Talk:Nautical mile
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[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)
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- Simpler only if you're using feet, fathoms and cables. Don't you mean "most U.S. navy and merchant vessels"? Jimp 2Nov05
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- Well all vessels use these measurements, as the metric system is very difficult to use to any effect.
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- 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)
[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)
[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."
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)
