What Is a 'Nautical Mile'? (2011)(science.howstuffworks.com) |
What Is a 'Nautical Mile'? (2011)(science.howstuffworks.com) |
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.
> Only if that minute of arc lies on a great circle route.
You missed out some context for that quote that already makes it clear that it's talking about a great circle:
> A nautical mile is based on the circumference of the planet Earth. If you were to cut the Earth in half at the equator, you could pick up one of the halves and look at the equator as a circle. ... A minute of arc on the planet Earth is 1 nautical mile.
That's all arcs. A line of lattitude is not an arc.
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.
Its not strictly true that 'proper' navigational planning happens entirely on ECDIS, especially on smaller vessels. My preference while working on offshore tugs was to work out the rough voyage on paper first, before transferring them to ECDIS; the 2nd mates I worked with in training had a similar preference.
That said, paperless navigation is becoming more and more common (and saves a ton of time on chart corrections), and in that case you don't have a choice.
I'm pretty sure to derive a nautical mile you have to use the equator, not just any great circle, because the earth isn't a sphere.
So you have the equator, and then 90 degrees north and south. Take one of those degrees, divide it by sixty (arc-minute), project it on the surface, and you have a nautical mile. That's regardless of where you are on the surface.
That's the original/historical definition; now it's 1852m.
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.
This makes measuring distance in nautical miles using a chart a lot more practical.
edit: of course you could pick off distance with any kind of measurement; but my point is that it converts directly into minutes of latitude
EDIT: that information is already in the article, making my comment redundant and unnecessary.
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.
The story in the article is the original, obsolete definition.
It’s a surface describing constant (magnitude of) the acceleration of gravity.
P.S.: Supply Corps, so never needed to know it.
Wrong. It used to be that way, but not anymore.
The article doesn't mention that it's not the current definition.
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".
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…
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
It is now been translated in relation to the metric system as 1852m.
I don't believe that is correct. The latitude scale of a Mercator projection chart will not be constant because the projection introduces some distortion. So 1NM at the top of the chart will not be the same distance as 1NM at the bottom of the chart. For this reason, you should always place your dividers roughly in the area you are going to be sailing in.
As I recall from my Yachtmaster course, the difference is not usually that great for day to day stuff, but for planning long passages on small scale charts it could be a significant error.