The Nautical Mile

J

Jaguar

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Something troubled me over Christmas. We all know that the nautical mile is equal to one minute of arc of latitude as measured over the earths surface. However, since the earth is squished at the poles, this would render the nautical mile somewhat shorter at the poles than the equator. Does anyone know how the value for the nautical mile was arrived at? If it truly is equal to one minute of arc, irrespective of measurement location, then the actual value of the Nm would be relative. This approach does not easily sit with my regimented view of the world order. I just like to know these things :o)
 
Try to visualise this is as follows:
1. Draw a circle
2. Draw a line from the centre of the circle to the 12 o'clock position
3. Draw a second line from the centre of the circle at 1 degree after 12 o'clock
4. Now repeat steps 2 and three but this time at 3 o'clock and one degree after 3 o'clock
5. observe that the distance between lines 1 and 2 and lines 3 and 4 is the same where these lines cut the arc of the circle.

ie the lines of latidude are not "squashed" at the poles. They are actually measuring the arc.

Dave L.
 
Aye in fact the further north ye gang the lager (sb langer but lager seemed apposite!) each mile seems ... hae ye no heard o' the lang scots miles?
 
What's the confusion? As you say the earth is not a perfect sphere, so the nautical mile varies slightly in length from 6046ft at the equator to 6108ft at the poles.This is not a problem because a Mercator chart has a built in scale of distances relating to that latitude.Instruments such as logs and radar are calibrated on a mean figue of 6076ft, now universally adopted as the International Nautical Mile
 
...Try drawing an elipse, with the major axis considerably larger than the minor axis (say, 200mm x 100mm) and then carry out the steps you described. If you then measure the line of arc (subtended by say, 5 degrees), you will notice that the line of arc as measured will be considerably longer at the major axis than that at the minor. Of course, the earth is not flattened to the same extent as I have descibed, but the effect will be the same. i.e. the length of one minute of arc of latitude (as measured across the earths surface) at the poles will not be the same as that at the equator.

JJ
 
Do a search on Mercator projection, what most of the charts used on based on, explains why ! minute latitude varies as you go north.

Brian
 
That is correct - the Nautical Mile does vary (a little) at the poles - but the difference is not really significance. There is also a standard distance the "Sea Mile" which is 6000ft - marginally shorter than the Nautical Mile.

To all practical purposes these measures are identical
 
Does anyone know by how much it does vary? As you say, I didn't use to think that it could vary much, till I went on an RN navigation course, where they were very strict about always using the part of the latitude scale at the same level as the distance that you were measuring.

Is this just a case of the Navy being over keen, or does it really make a difference?
 
If you read \"Leckys\" and..

many other books you will fin that yes the NM does vary in length from the equator to the pole.

The mean length is about 6,076.8 feet (plus a few decimals), often taken to be 6,080 feet (accurate onlt at 48 degrees N/S). At the poles one NM is about 6,109 feet and at the equator about 6046 feet.

For those interested in more I reccommend Captain Lecky's "rinkles in Practical Navigation", written in 1851 and still useful (but forget right ascension).
 
Primarily due to ..

the projection and distortions thereby introduced. That is why you must measure at the latitude at which you are positioned.

Look at a gnomonic chart, the type used for navigation at the poles and you will see latitude lines curved (and parallel) and longitude straight.
 
Re: Primarily due to ..

Gnomonic projection is also used on small scale charts, ie harbour detail, and insets.

Life is complicated, or just a challenge.

Brian
 
Well, well, someone else has a copy!

It is strange that this Bible of the keen merchant marine deck officer until the dawning of the age of e-whizz is so scarce. One seldom sees a copy. Full of good stuff, two or three bits of which I have actually used.

I always thought that the official "standard" nautical mile for log calibration was specified as, "measured in the English Channel, off the Nab Tower"? That is to say, 50 degrees, 40.05 minutes North, (and, for what it may be worth, 0 deg 57.7' West!)
 
ponapay said:
If you read \"Leckys\" and..

many other books you will fin that yes the NM does vary in length from the equator to the pole.

The mean length is about 6,076.8 feet (plus a few decimals), often taken to be 6,080 feet (accurate onlt at 48 degrees N/S). At the poles one NM is about 6,109 feet and at the equator about 6046 feet.

For those interested in more I reccommend Captain Lecky's "rinkles in Practical Navigation", written in 1851 and still useful (but forget right ascension).
Click to expand...
I have just been refreshing my memory ref the actual distances of a nautical mile at the Equator and the Poles. I am aware of the internationally recognised figure 1852m. The Earth is an oblate spheroid (flattened at the poles), therefore the distance from the centre of the Earth to the poles will be less than centre of the Earth to the Equator. Therefore the 1 minute of latitude subtended at the Equator will yield a greater figure for a nautical mile than the 1 minute subtended at the poles. Various sources including this post and Wikipedia seem to have this muddled up, or is it me?

Look forward to replies

Minesapint
 
The International Nautical Mile is 1852m.
IT's exactly that an any latitude and any altitude. It doesn't get bigger in a plane a 30,000 ft

Historically, nautical miles have be defined in various variations of a minute of latitude, and navigation has rarely been accurate to make the differences matter.
I don't know if any other official NMs are still defined?

Just a metre was once defined as 1/10millionth of the great circle from the North Pole to the equator through Paris, then that wasn't precise enough so it became a standard piece of metal, now it's defined by the second and the speed of light. The definition actually changed while I was at college.
 
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