Magnetic North - Deviation

[Wandering Star]

My understanding has always been that the earth spinning on it's N/S axis without any other influences would provide True compass bearings. However the influence of a shedload of Lodestone in the vicinity of Hudson Bay exerts a magnetic pull. The actual amount of variation at any position on the globe is further influenced by the latitude (because of the speed of rotation at higher latitudes) and the relative distance from Hudson Bay.

Not that it matters to my practical navigation skills as I lift variation off the chart but I'd like to know if I've been wrong all these years about my understanding of the cause of variation. My sons RYA teacher has told him it's due to the floating North Pole ice cap which I think is a load of bolleaux.

Cheers, Brian

 

[telford_mike]

"Complex fluid motion in the outer core of the Earth (the molten metallic region that lies from 2800 to 5000 km below the Earth's surface) causes the magnetic field to change slowly with time." .... apparently

http://www.spacecom.com/customer_tools/html/body_mag_dec.htm

 

[Skysail]

AFAIK ice is not magnetic, where did the 'instructor' get this from? But longitude also has a large influence, in New Zealand variation is 20 deg East....

On an aeronautical chart they show differing variation as you travel East or West at the same latitude.

 

[FWB]

This doesn't answer the OP but it is of interest http://www.pangolin.co.nz/almanac/magvar.php

My set up is to use the fluxgate compass and everything is shown as True.

 

[Doug_Stormforce]

Variation is the difference between the Mag North (place that a compass points to N) and true North (the place that a globe or chart assumes is North).

Magnetic North changes slightly as the earths magnetic forces changes, this is not caused by melting ice, nor it it really caused by the earth spinning in its axis (however this may indirectly encourage molten layers below the surface to move). Longitude does not specifically vary variation, it is the angular distance you are from the meridian that passes from the True North Pole, through Mag North and on to the True South Pole that dictates the volume of the error.

The term true is slightly misleading as it simply means the assumed but wrong North.

It is interesting to note that the magnetic South pole does not align itself exactly opposite the Mag North Pole, in fact its some 300NM or so out. This does not however cause a problem for most of us.

 

[Algol]

My understanding has always been that the earth spinning on it's N/S axis without any other influences would provide True compass bearings. However the influence of a shedload of Lodestone in the vicinity of Hudson Bay exerts a magnetic pull. The actual amount of variation at any position on the globe is further influenced by the latitude (because of the speed of rotation at higher latitudes) and the relative distance from Hudson Bay.

Not that it matters to my practical navigation skills as I lift variation off the chart but I'd like to know if I've been wrong all these years about my understanding of the cause of variation. My sons RYA teacher has told him it's due to the floating North Pole ice cap which I think is a load of bolleaux.

Cheers, Brian

What you're talking about is variation. Deviation is something different - ie the effect your boat has on your compass at different angles of heading

 

[halcyon]

AFAIK ice is not magnetic, where did the 'instructor' get this from? But longitude also has a large influence, in New Zealand variation is 20 deg East....

On an aeronautical chart they show differing variation as you travel East or West at the same latitude.

It's also on the chart's compass rose for the local area, and the amount it varies each year, which we all apply don't we.


Brian

 

[Wandering Star]

What you're talking about is variation. Deviation is something different - ie the effect your boat has on your compass at different angles of heading

I am talking about variation!

Cheers, Brian

 

[EastCoastChris]

I am talking about variation!

Not when you originally posted. You edited your post to correct deviation to variation. Unfortunately you can't edit the thread title.

I thought about being a pedant, but decided not to say anything. However, now you've started brandishing exclamation marks at passers by...

Chris

 

[tudorsailor]

AFAIK ice is not magnetic, where did the 'instructor' get this from? But longitude also has a large influence, in New Zealand variation is 20 deg East....

On an aeronautical chart they show differing variation as you travel East or West at the same latitude.

Day skipper would be interesting in New Zealand! Variation goes from 18 degrees at top of the North Island to 25 degrees and the bottom of the South Island

TS

 

[Wandering Star]

Not when you originally posted. You edited your post to correct deviation to variation. Unfortunately you can't edit the thread title.

I thought about being a pedant, but decided not to say anything. However, now you've started brandishing exclamation marks at passers by...

Chris

Yes, but I replaced "deviation" with "variation" within 1 minute of making the original post and after only 2 people had read the post. I didn't expect the error to be pointed out more than an hour later. The original error was the result of helping my son with his homework, we'd been discussing deviation more than variation so it was a mindset thing! Shame about the thread title or I would have changed that too and no one apart from the first 2 people to look at the thread would have been any the wiser!

Cheers, Brian.

 

[EastCoastChris]

Shame about the thread title or I would have changed that too and no one apart from the first 2 people to look at the thread would have been any the wiser!

It's the first rule of sailing, there's always someone watching when it all goes wrong

Chris

 

[fergie_mac66]

I believe that geological magnetic north and south flip every so often. guess that will be 180 degree variation then !

 

[BrendanS]

Variation

Yes, but I replaced "deviation" with "variation" within 1 minute of making the original post and after only 2 people had read the post. I didn't expect the error to be pointed out more than an hour later. The original error was the result of helping my son with his homework, we'd been discussing deviation more than variation so it was a mindset thing! Shame about the thread title or I would have changed that too and no one apart from the first 2 people to look at the thread would have been any the wiser!

Cheers, Brian.

Allow me, there, title changed!

 

[BrendanS]

Deviant Flobbergob

Or a more likely one!

 

[Cornishman]

An Explanation

My understanding has always been that the earth spinning on it's N/S axis without any other influences would provide True compass bearings. However the influence of a shedload of Lodestone in the vicinity of Hudson Bay exerts a magnetic pull. The actual amount of variation at any position on the globe is further influenced by the latitude (because of the speed of rotation at higher latitudes) and the relative distance from Hudson Bay.

Not that it matters to my practical navigation skills as I lift variation off the chart but I'd like to know if I've been wrong all these years about my understanding of the cause of variation. My sons RYA teacher has told him it's due to the floating North Pole ice cap which I think is a load of bolleaux.

Cheers, Brian

An excellent explanation appears in a book called The RYA Navigation Handbook by a regular contributor to these posts. It answers all your questions clearly and concisely.
It also corrects the erroneous assumptions made by your son's "RYA teacher" and one or two others here.
For a fuller and beautifully written explanation for those requiring more details of this fascinating subject I suggest a visit to the library and reference to Lecky's Wrinkles in Practical Navigation.

 

[AntarcticPilot]

My understanding has always been that the earth spinning on it's N/S axis without any other influences would provide True compass bearings. However the influence of a shedload of Lodestone in the vicinity of Hudson Bay exerts a magnetic pull. The actual amount of variation at any position on the globe is further influenced by the latitude (because of the speed of rotation at higher latitudes) and the relative distance from Hudson Bay.

Not that it matters to my practical navigation skills as I lift variation off the chart but I'd like to know if I've been wrong all these years about my understanding of the cause of variation. My sons RYA teacher has told him it's due to the floating North Pole ice cap which I think is a load of bolleaux.

Cheers, Brian

Basically, it's all garbage. The Earth's magnetic field originates in the fluid iron of the outer core, far below the influence of anything at the surface of the earth. Occasionally, large mineral deposits can cause small local deviations but the main field is from far deeper. The Earth's magnetic field is caused by complex dynamo effects in the convecting fluid of the outer core. Changes in the fluid dynamics cause the changes in the direction of the magnetic field.

Incidentally, the dynamo effect is not stable, and can flip direction over geologically short periods - probably a few hundred years, during which the magnetic field dies away and then re-establishes itself in the new orientation. We use observe these changes in direction in the preserved pattern of magnetic anomalies around spreading centres like the Mid-Atlantic Ridge. It was one of the earliest indicators that led to the development of Plate Tectonics.

 

[binch]

The magnetic north" pole" has shifted enormously during my time at sea and I believe, on no real evidence whatsoever, that it is on its way to the southern hemisphere.
I also conjecture that the movement is one of the initiators of climate change, but I haven't the slightest notion of why.
And now that someone has mentioned climate change, the cat is really among the pigeons.

 

[Algol]

The magnetic north" pole" has shifted enormously during my time at sea and I believe, on no real evidence whatsoever, that it is on its way to the southern hemisphere.

I bet you consider Polaris an upstart and hanker for the days of Kochab too

 

[AntarcticPilot]

The magnetic north" pole" has shifted enormously during my time at sea and I believe, on no real evidence whatsoever, that it is on its way to the southern hemisphere.
I also conjecture that the movement is one of the initiators of climate change, but I haven't the slightest notion of why.
And now that someone has mentioned climate change, the cat is really among the pigeons.

If we were in for a magnetic field reversal (which could happen), then the signs would be a weakening of the magnetic field - basically the magnetic field has two meta-stable states, north magnetic pole near the north geographic pole, or south magnetic pole near the north magnetic pole. The transition probably involves a dying away of the field to a small value, rapid migration of the pole, and then strengthening the other way up, so to speak. But we don't really know - the geological records are not on a fine enough scale for us to be sure of the exact process, and the magneto-hydrodynamic processes are not well enough understood to reliably model it.

And it is likely that the reversal would have some effects - but not on the scale we are seeing. Magnetic field reversals happen about every 250,000 years - we are overdue, as the last one happened about 780,000 years ago! But because they happen on a geologically frequent time-scale, we can be confident in saying that they don't have the major effects on climate and ecosystems that we see at present. Current extinction rates and so on are equivalent to those at major extinction events in the geological record.