The Nautical Mile

When I was about 15 years old in my yacht club navigation classes, I was taught to measure distance by putting my dividers on the latitude scale at the chart edge at the same rough latitude of the objects that I wanted to measure.

Thus, if I had a chart covering the whole of the North Atlantic and I wanted to measure distance near Greenland, I would plonk my dividers on the latitude scale across from Greenland. If I wanted to measure a distance in the Caribbean, I would plonk my dividers on the latitude scale across from the Caribbean. It's all self correcting. The Mercator charts compensate for the ovoid shape of the earth. (At least, that's what I was told and I've done a couple of successful Atlantic crossings without electronic aids.)
 
One of my more shameful memories is of the difficulty my table at a club quiz had in answering the simple question "What is the distance on the Earth's surface from the Equator to the pole, in NM?". It may have been the Chilean Merlot that blinded us.
 
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
 
I gave no experience of astro navigation (but I did get 5400 NM as the distance from the pole to the equator) so please forgive my confusion. What is the definition of latitude on a planet that is not a sphere? Do you just take a line from a point on the surface to the centre of the earth and measure the angle between that line and the axis of rotation? Is the line from the centre always vertical where it crosses the surface? Because the local vertical must be important when measuring angles above the horizon.
 
DJE said:
I gave no experience of astro navigation (but I did get 5400 NM as the distance from the pole to the equator) so please forgive my confusion. What is the definition of latitude on a planet that is not a sphere? Do you just take a line from a point on the surface to the centre of the earth and measure the angle between that line and the axis of rotation? Is the line from the centre always vertical where it crosses the surface? Because the local vertical must be important when measuring angles above the horizon.
Click to expand...
You'd think that there would be just one "centre of the earth", but even that is not the case. There are lots of "datums" which each define a centre, and I think WGS84 is the best known but not the only one. Even my ancient Garmin GPSII+ GPS provides several dozen for selection.
 
TernVI said:
IT's exactly that an any latitude and any altitude. It doesn't get bigger in a plane a 30,000 ft
Click to expand...

How does that work then? I have no idea about aviation nav, but surely the distance from the equator to one of the poles is further at higher altitude?

Is it fair to say that a NM is not a measure of distance, rather an indication of position relative to a known object?
 
MINESAPINT2 said:
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.
....
Click to expand...
Just as a 'foot' will always be the heel to toe length of my welly boot for the purposes of gardening.
My astro never got anywhere near good enough to worry about different definitions of NM.
 
Plymsea said:
How does that work then? I have no idea about aviation nav, but surely the distance from the equator to one of the poles is further at higher altitude?

Is it fair to say that a NM is not a measure of distance, rather an indication of position relative to a known object?
Click to expand...
An international NM is exactly a measure of distance.
For practical purposes, navigators can do a job of navigation by using it as a measure of angle, assuming the radius of the earth to be near enough constant.
It's a case of 'near enough for the job in hand'. Speed logs on boats have errors of several %. Distance logs may be a bit better.
For most nav purposes, it does not matter.
But if you were surveying the base line of a radio telescope or something it might begin to matter.

If you are a scientist and you measure something, you often want to know how accurate your measurement is. I read my voltmeter it says 13.75, I know that's +/- 1% +/- 1 digit. Other people might just take it at face value.
 
mjcoon said:
You'd think that there would be just one "centre of the earth", but even that is not the case. There are lots of "datums" which each define a centre, and I think WGS84 is the best known but not the only one. Even my ancient Garmin GPSII+ GPS provides several dozen for selection.
Click to expand...
Exactly.
You start off thinking the earth's a sphere, thinking that 'flat earth' is a silly idea
Then someone tells you 'its and oblate spheroid'
Then they refine that to 'geoid' which pretty means 'earth shaped'.
If you really look in to it, the damned thing changes shape with the tides!
 
As the earth is flatter at the poles than the equator with the surface nearer the centre at the poles than at the equator, then the definition of one minute of latitude is critical. If you define a Nautical MIle as the distance which gives a change of 1 minute in the angle that the line between you and the centre of the earth makes with the axis of the earth then this is smaller at the poles and bigger at the equator. If you define it as the distance which gives a change of 1 minute in the direction of the local vertical then this is bigger at the poles and smaller at the equator. .......... Er, I think.

Maybe this accounts for some of the differences between posters above.
 
DJE said:
As the earth is flatter at the poles than the equator with the surface nearer the centre at the poles than at the equator, then the definition of one minute of latitude is critical. If you define a Nautical MIle as the distance which gives a change of 1 minute in the angle that the line between you and the centre of the earth makes with the axis of the earth then this is smaller at the poles and bigger at the equator. If you define it as the distance which gives a change of 1 minute in the direction of the local vertical then this is bigger at the poles and smaller at the equator. .......... Er, I think.

Maybe this accounts for some of the differences between posters above.
Click to expand...
But the local 'vertical' will be offset by the same 'centrifugal force' which makes the geoid asperical?
Or will it?
 
TernVI said:
But the local 'vertical' will be offset by the same 'centrifugal force' which makes the geoid asperical?
Or will it?
Click to expand...
Yes - that's what I'm assuming so the local vertical is perpendicular to the spheroid.
At the poles the surface is nearer to the centre than at the equator but the radius of curvature of the surface is greater than at the equator (i.e. it's flatter).
 
TernVI said:
But the local 'vertical' will be offset by the same 'centrifugal force' which makes the geoid asperical?
Or will it?
Click to expand...
That certainly represents creative confusion. If you judge the vertical as being the direction that you have to throw a ball up in order for it to land on your head, you may be right, local gravitational anomalies permitting. I'm not good at throwing, so my vertical might consist of a large plank of plywood laid flat on the ground with a square block on it to sight up. Alternatively, and differently, I might prefer to use the direction that my cousin took when drilling from my antipodes to my position having passed through the Earth's centre, if we can agree where that is.
 
You must log in or register to reply here.

Share this page

Top