British Nautical Miles

[Gunfleet]

My Chambers Maths tables (school stuff, mid 60s) describes a British Nautical Mile and an International Nautical Mile. Does anyone know why there was a difference? Translating the numbers against each other, the British one is 4 ft longer. And who was Gunter? It says his chain was 22ft long. I thought all chains were 22 ft long.

 

[Salty John]

The sea mile was defined as the distance on earth corresponding to one minute of arc at the earth's centre. As the earth is not a true sphere the sea mile is different at the poles than at the equator. So both the Brits and the Europeans decided to fix it and call it the nautical mile. The Brits chose an imperial dimension, 6080 feet (1853.18 miles) and the Europeans (the International nautical mile) did it in metric, 1852 meters.
No idea about Gunter.

 

[BrendanS]

This is Gunter

http://en.wikipedia.org/wiki/Edmund_Gunter

http://en.wikipedia.org/wiki/Gunter's_measurement

 

[Gunfleet]

Thanks. Though your fingers have slipped here <<6080 feet (1853.18 miles) >> /forums/images/graemlins/ooo.gif

 

[Daedelus]

Good grief man, it's because British is best and bigger than...oh all right.

Loosely a sea mile is defined as 1 minute (1/60th of 1 degree) of arc around the world. There is a difference of opinion as to how far this is, partly caused by the fact that the earth is not actually round. Thus the length of 1 minute of arc varies from the equator (most) to the poles (least). There is also a variation due to which definition of the shape of the earth you use and a further one due to sheer bloody mindedness (alright I made the last one up).

Having a unit of distance that changes depending on where you are is less than ideal (and different ideas of length of the same measurement can cause problems - especially with GPS), so it has now been agreed that a nautical mile is exactly 1852 metres* which if my rusty calculator is correct is 2025.3472 yards.

* The metre has changed a fair bit since first defined as 1/10,000,000 of the distance between the equator and the N pole (via Paris). Unfortunately, the distance was not measured accurately and the first great survey of India, (using The Great Theodolite which weighed over a ton and had to be transported the length and breadth of India with tigers, cobras, malaria etc to contend with) not only showed the world to be a totally different size but demonstrated that mountain masses such as the Himalayas pulled the plumb bob off centre and that threw calculations out too.

Now that science that closely resembles magic has taken over the metre is defined as the length of the path travelled by light in a vacuum during a time interval of 1/299,792,458 of a second, should you ever wish to measure it for yourself.

I think I'll go and lie down now, my brain hurts.

 

[Oldhand]

[ QUOTE ]
The sea mile was defined as the distance on earth corresponding to one minute of arc at the earth's centre. No idea about Gunter.

[/ QUOTE ]

I think you mean one minute of arc subtended on the earths's surface, at the earth's centre it would have zero distance.

 

[Salty John]

Must be the wine!

 

[Salty John]

I'm sure you're right. I thought my description might be more understandable to the geometrically challenged, like me!

 

[mortehoe]

What a brillinant thread sofar

Just love that 1nm = 1853.18 miles .... ie "The Brits chose an imperial dimension, 6080 feet (1853.18 miles) and the Europeans (the International nautical mile) did it in metric, 1852 meters"

Eitherway both are wrong: one minute of arc is supposed to represent 1nm but as the earth is an oblate spheroid ..... well GPS's can't even work it out as the position gets closer to the poles.

IMO, the easiest method of remembering it is ~2000 yards and that's good enough for dead reckoning. And if you have to do it in metric then all you have to remember that 1" is exactly 25.4 mm /forums/images/graemlins/smile.gif

 

[Salty John]

Re: What a brillinant thread sofar

Err, the original poster asked about how we have two 'official' nautical miles. The answer I gave is correct, except for my 'miles' instead of 'meters, mistake, a slip of the fingers. I don't understand the reference to 'the best way of remembering...' What are we trying to remember and why does 1" equals 25.4mm come into the equation?
Because the earth is an oblate sphere and the sea mile, one minute of arc, varies from 6064 feet at the equator to 6108 feet at the poles the authorities at some time in the past decided to standardise the nautical mile at 6080 feet (the British authorities) or 1852 meters ( the International panel). Because one selected a round number of feet and the other a round number of meters there is a 4' difference. What part of this is complicated and cause for amusement, other than I typed miles instead of meters?

 

[Gunfleet]

Re: What a brillinant thread sofar

Tsk. You're usually very difficult to rile John. Did you read the Gunter thing? Now there's a very clever fellow.

 

[Salty John]

Re: What a brillinant thread sofar

You're right. My mask of infinite tolerence slipped momentarily. I apologise.
That Gunter bloke was quite the thinker wasn't he? I wonder if he came up with the Gunter rig? Perhaps as a break from spheroid geometry he turned his mind to thoughts of a more efficient way of propelling a boat by the power of the wind. You never know.
I'm sure I'll be corrected if this is not the case.

 

[shmoo]

Here is what the Admiralty Manual of Hydrographic Surveying (1965) had to say on the matter. No mention of nautical mile, only sea mile. The foot notes are important too (I think...) so I have included them. Seems to suggest that there is really not fixed value.

 

[Salty John]

Yes, the sea mile is as so defined - a variable dimension. The Nautical Mile was the agreed fixed dimension for a sea mile to avoid confusion.
I actually know very little about this stuff but being a keen navigator before the welcome arrival of GPS I was forced to learn the basics - such as the nautical mile. Perhaps Gunter himself will be along shortly to provide a more authoratitive answer and get me off the hook!

 

[shmoo]

Forgive me. I misunderstood the question. (actually, sloppy reading of the question but I am not admitting to that)

 

[bilbobaggins]

That's all very well, and some of us are still 'all at sea'.

I suppose it's 'close enough for government work' on the sea surface, but what about air travel? Does the air nautical mile expand as an aircraft ascends 6 or 7 miles up into the troposphere and if so, how? Or does the International Nautical Mile get used for the whole distance between e.g. London and Sydney and are they a few miles short - or overshot - at the other end?

How does this affect my collection of Air Miles? Am I being short-changed by those thieving Eurocrats again?

"Hello? Is that Weights and Measures.....?"


/forums/images/graemlins/smirk.gif

 

[Danny Jo]

I know nothing about this, but that has never stopped me before.

Surely air navigators use analogous concepts to those that we use - they will distinguish between air speed and ground speed. The ground speed in a plane flying 7 miles up will never be quite as much as the air speed, even if there is no wind.

 

[shmoo]

Assuming the sea level distance around the equator is 24000 miles.

If you go all the way round at 7 miles up you will do (in whole numbers) 24044 miles.

You are going round a sphere of 14 miles greater diameter.

So
24000/3.14 = diameter
(diameter + 14) * 3.14 = 24044

(I think, and subject to a couple of G and Ts after a day's sailing)