Talk:Nautical mile/Archive 1

Archive 1

Wikipedia abbreviation for nautical 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)

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)

I cannot add clarification to this or a reference. I can agree with below - exactly 2000 yards to is a close approximation, simplify math. However I believe No mariners of any country use meters & kilometers at sea. Matt Wilkens defines cable and fathom as sub-units of nautical mile, I do not think that is the orginal definition of those units, instead I suspect original cable and fathom definition was perhaps as 6 feet = 1 fathom, 1 cable = 100 fathoms. Amongst mariners the 6000 feet = 1 nmi is accepted. 74.214.43.199 (talk) 01:41, 21 October 2010 (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)

One tenth nautical mile was a stadion of 185m. The Greeks measured 8 stadions (cables) to a thousand of land (mia chilioi) and 10 stadions to a nautical mile. The Romans measured 75 mille passus or milliare to a degree. 12.187.95.196 (talk) 10:30, 19 September 2013 (UTC)

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)

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. — Preceding unsigned comment added by Bingbongbelgium (talk • contribs) 20:12, 12 March 2007‎

Seeing that the sea mile is 6000 feet exactly, it can hardly vary from place to place. It must be the metre (an other geographic unit, equal to 0.1 centisimal second), that varies. A nautical mile represents a degree at the surface of a sphere approximating the earth. Wendy.krieger (talk) 07:50, 3 June 2012 (UTC)

It sounds like the sea mile is defined in different ways by different organisations (a really good reason not to use this unit). The article should start by pointing out the ambiguity, and then provide the various conflicting definitions. Dondervogel 2 (talk) 10:35, 3 June 2012 (UTC)

A meter is a unit of length that does not vary anywhere on Earth's geoid. If it was oriented along the equator it would subtend 2.16 milliseconds of time. If it was oriented parallel to any other line of latitude (all small circles), it would subtend greater periods of time at greater latitudes. At 89° it would subtend about 124 ms. A nautical mile is one arcminute (not one degree) of any great circle on the surface of the Earth, including along the equator and along any meridian/antimeridian, but not along any parallel of latitude other than the equator. Using this definition it varies no more than 0.5% from 1852 meters, the International nautical mile.

The "sea mile" definition of 6000 feet is due to Richard Norwood in his extraordinarily popular Seaman's Practice (1637) which was still being sold in 1776, and quoted as an authority in 1822. He personally observed the altitude of the Sun in London at the summer solstice of 1633 and in York at the summer solstice of 1635, and measured the meridional distance between them using chains and pacing. He then calculated that a degree of latitude was 367,196 English feet, but rounded this to 360,000 feet per degree or 6000 feet per arcminute. All commentators until the mid 19th century noted that 6000 feet was a rounded value. — Joe Kress (talk) 05:54, 4 June 2012 (UTC)

Joe Kress says:

"A nautical mile is one arcminute (not one degree) of any great circle on the surface of the Earth, including along the equator and along any meridian/antimeridian, but not along any parallel of latitude other than the equator."

That's not correct. As the article makes clear, a nautical mile is not one minute of any arc, but is defined internationally as 1852 meters exactly.

By the way, Richard Norwood's A Seaman's Practice is still in print today, available from Amazon.

Dondervogel 2 says:

"It sounds like the sea mile is defined in different ways by different organisations (a really good reason not to use this unit). The article should start by pointing out the ambiguity, and then provide the various conflicting definitions."

The article is about the modern term "nautical mile", not about the disused term "sea mile", so it correctly starts out by defining the former. The latter is taken up in the second section, where it is clear that there are at least three definitions of "sea mile", two of them official, and one of which has changed since 1966. . . Jim - Jameslwoodward (talk to me • contribs) 10:53, 4 June 2012 (UTC)

Jim completely ignored my statement "Using this definition it varies no more than 0.5% from 1852 meters, the International nautical mile." so I was obviously refering to the nautical mile's historical definition, which is correct. Most of my definition even appears in the lead paragraph, prefixed with "about" to account for the difference I explicitly mentioned. The article is not only about the modern term nautical mile, it is about all historical definitions as well.

I see two print versions of Richard Norwood's Seaman's Practice available, from BiblioBazar and Eebo editions. Both appear to be printed versions of microfilm/microfiche editions published during the last half of the 20th century. As such they contain all defects of the microfilm/microfiche editions, including off center and cropped pages and illegible letters, words, paragraphs or pages. The second, Eebo editons, is obviously a copy of the copy in Early English Books Online viewable at many libraries (one page at a time). — Joe Kress (talk) 01:45, 5 June 2012 (UTC)

We're quibbling over a very small point, and since I started it, I'll take the blame. The problem here is that many readers do not understand this subject very well -- read all of this talk page, including the claim below that a nautical mile is a minute of longitude, not latitude, and you will see what I mean. Therefore, I react when someone makes a statement that may mislead other readers -- it is absolutely correct that a nautical mile is very close to a minute of any great circle -- that is, after all, the whole point of using the special unit for navigation -- and I did note your comment that it varied by less than half a percent, but you must agree that your flat statement, without qualifiers, is not correct.

As for the Norwood, certainly it's a copy of a microfilm, but it interested me that it was readily available, in stock at Amazon. Of course old sea books have long lives -- Bowditch has been continuously in print for more than 200 years. Of course, unlike Norwood, Bowditch has been revised more or less continuously over its entire life. . . Jim - Jameslwoodward (talk to me • contribs) 23:08, 5 June 2012 (UTC)

Early English Books Online is the online version of four microfilm collections issued between 1938 and the present. One collection is Early English Books by Donald Wing, which contains Mr. Richard Norwood's Works, which contains the 1670 ninth edition of Seamans Practice. Although the online version has a few duplicate pages, and many pages that tilt toward each other, none of this affects its legibility. I did not consult either printed version.

After observing the altitude of the Sun from both York and London and measuring the length between them in chains, Norwood concluded on page 5 that one degree of a great circle was 367196 English feet or 367200 feet "lacking 4 feet, which here we regard not." Norwood stated that the latter made a degree 69 English miles 4 furlongs 14 poles [69.54375 statute miles, where 8 furlongs/mile × 40 poles/furlong × 16.5 feet/pole = 5280 feet, which conflicts with 367200 feet / 5280 feet/mile = 69.54545 miles. Norwood mentioned "and about one half [of a pole]" earlier, but not here. He should have specified an additional 9 feet beyond 14 poles for 367200 feet (he mentioned 5 feet earlier for 367196 feet)]. On page 48 he assigned to a degree only 360000 feet so a mile was 6000 English feet. This was intentionally shorter than the length of the degree that Norwood measured so that reckoning by a log line with knots would indicate that the ship had sailed its intended distance before it reached its intended destination to avoid surprise, and "for the rotundity of the number". Norwood never used the terms "sea mile", "geographical mile", or "nautical mile". He only used "mile" for each of the 60 miles in a degree. — Joe Kress (talk) 05:10, 23 June 2012 (UTC)

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)

In one section ( toward the bottom ) the Wiki page lists a nautical mile as exactly 1,852 meters. In another section, we're told "The Imperial (UK) nautical mile, also known as the Admiralty mile, was defined in terms of the knot such that one nautical mile was exactly 6080 feet (1853.184 m):[5] it was abandoned in 1970[5] and, for legal purposes, is now converted to metres on the basis of one UK nautical mile = 1853 metres exactly.[6]" —Preceding unsigned comment added by 70.91.201.209 (talk) 23:26, 15 January 2010 (UTC)

Both are correct. The international nautical mile is exactly 1,852 m. The Imperial nautical mile was defined as 6080 feet, which is equivalent to 1853.184 m. However, when the UK decided to abandon the Imperial nautical mile, they also decided that if old references to it need to be converted to SI, the old references should be converted with a conversion factor of 1853 meters to the Imperial nautical mile. --Jc3s5h (talk) 23:50, 15 January 2010 (UTC)

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)

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)

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.

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)

The quick-and-dirty approximation used for practical navigation is that a nautical mile is 15% longer than a statute mile:

5280 ft X 1.15 = 6072 ft ≈ 6,076.1

This is also known as "slide rule accuracy" — the kind of precision one would get with a traditional E6B Flight Computer. —Quicksilver^{T @} 19:11, 8 January 2009 (UTC)

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)

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)

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) There should be a short and absolute definition in the beginning of the main article text of that what a nautical mile equals to meters. This is the main inadequacy of the article.It must be clearly expressed that a nautical mile e quals to 1852 m. at the very begining. because many people may want to obtain shortly the meter equivalent of nmi.so i am writing down this knowledge at the main definition paragraph .yes it is existing in the frame but it must also be in text either. —Preceding unsigned comment added by 85.108.76.2 (talk) 03:02, 12 February 2010 (UTC)

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)

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)

As it stands now the lead mentions the abbreviations M, NM and nmi, and the Unit Symbol section mentions M and nm. I think the Unit Symbol section should mention NM and nmi as well. The discussion mentioned above has been archived. Ulflund (talk) 11:47, 8 December 2011 (UTC)

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)

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)

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)

Circular Definition

The knot article defines a knot as one nautical mile per hour. The nautical mile definition says a nautical mile is defined as one knot divided by one hour. Somewhere there has to be an original definition for the (admiralty) nautical mile, but what was it? Rhialto (talk) 08:56, 7 March 2011 (UTC)

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)

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)

Good point. That's something I noticed a while ago, then completely forgot to do anything about. When I find my copy of the Mariner's Handbook, I'll have a go at properly defining sea miles. There will probably have to be a mention made on Mile as well, and a redirect page created for Sea mile. Wardog (talk) 15:34, 18 June 2008 (UTC)

I find the definition of sea mile in this article confusing. When I went to the source (The Admiralty Manual of Navigation) I was not confused. I made an ill-advised edit, which I have now reverted. It's the apparent (but not actual) redundancy of "1' of arc of latitude ... along the current meridian" that throws me off track. OK. This is looking more and more like a problem with the way my brain is wired, and less and less like a problem with the writing. But for me, I think I could understand it better if the first bit just said 1' of arc along the current meridian at the current latitude. Then a second sentence could state this corresponds to 1' of latitude and briefly explain why 1' of latitude varies as a function of current latitude. In the source, when it was separated out, I found it easier to follow. Does anyone object to my trying to do a small rewrite in the next few days? Susfele (talk) 17:06, 20 June 2010 (UTC)

Thank you, Jameslwoodward, for rewriting the sea mile definition. It's both more succinct and more understandable. Susfele (talk) 00:31, 22 June 2010 (UTC)

Sea Mile /Nautical Mile

The Mariner's Handbook defines the International Nautical mile as 1852m; and the Sea mile as "the length of one minute of arc measured along the meridian in the latitude of the position; its length varies both with the latitude and with the dimensions of the spheroid in use". It defines the geographical mile as "the length of one minute of arc measured along the equator; its value is determined by the dimensions of the spheroid in use".

The Admiralty Manual of Navigation Vol 1 (1987) says that the abbreviation for a nautical mile is "n.mile". It gives the abbreviation for sea mile as "M" on charts and '(as used for minutes of arc) elsewhere.

Tim Bartlett —Preceding unsigned comment added by 82.153.197.217 (talk) 12:50, 2 January 2009 (UTC)

Radar Mile?

I'm surprised to see 'radar mile' here. I think it is in the wrong article.

The point is, a radar mile is measurement of time, not a measurement of distance. As stated it is the time it takes for a radar (RF) signal to go a mile, strike an object and be reflected back to its origin. A radar mile is therefore 12.36 micro-seconds.

It's a bit like calling 100 'sprint' metres, 20 seconds. (the time to run 100M and run back again) I'm being picky but it there is no other connections this could be deleted This information only belongs in the Radar article (or maybe in the Mile article —Preceding unsigned comment added by 220.101.28.25 (talk) 01:01, 25 October 2009 (UTC)

The radar mile is a unit of time that it takes radar to travel one mile. This is the time it takes radio waves to go out and back, one mile. It is a unit of time, but as 'knot' is here as a derived unit (nm/hr), so should this unit (nm/radar). --Wendy.krieger (talk) 07:10, 30 August 2010 (UTC)

Gents, the lead-in line to the main page is incorrect.

It states "The nautical mile (symbol M, NM, Nm or nmi) is a unit of length corresponding approximately to one minute of arc of 'latitude' along any meridian. By international agreement it is exactly 1,852 metres (approximately 6,076 feet).”

However, it is not latitude, but longitude that one measures a nautical mile from (on a chart), as the length (or distance apart) of the minutes of latitude vary depending on how far from the equator one gets, whereas minutes of longitude do not. Hence, a nautical mile measured from a line of latitude near the Pole would give you a much shorter nautical mile than if measured along a line of latitude at the equator. However this variation does not occur in the lines of longitude and hence why longitude is ALWAYS used to measure a nautical mile.

So that first line, and any others of similar nature referring to ‘latitude’ as the measure of a nautical mile in the article needs to be changed. — Preceding unsigned comment added by 124.150.97.91 (talk • contribs) 23:31, 15 February 2011

Sorry, but you have it backwards. Perhaps you have been mislead by looking at a Mercator projection which makes it appear as you describe, but a minute of longitude is approximately one nautical mile at the equator and zero at the poles. . . Jim - Jameslwoodward (talk to me • contribs) 17:55, 16 February 2011 (UTC)

According to the articles aboutlatitude and longitude I would say James is wrong. Do you have a link that would explain your statement? 130.126.28.48 (talk) 22:43, 30 November 2011 (UTC)

He said a minute of longitude decreases from a nautical mile at the equator to zero at the poles; are you saying he's wrong about that? Aside from some semantic quibble it's hard to see how he could be. Tim Zukas (talk) 22:03, 1 December 2011 (UTC)

I also took a few minutes to work this out... not sure that "approximately the length that spans one minute of arc on the surface of the Earth, measured along any meridian" is a helpful first sentence for a casual reader! Also unsure of the best way to explain a nautical mile, but the technical side of it should probably come later... and with lots of diagrams! ‑‑Yodin^T 15:35, 9 November 2015 (UTC)

It is meridians of longitude and parallels of latitude (see Macquaire Dictionary for def of meridian). Therefore, the lead is incorrect. (My simple memory jog is that lats are long - they go around and are endless) The circumference of a parallel of latitude gets markedly shorter, the closer you are to the pole (from a maximum at the equator to zero at the pole). Cinderella157 (talk) 05:15, 10 November 2015 (UTC)

So your amendment was wrong. A minute of longitude is a different distance depending on what the latitude is at that point. The nautical mile is broadly equivalent to a minute of latitude, and you measure that as you come down a meridian, which, as you say, is a line of longitude. --David Biddulph (talk) 08:19, 10 November 2015 (UTC)

The picture in the history section of the article shows the measurement of a minute of latitude. If you are still confused, look at a globe with the lines of latitude and longitude marked on it. --David Biddulph (talk) 08:26, 10 November 2015 (UTC)

I don't think readers should have to click on links, scroll down to other sections, or look terms up in order to understand the basic concept. As such I've had a go at changing the lead: hopefully this will reduce the number of us puzzled folk turning up here on the talk page. ‑‑Yodin^T 12:41, 10 November 2015 (UTC)

@David Biddulph If it is a meridian of longitude (per my ref and the diagram) then I think it is not me that is wrong. See also Meridian (geography). @Yodin I don't disagree with your edits per se but with the moves in the figures at the same time, the changes to text aren't clear. Perhaps you might do these separately given there is some contention on the issue? Cinderella157 (talk) 13:27, 10 November 2015 (UTC)

@Cinderella157: Your edit was most definitely wrong. You said "The nautical mile is nearly equal to a minute of longitude on a chart". You said yourself that "The circumference of a parallel of latitude gets markedly shorter, the closer you are to the pole (from a maximum at the equator to zero at the pole)." The circumference of a parallel of latitude is 360 degrees of longitude, hence if you measure the distance of a degree (or a minute) of longitude it is smaller if you measure it nearer to the pole. (For example, a minute of longitude at 60 deg of latitude is half the distance of a minute of longitude at the equator.) That is why the article said, correctly, that "The nautical mile is nearly equal to a minute of latitude on a chart". From a quick glance it looks as if Yodin's rearrangement makes sense, so hopefully people in future may not be so inclined to get confused. --David Biddulph (talk) 14:03, 10 November 2015 (UTC)

• Apologies Cinderella157, I'd never thought of that way of getting around the loathsome diff engine before! I don't mind reverting and making the changes again one at a time if that helps? ‑‑Yodin^T 14:28, 10 November 2015 (UTC)

Questioning the conclusion

Nautical Mile: If a nautical mile is greater at the poles than at the equator, how can the earth be considered an oblate spheroid rather than a prolate spheroid? How can it be said that we know the earth is wider at the equator than at the poles given the length of a nautical mile is longer at the poles than at the equator?Goodhayman (talk) 12:33, 14 April 2011 (UTC)

A frequent source of confusion. What you need to do is think about what latitude is. Suppose the Earth were much more oblate-- still an elliptical cross-section, but pancake-shaped. Where would 45 degrees latitude be? Near the equator, or near the poles? Tim Zukas (talk) 16:26, 21 October 2011 (UTC)

Also the elliptic shape of Earth is a few meters of difference, or so - crossing waves at the oceans or sailing where the tides are strong means more. Not to speak of mountains on land. Boeing720 (talk) 02:53, 16 January 2015 (UTC)

Telegraph mile

Unless there is an objection, I am going to remove this section. I can't find any substantive reference for a "telegraph mile" or "telegraphic mile" except a few sites on Google that provide conversions into or out of it. They may simply be built off of the definition here. It is certainly of less importance than many other uses of "mile" that we do not include here. . . Jim - Jameslwoodward (talk to me • contribs) 13:01, 17 February 2012 (UTC)

There being no objection after waiting 2+ weeks, I have removed this section. . . Jim - Jameslwoodward (talk to me • contribs) 14:24, 7 March 2012 (UTC)

1,852.00 - wrong

From a mathematical point of view are the zeros incorrect. They suggest an accuracy of six digits. But the true value (modern definition) is 1,852 meter exactly. It's an integer value, not a floating one or "with decimals". There is no call for adding the zeros. Actually 1,852.00 means a value somewhere between 1,851.995 and 1,852.004. On the other hand in order to point out this is a four digit integer, it's expressed 1.853 x 10³, but that is an exaggeration. But not the added zeros. And I'm not a mathematican, just someone that has studied mathematics to a cetrain level (some lower university courses) a long time ago. Boeing720 (talk) 02:45, 16 January 2015 (UTC)

Q: What is the length of a minute of longitude?

A: Read this and draw your own conclusions. Dondervogel 2 (talk) 09:22, 28 December 2015 (UTC)

That page simply shows the futility of trying to explain earth-centric nautical miles in SI metric or imperial units! For example, is a nautical mile on the moon equal to 1852m? Santamoly (talk) 09:31, 4 March 2016 (UTC)

One nautical mile is equal to 1852 m, by definition. It doesn't matter whether you are on Earth, Mars or Alpha Centauri. But that's not what this thread was about. Dondervogel 2 (talk) 17:05, 4 March 2016 (UTC)