longest at the pole and shortest at the equator as is the nautical mile". So the statement of Nautical_mile#History: "According to WGS84 the length of one minute of arc along a meridian on the Earth's surface varies from 1852.2 m near the poles to 1855.3 m near the Equator." cannot be true. Nor does the article WGS84 support it. Bo Jacoby 13:14, 21 July 2006 (UTC)

The length of a Sea Mile is the shortest at the Equator (1842.9m) and the longest at the Poles (1861.7m). An average value of 1852.3m is at 45 degrees Latitude. (IYT YM Ocean handbook). A cable, being a tenth of a mile, equals 185.2m or ROUGHLY 200 yards.

[edit] Precise length

In the discussion on the Knot (speed) page, someone says regarding the metric conversion to km: "1.852 is a round up (the actual precise number being 1.851999985024)" Anyone? Fizzybrain 12:38, 17 April 2006 (UTC)

I just looked at the BIPM reference. It says 1852 not 1852.5 meters. Has our definition really been wrong all along? I just corrected it. 16:09, 13 December 2006 (UTC)

I modified the conversion section to indicate which conversions have exact (rational) values, and also grouped the two approximate values together at the end. Lacking any more precise definition of geographical mile than one arc minute at the Earth's equator, this can only be as exact as the current estimate of the latter, and likewise for the arc minute itself, which certainly is not exactly one nautical mile given the SI standardization to 1852 m. --Vaughan Pratt 01:02, 15 August 2007 (UTC)

[edit] What is "SI"?

This entry never explains what SI is -- perhaps whoever added it could include it? It's not very clear to me what SI is from the context.

International System of Units, now Wikilinked in the second sentence.
Atlant 16:58, 5 June 2006 (UTC)

[edit] US Customary Units box

In the article box where it has conversions is states 1 nautical mile = 1088.259 miles, which can't possibly be right. However, I know nothing about nautical stuff so is there something I'm missing here? --Cammy 19:15, 11 August 2006 (UTC)

Somebody has screwed up the {{template:units of length}} template.
Atlant 00:02, 13 August 2006 (UTC)

[edit] citation on etymology of knots needed

"The term 'knot' derived from the practice of using a knotted rope as a method of gauging speed of a ship. The rope would be thrown into the water and the rope trailed behind the ship. The number of knots that passed off the ship and into the water in a given time would determine the speed in 'knots'."

This really sounds like nonsense folk etymology. I'd always had the impression 'knot' was simply a respelling of 'nauts', short for 'nautical miles (per hour)'. This thing about dropping knots sounds like nonsense. 192.128.167.68 11:00, 25 August 2006 (UTC)

Nope, that's the exact etymology. So many knots on so many seconds.
Atlant 18:20, 27 August 2006 (UTC)
It's correct. The only early way to know a ship's speed was to use what is called a log line. Check dictionary definition 10b. ericg ✈ 19:01, 27 August 2006 (UTC)
Sorry, I've never included a cite before. If someone else doesn't mind doing it this is an excellent explanation of the nautical term "knot." It even explains that "naut" as in nautical and "knot" as in a marker in a rope is purely coincidental. The use of a wooden wedge is explained, to serve as a sea anchor, thus insuring that the rope would play out properly, along with a 30 second "hourglass", and a length of knotted (not nauted) rope. The process involved three persons. The timekeepr, the knot counter, and the rope player-outer.

http://www.onlineconversion.com/faq_07.htm

This link http://www.tallshipbounty.org/Demos_ChipLog.html includes photos and further explaination of the "chip log."

Wjbean

It all started with logs being thrown overboard over the bow of a ship on a mark and someone counting the seconds that passed until the log passed the stern. The vessel's size was known and this way the speed could be calculated and entered in the LOG-book. Later they tied a rope to the log so they could re-use the same log over and over again and thus saving valuable storage space. Eventually they ended up with the knotted rope.

[edit] Conversion to Geographical miles

The articles states that 1 nautical mile is equal to 1.1507794 geographical mile. Yet, the geographical mile article states that geographical mile is 1855 meters, which means that 1 nautical mile is equal to 1852/1855 = 0.9984 geographical mile. Hence, (at least) one of the two statements has to be wrong, though I do not know which one.
--158.38.82.84 12:50, 12 January 2007 (UTC)

Changed, I assume that one nautical mile is equal to 0,9984 geographical mile (according to the [Geographical mile] definition).
Tatrgel 13:52, 14 January 2007 (UTC)

[edit] Conceptual Error

I have almost by chance noticed that the radius at the poles were shown to be greater than the radius at the equator, which is of course wrong; it is a known fact that the earth is bulging at the equator. Also the corresponding lengths of one minute of arc was wrong. So I consulted the WGS 84 for the radii and made the necessary calculations of the arc myself. --zumanon 14:05, 25 January 2007 (UTC)

Actually I have seen the same error in various websites from which I suppose the main body of the article has been copied. If I have time I will revisit this article and check other figures at least for conceptual errors.--zumanon 14:10, 25 January 2007 (UTC)

No error, the radius of curvature of a meridian attains its minimum at the equator and maximum at the poles. You are probably thinking of another radius, namely the distance from the surface to the center of the earth, which as you say is the other way round. Take a look at the geodetic constants in http://www.jqjacobs.net/astro/xls/aegeo.xls. The relevant ones here are sma (semimajor axis or equatorial radius), smi (semiminor axis or polar radius), fr (flattening reciprocal, = smi/sma), and rcp (radius of curvature, polar), all IUGG values (for consistency). Missing is rce (radius of curvature, equatorial). You can get all of these from just sma (6378137 exactly) and fr (0.996647189318820) alone, using smi = sma×fr, rcp = sma/fr, and rce = sma×fr2. So smi and rcp go in opposite directions from sma, while rce/rcp, the ratio of the two curvatures, is fr3 = .989975, i.e. the radius of curvature decreases by 1.0025 per cent going from pole to equator. Memorizing this as one percent is all the accuracy you'll ever need in practice (oblateness works in mysterious ways). --Vaughan Pratt 22:33, 16 August 2007 (UTC)

[edit] Proposed WikiProject

Right now the content related to the various articles relating to measurement seems to be rather indifferently handled. This is not good, because at least 45 or so are of a great deal of importance to Wikipedia, and are even regarded as Vital articles. On that basis, I am proposing a new project at Wikipedia:WikiProject Council/Proposals#Measurement to work with these articles, and the others that relate to the concepts of measurement. Any and all input in the proposed project, including indications of willingness to contribute to its work, would be greatly appreciated. Thank you for your attention. John Carter 20:56, 2 May 2007 (UTC)

[edit] Definition

Anonymous editor 74.161.41.234 keeps changing the definition to:

Unit of distance used in navigation, an internationally agreed standard (since 1959) equaling the average length of one minute of arc on a great circle of the Earth, or 1,852 m/6,076 ft.
Refer to: http://geodesy.noaa.gov/PUBS_LIB/FedRegister/FRdoc59-5442.pdf

However, the cited reference does not mention "one minute of arc" and it gives a much more precise length than 6,076 ft. The first is already mentioned in the 'definition' as an approximation and the second is given almost as precisely as that in his ref in a list below the definition (Conversions to other units). — Joe Kress 22:45, 27 August 2007 (UTC)

The anonymous editor is also wrong that it became an international standard in 1959. Various sites, including the BIH site, state that it was internationally accepted in 1928. 1959 is the much later year that the United States accepted it. I left a note on his talk page (User talk:74.161.41.234) requesting him to respond here. Because all of his edits have used the same numeric IP address, he should see an alert that he has a message on any Wikipedia page. — Joe Kress 23:50, 28 August 2007 (UTC)

[edit] Unsourced remarks

I reformatted the reference in the lead sentence to point directly to Table 8 in the BIPM brochure, rather than a section that contains several tables. I also removed some unsourced remarks from the footnote. In particular, the footnote contained the quotation "expected to continue to be used for many years", but that phrase does not occur anywhere in the BIPM brochure (unless there is some quirk that prevents the search facility in Adobe Acrobat Reader from finding it). --Gerry Ashton 18:01, 30 August 2007 (UTC)

Sorry for the misquote. The phrase is actually, "continue to be used for many years", which occurs in the first paragraph of section 4.1: "Tables 8 and 9 contain units that have exactly defined values in terms of SI units, and are used in particular circumstances to satisfy the needs of commercial, legal, or specialized scientific interests. It is likely that these units will continue to be used for many years." This quote is also the source for the excised statement that it is "an exact SI definition".
I've not been able to confirm the original statement that "At one time, the nautical mile was discouraged for use by the BIPM" due to a lack of access to early editions of the SI brochure. The 7th edition notes that in 1969 the CIPM "listed three categories of non-SI units: units to be maintained; to be tolerated temporarily; and to be avoided", but fails to state which category contained the nautical mile. An early table containing the nautical mile from an unknown edition was entitled "other units outside the SI that are currently accepted for use with the SI, subject to further review". The table in the 6th edition (1991) is entitled "units temporarily accepted for use with the SI" while the table in the 7th edition (1998) is entitled "other non-SI units currently accepted for use with the International System" (but its "use is not encouraged"), compared to the 8th edition (2006), which only has "other non-SI units". Although section 4.1 is entitled "non-SI units accepted for use with the SI, and units based on fundamental constants", even the preface to table 7 states that its units "are not generally used with SI". — Joe Kress 05:56, 1 September 2007 (UTC)

[edit] abbreviation for nautical mile

If you have a view on what abbreviation(s) should or should not be used, you may be interested in reading this discussion. Thunderbird2 20:55, 4 September 2007 (UTC)

[edit] Clarity regading nations

In the history section, the fourth paragraph begins: "Other nations had different definitions of the nautical mile." I infer from this phrase that the preceding three paragraphs have been refering to one or more specific nations, yet none is mentioned. --Jamestowell 19:27, 9 September 2007 (UTC)

You're right. It doesn't make sense. Feel free to improve the wording yourself when you spot something like this. Thunderbird2 19:49, 9 September 2007 (UTC)

[edit] Simplification of length of minute of latitude

I have simplified the passage that explains the length of a minute of latitude. Since nautical miles are ordinarily used in navigation, it is appropriate to round to the nearest meter. Also, since nautical charts use geodetic latitude rather than geocentric latitude, I removed the passage about geocentric latitude.

Also, I added a reference to the Explanatory supplement to the Astronomical almanac. --Gerry Ashton (talk) 19:59, 6 February 2008 (UTC)

[edit] Admiralty mile

When was it adopted? Septentrionalis PMAnderson 16:09, 3 March 2008 (UTC)

N.A.M. Rodgers, in The Wooden World: An Anatomy of the Georgian Navy, makes the following statement, "Commodore Frankland... reported dangerous variations in marking the log line, and consequently in reckoning distance run: 'The Winchester, by allowing only forty-two feet to a glass of thirty seconds, overrun her reckoning by near a hundred leagues between Madeira and this island [Barbados]. 35 He asked for an Admiralty order fixing the length of the log line.
Endnote 35, Public Records Office, Letters of the Admiralty, T. Frankland, 18 Nov 1755.
Now the above is far from a statement that in 1755, as a consequence of variations in marking the log line, the Admiralty fixed the nautical mile. However, as this period saw an increased cognizance in the import of precise and reliable navigation, I believe we are getting close, if you will. Would anyone care to opine? --Crusher1 (talk) 03:33, 20 April 2008 (UTC)

[edit] What is "so"?

The article title is "Nautical Mile so", and the page for "Nautical Mile" redirects here, but nowhere in the article is the "so" part defined. What does it mean? 150.101.166.15 (talk) 23:41, 27 March 2008 (UTC)

The page has been vandalised, but I don't know how to fix it. The "so" is someone's idea of a joke. Does anyone know how to retrieve the correct name please? Thunderbird2 (talk) 10:14, 28 March 2008 (UTC)
I have reported this vandalism WP:AIV. I hope an administrator will know the best method to undo this problem. --Gerry Ashton (talk) 14:20, 28 March 2008 (UTC)
User:Skomorokh has fixed the problem. --Gerry Ashton (talk) 14:50, 28 March 2008 (UTC)
Thanks. Thunderbird2 (talk) 15:36, 28 March 2008 (UTC)

[edit] Sea Mile /Nautical Mile

The Admiralty Manual of Navigation and the RYA Navigation Handbook both make a helpful distinction between these two terms. A Sea mile is the length of one minute of arc, along a meridian. It is actually calculated as the angle between two intersecting normals, it does not make reference to the centre of the earth. A "normal" is a line at right angles to a tangent and running through the tangent at the point it touches the curve. Since the earth is not a sphere, and its cross section not a circle, then the length of a Sea Mile does alter according the latitude it is taken. When a navigator takes a distance measure from the vertical edge of a chart, he is measuring sea miles.

Nautical Miles are an attempt to create a "standard" or average Sea Mile, one which is the same no matter the latitude it is used. This is vital for specifying speeds, unless a knot is to have a slightly differing value at different latitudes. This is largely a matter of agreement between various maritime authorities.

To sum up, this excellent article would be improved still further if the diagram were to be changed to say "sea miles", and distinction between the two be made more clearly.

Andrew A Powell (talk) 09:07, 11 April 2008 (UTC)

The variation of the length of a degree of latitude (60 minutes) relative to an ellipsoid is discussed at Latitude#Degree length. — Joe Kress (talk) 02:46, 12 April 2008 (UTC)