Nautical miles and kilometres.

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[Traddles]

I have a real gripe about the BBC when it comes to programmes about the sea. They are so taken with metric measures that programmers talk of nautical distances in kilometres!!! As any seafarer knows, nautical distances cannot be measured in this way. The nautical mile is not an arbitrary measurement but a geometrical one. The definition of nautical mile is the distance subtended at the surface of the earth by an angle of 1 minute of arc, (1/60th of a degree) at the centre of the earth. As the earth is not a true sphere but a spheroid, that is to say, squashed a bit at the polar extremes, the nautical mile is shorter at the poles than at the equator. However the circumference of the earth in nautical miles is a constant. That to say 360degrees X 60minutes = 21,600 nautical miles. A simple way to think of it is that one degree of latitude is equal to 60 nautical miles. I know this is a bit of a niggle on my part, but I wonder do our other members from different countries have the same problem.

[Brinch]

I have a problem with windspeeds i km/h, I have to do the math or else it's very hard to relate to. Why not mention windspeeds in either m/s or knots...

[Traddles]

I am glad to see that VStep use Beaufort notation in Knots. Good for them!!!

[Stuart2007]

I'm not as 'wisened in years' as you are, and was brought up under a mixture of metric and British Imperial measurements.

I get really wound up by the BBC though as I have to re-calculate everything back into imperial. As much as metric is a much better system, while it is forced on us, we will never surrender. If the government asks nicely then we probably would go for it in time.

Or we could keep on prosecuting- and hounding market traders to death for selling by the lb, as with Mr Steve Thorburn.

Stu

[mvsmith]

The main trouble with kilometers is that few pronounce it correctly. I've even heard a Greek seaman pronounce it "kill AH meter", go figure. There are some on BBC who pronounce it correctly, but listeners in the US probably think it's another wierd Britisism.

[Traddles]

I would really like to see someone work out the days run in kilometres, without messing about with converting the answer from nautical miles to kilometres. What about the great circle distance from Lands End to North Point, Barbados? Best of luck to them.   The BBC has probably never heard of Departure, Meridional Parts let alone a Haversine?

[dharl]

Totally agree, but i imagine that it is easier for the BBC to only give the General Public a distance in KM's rathern then try and introduce a third more complicated system to measure disctance and speed!   

[Traddles]

I always thought the BBC was there to inform and educate as well as entertain. Before decimalisation no one was bothered about distinguishing between statute miles and nautical miles so what has changed? Political Correctness let loose, but then I shouldn't be surprised, half the people we see and hear on TV cannot speak English properly anyway!!  Oops, I'll have to be careful or I'll be in trouble .

[[RWP]DJM]

I agree Traddles, Political Correctness.....not much to say to that except "pfftt, who needs it"

I better be careful too eh

Regards.

DJM.

[Stuart2007]

I better be careful too eh

Why?  OK OK. I know we can't get into an arguement here... but if anyone would like to start a forum elsewhere for discussion of this nature... let me know 

www.notopoliticalcorrectnessandotheruselessblairisms.com

Stu

[budbud]

As the earth is not a true sphere but a spheroid, that is to say, squashed a bit at the polar extremes, the nautical mile is shorter at the poles than at the equator

Hi,
Maybe should I have a look again on my lessons of navigation but I thought that the mile was the length of 1 minute of arc at the equator and so had a constant value:
1 mile=1852 m at any latitude.
Am I wrong or didn't I understand the sense of the post from Traddles?

[budbud]

Oups sorry I have just found this piece of information:
The official value for 1 nautical mile has been fixed to 1852m by the international hydrographic conference in 1929 at Monaco. So what I said in my last post was wrong, sorry, forget it.

[Traddles]

Hi budbud,

What you say is correct up to a point, but the nautical mile IS the distance subtended at the surface of the earth ANYWHERE by an angle of one minute of arc at the earths centre. The international agreement of 1929 is also an abitrary agreed measure but in fact what I said about the differing lengths due to the earth being a spheroid is also true. Hope that clears it up a little.

[budbud]

Hi Traddles,
Thanks for the explanation.
I read again your post and I had a look at different documents about the definition of the nautical mile.
My confusion came from the fact at school we learnt (probably to simplify) the definition i wrote in my first post. And in navigation we never use metric systems. In fact I'm using it only to convert the distances for the "terrians" so know I completely agree with you definition.

[Traddles]

Hello again budbud,
I call them Earth People, poor folks do not know how we Jolly Jacks operate. I mean they think the earth revolves round the sun,but we seafarers know that is not right!! The sun, moon,stars and planets ALL rotate round the earth stuck to the inside of a great big glass ball called the Celestial Sphere which has the earth at its centre.

[mvsmith]

Hi budbud,

the nautical mile IS the distance subtended at the surface of the earth ANYWHERE by an angle of one minute of arc at the earths centre.

That French thingy is one ten millionth of the distance from the North Pole to the Equator along the Prime Meridian (The one that passes through Paris).
The yard is the distance from the tip of HM nose to the tip of HM right index finger.
Both are examples of units that have been redefined from their original concepts to a more precise value. So it is with the nautical mile, which is the average length of one minute of arc on a great circle of the earth. Its value was fixed in 1929 as1852 m (6076 ft).
By your definition, Traddles, the length of the nautical mile would change from place to place, and depend upon heading. It would be useless for navigation.
Marty

[Traddles]

It is in fact as I said, regardless of the STANDARD nautical mile, and it only alters with latitude not heading. I know that it has been  defined in 1929 as mentioned. However if you work out the great circle distance between two points the answer is given in the real nautical miles and not the standard ones. There would of course only be a very small difference, but there would be a difference even so. In my day the prime meridian passed through Greenwich Observatory and still does! Where is zero degrees of longitude? Certainly NOT Paris.

[LucAtC]

Hello Traddles, 
It is surely like you say, but following your definition (and a definition is always correct, even if not agreed), to compute the great circle distances, on uses formulas based on the distances on the surface of a sphere, not even on the revolution ellipsoid of reference, and surely not at the surface of the theoretical geoid. So the formulas cannot give correct results, anyway, whether in angle or distance.
And like  you say, zero degrees of longitude through Paris is an obsolete agreement, just like abandoned definitions of the nautical mile.
Regards,
Luc

[mvsmith]

The equatorial circumference of the Earth is 40,076.5 km; the polar circumference is 40,008.6 km. Each of those great circles has 21,600 minutes, so the length of a minute of arc—and a nautical mile defined that way—is different for each.

For a ship on the Equator, a nautical mile has a different average length on a heading of 0 degrees than it has on a heading of 90 degrees.

The standard GPS geoid, on which modern sea navigation is based, is lumpy—rather like an orange that has been dropped on the deck multiple times.
The local diameter—and the length of a minute of arc—of a great circle on that geoid varies. A nautical mile based on a minute of arc will have a length that depends upon on what great circle it lies. The particular great circle you are on depends upon your position and your heading.

The 1929 fixing of an absolute value on the nautical mile recognized the absurdity of trying to navigate with a mile whose length varied along one’s track.

At the time that the French invented the meter, the Prime Meridian did pass through Paris—at least as far as they were concerned.

Marty

[Stuart2007]

So what you're saying, Marty, is that where Traddles thinks the universe is centred on this glass celestial sphere, you think the universe revolves around Paris? 

I thought the world revolved around me... But that's another story   

Stu

[simplayer]

One can say that Being "Belgian" is a great thing (apart from the politics) if youre a mariner
We're used to learn these things in different ways, Km/miles, liters/gallons, and the english language of course , we were brought up with BBC television, not to mention that some of us speak 4 languages.
I'm not assuming were all "einsteins" i'm mearly pointing out the advantages we have... (if you want too)

[mvsmith]

You forgot to mention Helmut Lotti.

[Stuart2007]

One can say that Being "Belgian" is a great thing (apart from the politics)

Yes, putting the EU parliment in Belgium is like drawing a moustache on the Mona Lisa or spilling a can of Stella Artois

Stu

[simplayer]

Now there's something to be proud off, and i do mean "real proud"...
And Helmut Lotti....  never heard of him...

[mvsmith]

In my day the prime meridian passed through Greenwich Observatory and still does!

Actually, the Prime Meridian no longer passes through Greenwich Observatory. It passes though the National Maritime Museum of which the house of Flamsteed, Halley, and Maskelyne is now a part.
Greenwich Observatory is now located at the University of Cambridge's Institute of Astronomy.
For improved seeing, most of the equipment and operations of the observatory were moved, at the end of WWII, to Herstmonceux Castle, which is about 97 km SE of the original spot.
Marty