Magnetic north

[Coldicote]

I've always understood that the earth rotates on its own axis and that the point of the axis is the north pole. What I don't understand is why magnetic north should be any different. Where and what is it that attracts a magnet/ compass to point somewhere else? What happens with a compass in the southern hemisphere? Thanks to anyone who can enlighten me.

[pug100]

Quite a lot to answer here but try wikipedia for in depth explanations.

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

and

http://en.wikipedia.org/wiki/North_Magnetic_Po le

Basically the reason we have a magnetic north is that the earths core is made of liquid metals such as nickel and iron.
To quote from the above links

In early times European navigators believed that compass needles were attracted either to a "magnetic mountain" or "magnetic island" somewhere in the far north, or to the Pole Star.[4][5] The idea that the Earth acts as a giant magnet was first proposed in 1600 by Sir William Gilbert, a courtier of Queen Elizabeth I. He was also the first to define the North Magnetic Pole as the point where the Earth's magnetic field points vertically downwards. This is the definition used today, though it would take several hundred years before the nature of the Earth's magnetic field was properly understood.[4]

[New Judge]

This is quite a complex matter, but pure simply the material which creates the earth�s magnetic field (and so causes compasses to point �north�) is not located at the geographic North Pole.

Furthermore, the location of �magnetic� north is not constant and varies (in geological terms) quite rapidly. Another phenomenon known as �geomagnetic reversal� has, in the past, caused the earth to reverse its magnetic polarity. This is thought to be due to movements in the earth�s liquid core.

Because the generation of the earth�s magnetic field is also influenced by the Sun, it can also vary over quite short timescales and �magnetic north� can even be in a different position at different times of the day.

[Coldicote]

How I envy your knowledge! Thank you both for putting me on the road to further understanding. By the way, what happens 'down under' - does a compass point to the South pole?

[Rollo]

The compass will always point to the North Magnetic pole, except if you are situated at the spot on the Southern hemisphere where the Magnet North pole is in a straight line from the compass to the pole through the centre of the earth.

There the compass will poinr in every direction - that is it will go "haywire", probably continuously revolving.

[dundurn]

And just to further enlighten (or confuse you) opposite poles attract, so since the 'North' pole on a compass points to the North magnetic pole, the North magnetic pole is actually a South pole! (Proper nomenclature is actually to call the North pole of a magnet a North-seeking pole.)

[R1Geezer]

further to the above:

"... What I don't understand is why magnetic north should be any different"

It's just as valid to point out that there's very little reason as to why it should be the same. Indeed there is no reason why magnetic north is quite close to geographic north. It could just as easily have been on the equator.

[Rollo]

In fact the magnetic pole has been in the Southern hemisphere a number of times during Earth's history. No doubt it will return there sometime in the future.

[Coldicote]

This is all very interesting - I'll save a copy for further reading. If I wanted to find the North Pole, i.e. the point of the earth's axis, it seems a compass wouldn't be much help with magnetic north not being constant (New Judge). I wonder what adventurers mean when they say they are trekking to the north pole and how do they and know when they've got there. Not that I want to go there - it's cold enough here just now!

[R1Geezer]

They use GPS these days, when it says 90 degrees latittude, your at a pole. Oddly enough there is apparently a marker that has to be moved to take account of ice shifting etc.

[Coldicote]

A very informative science lesson and points well answered, thank you everyone. I'm signing off now.

[dundurn]

To quote QI

'The Earth has 11 poles - two geographic poles, two magnetic poles, two geo-magnetic poles, two poles of inaccessibility, two celestial poles and a ceremonial south pole.'

[mibn2cweus]

"If I wanted to find the North Pole, i.e. the point of the earth's axis, it seems a compass wouldn't be much help with magnetic north not being constant (New Judge). I wonder what adventurers mean when they say they are trekking to the north pole and how do they and know when they've got there."

Ho! ho! ho!


D'oh!

[Coldicote]

I wasn't intending to come back again on this topic, but the thought of earth having eleven poles (dundurn) made my mind boggle. Have to admit the science of it is beyond me but I'm tempted to add 'up the pole' to make it twelve!
Mibn2cweus your two links were spot on and delighful thank you. Happy Christmas everyone.

[nix-j-c]

so do explorers use geographical poles ie 90' north or south or magnetic