Talk:Nautical mile

Gunter's definition

The article says that "Gunter used Snell's circumference to define a nautical mile as 6,080 feet", and that is very close to modern values. But the article on Snell says that his circumference was too small by 3.5%. So it is hard to see how Gunter was able to get an accurate result. This needs to be verified against additional sources. JonH (talk) 15:05, 12 January 2018 (UTC)

To quote from "Edmund Gunter (1581–1626)" by C. H. Cotter (Journal of Navigation, Volume 34, Issue 3, September 1981, pp. 363-367), "he concluded that a one-degree arc of a meridian is equivalent to 352000 feet. This gave 5866 feet per minute of arc, a figure considerably in excess of the generally accepted value of 5000 feet. ... He drew the attention of English mariners to the need to improve their navigational practice by adopting a more accurate measure for the nautical mile. ... Gunter suggested the division of a degree-arc of the Earth's surface into a hundred parts or 'centesmes'." So it seems that he did not invent the nautical mile, nor define it as 6080 feet, and that someone later must have decided on 6080 feet. Some revision of the article is needed. JonH (talk) 18:32, 13 January 2018 (UTC)

No-one has provided any sources for this claim that Gunter defined a nautical mile as 6,080 feet, so I still think some revision of the article is needed. JonH (talk) 20:23, 25 August 2019 (UTC)

Physchim62 provided the answer to the problem, but his addition to the article on 22 May 2009 was removed by Cornellier on 10–11 June 2016, who also added your objectionable phase during his major rewrite.

Physchim62's addition to the article stated that the nautical mile was an average minute of arc on the Clarke 1866 ellipsoid according to A Dictionary of Physics by Richard Glazebrook, vol. 1, 1922, p. 587, which states "According to the definition adopted in England and the United States, the nautical mile is equal to the length of one minute of arc of a great circle on a spherical earth assumed to have the same area as Clarke's ellipsoid." Neither the year of Clarke's ellipsoid nor its radii are readable. However, Map Projections: A Working Manual by John P. Snyder, 1987, p. 16, states that the radius of the sphere having the same surface area as the Clarke 1866 ellipsoid (its authalic radius) is 6,370,997.2 m, yielding an arcminute of 6080 feet. — Joe Kress (talk) 07:42, 28 August 2019 (UTC)

Thanks for pointing this out. I cannot find a book or journal linking Willebrord Snellius's calculation of the circumference with Gunter's notion of using a arcs of latitude as a measure of distance. I did find this but it may be a case of WP:CITOGENESIS. The "objectionable phase" has been removed. --Cornellier (talk) 13:21, 28 August 2019 (UTC)

"old definitions are still in use" ... really?

The article says old definitions are still in use [by the RYA who says] "For most practical purposes distance is measured from the latitude scale, assuming that one minute of latitude equals one nautical mile". I wouldn't interpret that as the RYA is saying anyone should define the mile as such. Rather they are saying that one minute of latitude is a good enough approximation "for most practical purposes", a subtle difference, but the article implies otherwise. --Cornellier (talk) 23:10, 11 February 2019 (UTC)

Of course the RYA must speak for itself. However I doubt that the RYA is really interested in technical legal pronouncements, they are more interested in ensuring safe and practical navigation. For all navigational purposes one minute of latitude is a mile, any difference is trivial compared to the inaccuracies of a log or even GPS. Indeed, it becomes even less important than that; whether you call a minute one "nautical mile" or one "fibbergist" doesn't matter, you navigate in the historic units! Martin of Sheffield (talk) 23:18, 11 February 2019 (UTC)

The article says the Admiralty mile was about 4 feet shorter than the modern definition. According to gps.gov, global average user range error (URE) was 2.3 ft 95% of the time in 2016. NOAA says that some chart data may have originated from Captain Cook in 1778. If using a chart and compass 4ft does not matter in terms of significant figures. However the fact is that the GPSr is using the modern definition. So I'm not sure what the statement adds to the article. Is the RYA saying a 4ft error doesn't matter? Also I don't think one single quote from a book is enough to back up the statement that "old definitions are still in use". --Cornellier (talk) 11:12, 12 February 2019 (UTC)

Yet again, I cannot speak for the RYA. I don't see what the problem you are having is. "Despite the existence of precise modern definitions, in the early 21st century the old definitions are still in use.", they are in use. If you really want WP:CITEOVERKILL I can add two books recommended for the day skipper theory, the official RYA day skipper manual, the official RYA course notes for day skipper and the recommended yachtmaster theory course book, all of whom state explicitly or by direct implication that 1 nm = 1'. Uncitable perhaps, but I could quote the instructors on such courses.

If you don't trust the RYA, then Admiralty charts (who have been publishing charts for 200 years and which are used on 90 % of world shipping) in publication NP5011 states ""On smaller scale charts the latitude border should be used to measure Sea Miles and Cables". Of course you may be of the opinion that UK bodies know nothing about navigation, so perhaps a quote from the US NOAA might be of use: "A nautical mile is based on the circumference of the earth, and is equal to one minute of latitude. It is slightly more than a statute (land measured) mile (1 nautical mile = 1.1508 statute miles). Nautical miles are used for charting and navigating". Martin of Sheffield (talk) 20:35, 12 February 2019 (UTC)

The day skipper theory, the official RYA day skipper manual, the official RYA course notes for day skipper and the recommended yachtmaster theory course book are all wildly inaccurate (and therefore not reliable sources): One nm is not even close to 1'. Dondervogel 2 (talk) 21:22, 12 February 2019 (UTC)

Are you perhaps attempting humour? It is the only reason I can think of that in this context you can't tell the difference between a nautical mile and a nanometre, or between a minute and a foot. Having objected to "one single quote from a book" I have pointed you at multiple references from multiple highly authoritative agencies. Martin of Sheffield (talk) 22:17, 12 February 2019 (UTC)

My original point, which I perhaps did not express clearly, is as follows: the article states that "old definitions are still in use" followed by a quotation from an RYA manual. With the WP:PRIMARY reference given, the article can only say "an RYA publication uses the old definition". But the current text goes beyond that. It vaguely implies that the old definition is in general use. According to WP:OR, to make a general statement like that, a WP:SECONDARY is required, and not WP:CITEOVERKILL as proposed above. --Cornellier (talk) 15:08, 13 February 2019 (UTC)

Oh forget it. Three independant sources on both sides of the atlantic and you still won't accept a fact that any sailor knows. Conversation terminated. — Preceding unsigned comment added by Martin of Sheffield (talk • contribs) 15:24, 13 February 2019 (UTC)

I'm posting the text in question here:

Despite the existence of precise modern definitions, in the early 21st century the old definitions are still in use. The Royal Yachting Association says in its manual for day skippers: "1 (minute) of Latitude = 1 sea mile", followed by "For most practical purposes distance is measured from the latitude scale, assuming that one minute of latitude equals one nautical mile".

One tenth of a nautical mile is a cable length.

References

—  AjaxSmack  02:24, 1 July 2019 (UTC)

Why 1852m in 1929?

The arcminute based on the Clarke 1866 ellipsoid is 1853.2480 metres, and was used to define the nautical mile by Britain and the US. But the 1929 Hydrographic Conference in 1929 went with 1852 metres. Was this because France and other countries were already using the arcminute at 45 degrees, i.e. 1852? Or did they have newer data? Perhaps the international ellipsoid that was developed by John Fillmore Hayford in 1910 and adopted by the International Union of Geodesy and Geophysics (IUGG) in 1924, which recommended it for international use? I haven't investigate what arcminute it gives. Perhaps 1852 was the generally accepted mean value by that time? --Cornellier (talk) 19:01, 30 August 2019 (UTC)

I just investigated that by checking many issues of the Bureau des Longitudes' Annuaire on Gallica. From 1848 until 1909 1852 m was given as the length of a mille marin (nautical mile) without mentioning any ellipsoid (ellipsoids were described elsewhere without mentioning a nautical mile). From 1917 until at least 1950 1952 it was stated that since 1906 the legal nautical mile for the French Navy has been 1852 m, calling it a "conventional value". Since 1933, this description has been amplified by stating "The nautical mile is in principle the length of the sexagesimal minute of a meridian at a latitude of 45°." However, it notes that that depends on the specific ellipsoid. Their last explanation is the real reason, "If we admit that the meter is exactly the ten-millionth part of the terrestrial quarter meridian, it would be equal to 1851.85 m." That has been true since about 1799. That value, rounded to 1852 m, was rubber-stamped by the 1929 International Hydrographic Conference. — Joe Kress (talk) 03:57, 31 August 2019 (UTC)

If you want to check other issues, a convenient clickable list of all Annuaires is at [1], accessed by clicking "see all issues" at lower left. After selecting an issue, click on the spyglass on the left edge, and enter the search term mille marin. The calculation is 10,000,000/90×60 = 1851.85 m, thus not dependent on any ellipsoid. — Joe Kress (talk) 06:13, 31 August 2019 (UTC)

Thank you to Joe Kress and Cornellier for acting on my question above (#Gunter's definition). Seeing your replies, I Googled for "nautical mile 6080 clarke" and found an article (The Nautical Mile PDF) by A. B. Moody in the International Hydrographic Review dated 1950. It is a very clear and readable explanation of the history of the nautical mile, and it perhaps has other information which can be used to improve this article. JonH (talk) 14:49, 31 August 2019 (UTC)

Significant figures in conversions

A nautical mile is 1852 metres. Since the SI definition of the metre has nine sig figs, one could in theory convert a nautical mile to 6,102.4 feet and 1.150779 miles. However, WP:OPCOORD states that the larger the object being mapped, the less precise the coordinates need to be, and recommends that a minute of arc be rounded to the nearest metre. Therefore the nautical mile should be expressed as "about 6102 feet" and "about 1.2 miles". --Cornellier (talk) 14:47, 1 September 2019 (UTC)

since no opposition, updated article accordingly. --Cornellier (talk) 13:56, 29 September 2019 (UTC)

WP:OPCOORD has no bearing on the question – it applies only to the size of objects on a map, not the units used to measure them. In this case the article has reference 16 by Astin and Karo giving the legal equivalent as 6076.11549 feet, so that should be in the article, citing reference 16. This is repeated by reference 2 alongside 1.1508 statute miles, so the latter should be in the article, citing reference 2. Reference 10 by Jerrard [2] defines the cable as 608 feet (1/10 nautical mile} at least one of which is wrong, so cable should not be in the article. — Joe Kress (talk) 01:37, 30 September 2019 (UTC)

I missed this proposal when first posted. Seeing as the nautical mile is exactly 1852 m, there's a clear justification IMO to quote more than 4 significant figures (not fewer) in any conversion. I think it would be helpful to mention an exact number of miles (or feet) somewhere in the article. For the info box I suggest using 5 sig figs: 1.1508 mi and 6076.1 ft. Dondervogel 2 (talk) 09:26, 30 September 2019 (UTC)

Contradiction

There is just something that doesn't add up between the textual and the graphical versions of the historical definition of a nautical mile presented in this article. They appear to be contradictory. The text (in the lead) states the historical definition to be "the meridian arc length corresponding to one minute ( 1/60 of a degree) of latitude at the equator". However, the image portrays it as one minute of latitude along a meridian. The textual definition doesn't even make sense. If you move a minute of distance along the equator, that would be a minute of longitude and not latitude. Latitude refers to the north-south direction on the Earth. Lastly, as a result of the Earth not being a perfect round sphere (but rather an ellipsis), the polar circumference length of the Earth is not equal at the equator and through the poles and they thus aren't interchangeable to determine the length of a nautical mile.

Something is clearly incorrect here and I wonder which version is accurate. Tvx1 21:20, 12 February 2024 (UTC)

Nowhere does it say along the equator. The definition refers to a minute of latitude (i.e., along a meridian) at the equator. I see no contradiction. Dondervogel 2 (talk) 21:32, 12 February 2024 (UTC)

The contradiction is in the textual definition and the visual representation. The visual representation portrays it as random minute along a meridian. The image does not use the equator as either the starting or finishing point of the minute. Tvx1 06:38, 13 February 2024 (UTC)

I see what you mean. The graphic gives the impression of a perfect circle, when it is not. On the other hand it's clearly not to scale, so perhaps readers are expected to take it with a grain of salt. I agree the article would benefit from an improved graphic. Dondervogel 2 (talk) 07:53, 13 February 2024 (UTC)

I check the history of the article and I found that the "at the equator" part was added unilaterally by someone last summer without any discussion. Since this part was not in the historical definition, I think it should be removed.Tvx1 17:54, 14 February 2024 (UTC)

In that case perhaps we need to dig into the correct historical definition. I found 3 links that seem relevant

• On the history of the nautical mile (1934)
• The origins of the nautical mile (2008)
• The nautical mile (2023)

Dondervogel 2 (talk) 19:27, 14 February 2024 (UTC)