The Nautical Mile

[DJE]

Is there a practical use for this discussion ? or is it just a theoretical one ? just curious.

Brian

Well there seem to be two opposing views. It would be nice to know which one is right.

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

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).

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

Since posting a couple of days ago (post no 19) I feel the point I was trying to make has not been understood.

A nautical mile was, is and always will be 1 minute of arc anywhere on the Earths surface as used today in sight reduction. There are an infinite number of nautical miles each measuring slightly more or less within a few metres. Due to the Earth being fatter round the Equator nautical miles measure more around the Equator and less at the poles. The slight differences in measurements may not be important but this is no reason for not wanting to have an understanding of the facts.

When I looked on Wikipedia I noticed the actual distances for a nautical mile were quoted incorrectly giving a shorter distance for a nautical mile at the Equator and longer at the poles. I then also noticed the same figures incorrectly quoted in 2 posts in this thread and elsewhere on the internet. If people are putting information on the internet it would be helpful to all of us if they ensured its accuracy. Of course it is possible and I have not checked that Wikipedia has been corrected and the incorrect posts above have been edited.

As indicated in my post no 19 I am aware of the Internationally recognised figure of 1852m but this is a different issue.

Mike

 

[anoccasionalyachtsman]

@MINESAPINT2 is confusing what would certainly be the case with two spheres of differing sizes and what happens with ellipsoids, which give the counterintuitive result that he's querying.

 

[Daedelus]

What no-one appears to have taken account of is that measuring the distance will inevitably be distorted by the size of the waves on the sea.

 

[newtothis]

What no-one appears to have taken account of is that measuring the distance will inevitably be distorted by the size of the waves on the sea.

And whether you're tacking up wind or on a straight run.

 

[anoccasionalyachtsman]

What no-one appears to have taken account of is that measuring the distance will inevitably be distorted by the size of the waves on the sea.

Or ice.

 

[Bodach na mara]

The comment about a plane at an altitude of 30 000 ft reminds me of a question set by a maths teacher. An engineer runs a cable right round the earth at the equator. Just before he cut the cable, his boss said that it should have been on pylons 10 metres above the surface. How much extra cable is needed?

I should warn you that there is a bit of a trick in getting the answer.

 

[newtothis]

The comment about a plane at an altitude of 30 000 ft reminds me of a question set by a maths teacher. An engineer runs a cable right round the earth at the equator. Just before he cut the cable, his boss said that it should have been on pylons 10 metres above the surface. How much extra cable is needed?

I should warn you that there is a bit of a trick in getting the answer.

Okay clever clogs, for those of us who can barely count on our fingers, how much? Please show your workings.

 

[mjcoon]

Okay clever clogs, for those of us who can barely count on our fingers, how much? Please show your workings.

Two*pi*(R+10) - Two*pi*R
So the value of R (radius of Earth, Mars, or wherever) makes no difference. Answer is always 2 * pi * 10

 

[penberth3]

The comment about a plane at an altitude of 30 000 ft reminds me of a question set by a maths teacher. An engineer runs a cable right round the earth at the equator. Just before he cut the cable, his boss said that it should have been on pylons 10 metres above the surface. How much extra cable is needed?

I should warn you that there is a bit of a trick in getting the answer.

You need to give a circumference at the Equator?

Ah, mjcoon says you don't!

 

[anoccasionalyachtsman]

The comment about a plane at an altitude of 30 000 ft reminds me of a question set by a maths teacher. An engineer runs a cable right round the earth at the equator. Just before he cut the cable, his boss said that it should have been on pylons 10 metres above the surface. How much extra cable is needed?

I should warn you that there is a bit of a trick in getting the answer.

We all know that the outside wheels of a car have to turn faster than the inside ones in a turn (otherwise there would be no need for a differential in the axles). But how many more turns would your left hand wheels do if your drove clockwise around the coast of Britain?

 

[penberth3]

We all know that the outside wheels of a car have to turn faster than the inside ones in a turn (otherwise there would be no need for a differential in the axles). But how many more turns would your left hand wheels do if your drove clockwise around the coast of Britain?

Same calculation with wheel track (say 2 metres) as the difference? Then divide by the wheel circumference for number of turns.

 

[anoccasionalyachtsman]

Same calculation with wheel track (say 2 metres) as the difference? Then divide by the wheel circumference for number of turns.

Track is averagely 1.5m, so even less than that... maybe 5 turns.

 

[newtothis]

Two*pi*(R+10) - Two*pi*R
So the value of R (radius of Earth, Mars, or wherever) makes no difference. Answer is always 2 * pi * 10

That wooshing noise is the sound of your answer going over my head.

 

[johnalison]

That wooshing noise is the sound of your answer going over my head.

He's saying that the answer is the same for all sizes of the Earth's circumference, even zero, when the circumference of this circle of radius 10 miles is given by his formula, or 2piR as it was in old money.

 

[newtothis]

He's saying that the answer is the same for all sizes of the Earth's circumference, even zero, when the circumference of this circle of radius 10 miles is given by his formula, or 2piR as it was in old money.

So more cable, right?

 

[mjcoon]

So more cable, right?

Even the question takes it for granted that more cable is needed!

 

[Bodach na mara]

The following week the answer was given. The teacher reminded us of the question and asked where it was stated that the planet was Earth. Without going through the formula approach, he postulated that the planet radius was zero and thus the extra length was the circumference of a circle of radius 10metres :- 62 . 8metres

 

[alan_d]

It's always worth spreading the confusion more evenly...

Ah, but would the confusion remain evenly spread?

 

[mjcoon]

The following week the answer was given. The teacher reminded us of the question and asked where it was stated that the planet was Earth. Without going through the formula approach, he postulated that the planet radius was zero and thus the extra length was the circumference of a circle of radius 10metres :- 62 . 8metres

I think you need a powerful intuition to deduce that a bigger planet doesn't need extra girth...

 

[mjcoon]

Ah, but would the confusion remain evenly spread?

You mean it coagulates by a sort of gravity of a density gradient of the thickest people?