Magnetic North Question?

[Medskipper]

My son said to me last night, if there is a magnetic north surely the south pole must be magnetic as well! Its something that I have never in my life questioned! I have always just accepted that the North pole is magnetic! does anyone out there know the answer? is the south pole magnatised as well or is it like the opposite end of the magnet?

Barry



<hr width=100% size=1>I just want to retire with my boat to the Med!

 

[Keen_Ed]

Yup, the South Magnetic pole. The magnetic poles are not in the same place as the geographic poles (hence compass variation). Currently, the south magnetic pole is at about 65°S 139°E, moving 10 to 15 km north-westerly each year.

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[Chris771]

Hi Barry,

The earth is just a big magnet, the lines of flux are not entirely uniform and the mangetic mass in the core flows over time. This is why charts are marked with a variation and also an indication or how much it increases or decreases annually with respect to true north. There are areas of local magnetic anomaly distributed all over the earth.

It is possible for aircraft with accurately calibrated magnetometers to locate the position of submerged submarines due to the local disturbance of the earth's magnetic field.

When I worked on underwater weapons research, we used to calibrate the magnetic fluxgates in a wooden shed a few hundred metres away from all other buildings, with no electricial supplies or metallic tools. Thje nearest road was a dual carriageway about a quarter of a mile away. With experience it was possible to tell the direction of big articulated lorries along the road if the weapon was correctly orientated towards the road. The secrets act prevents me saying more, but magnetic fields can be quite fascinating.

On a level more useful to boaters, last year my auto pilot was very sluggish and the boat yawing all over the place, couldn't understand why. When checking the fluxgate found that someone had put a steel baking tray (which had been used in an electric oven - strong magnetic field - hence magnetised) in the locker under the fluxgate. Removed tray and auto pilot rock steady again. Since then I have made sure that no ferrous metals get put within a couple of metres of the flux gate.

I suppose the attraction of the North Pole was that it was reasonably "handy" for northern Hemisphere explorers wanting to go and look for it. The North and South Magnetic Poles do not coincide with the true poles which are the centre of rotation of the earth, but are slightly offset.

If you were to take your compass to the North Magnetic Pole Compass north would be vertically downwards and the sky would be south. When you started walking away from the pole, any straight line direction you walked in would be south. If you walked in a circle centred on the pole you would be going due east or west. Seems that I did not forget everything that I learned at Nautical College all those years ago, just most of it!

Chris

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[jonjo]

Is it true that as well as having moving magnetic poles, the whole magnetic field of the earth is weakening as we approach one of those complete north south flips?

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[Chris771]

Forgot to add the North Magnetic Pole is about 78N and 104W, was located by Sir James Ross in 1831. It is in the Queen Elizabeth islands of Northern Canada and over a period of time moves in a circle of around 100 miles diameter.

Apparently the Earth's magnetic field has reversed polarity 171 times in the last 76 million years - heaven knows how geologists figured that one out, something to with the permenantly magnetised magnetite in volcanic rocks. They must have read the date of the rocks and diretion of magnetisation!

Chris

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[NorthernWave]

This reminds me of an argument I had with a Croatian in a U.S. bar who thought that the Equator went through the middle of europe. I insisted not and to prove it asked an american sat at the bar, he didn't know what the equator was, so asked another....... nor did he.

Anyone seen where it might have gone???

Chris

<hr width=100% size=1>In Ft Lauderdale

 

[Medskipper]

Well done Chris, thanks very much, very comprehensive answer!

Sounds like you have had a very interesting career!

Regards Barry



<hr width=100% size=1>I just want to retire with my boat to the Med!

 

[npf1]

mmm. So how could you tell if you were at the geographic poles?

<hr width=100% size=1>Have boat. Will travel. Maybe cruising in 2005 - wanna join me?

 

[Chris771]

Quiite easy, if it is summer lie under a sheet of perspex and plot the course of the sun, it it moves in a circle iyou are at the Geographic Pole, if it moves in an ellipse you are not. Move from the perigee of the ellipse towards the apogee (highest point of elevation) and try again tomorrow.

Or , lie there for six months, wait for darkness and do the same with the stars.

Chris

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[NorthernWave]

Sorry Chris, a little off subject.

Chris

<hr width=100% size=1>In Ft Lauderdale

 

[Wiggo]

I doubt you'd have had any more success asking them if they knew what Europe (Yurp) was, either...

<hr width=100% size=1>Nihil curo de ista tua stulta superstitione.

 

[Chris771]

Chris,

It was a weak attempt at providing a humourous answer, obviously missed !

The principle is correct though, if at the centre of rotation the sun would describe an accurately centred circle over a 24 hour period. If off the centre of rotation, it would provide an ellipse with the point of highest elevetion being towards the centre of rotation. This could be measured with a sextant and forms the basis of astral navigation, I just oversimplified it by saying draw it from under a sheet of perspex, which would also work.

Best if I keep my attempts at humour to myself methinks......for now anyway


Chris

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