North Magnetic Pole

Poulgorm

Why are the magnetic poles not located at the north & south poles?

And why is magnetic north pole always moving ?

Cookie_Monster

the earth's axis is tilted off vertical but also is not perfect rotation and wobbles a little. This affects the magnetic field also

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

jumpjack

Poulgorm wrote: »

Why are the magnetic poles not located at the north & south poles?

And why is magnetic north pole always moving ?

Because Earth has not been designed, it was just "born" by stardust...
So it's not a perfect sphere, its crust has no constant thickness, its iron nucleus has not a constant density,...

Capt'n Midnight

not only does it move sometimes it reverses so north becomes south and visaversa

since we have maps, radar, radio beacons and GPS this won't affect us in the long run

Cookie_Monster

that flip is due is it not?

I read there some spot in South Afric (iirc) that compases don't work as it's being affected by the start of the flip of the poles

jumpjack

Capt'n Midnight wrote: »

not only does it move sometimes it reverses so north becomes south and visaversa

since we have maps, radar, radio beacons and GPS this won't affect us in the long run

It's yet unknown why poles periodically reverse.
And I do not think at all it will be just a matter of "crazy compasses": magnetic field is our shield against sun: if it reverses by passing through 0, we'll be for "some" time (hours? years? we don't know) without any defence from cosmic rays.

And if it does not reverse by passing through 0 but due to physical crust displacement... well, it will be a huge problem.

Official site about poles status:
http://www.iers.org/IERS/EN/DataProducts/EarthOrientationData/__Function/Plots__EOP05C04__2000/generischeTabelle__Diagramm.html

Capt'n Midnight

we'll have years of warning

jumpjack

Capt'n Midnight wrote: »

we'll have years of warning

There are no recordings of previous such events at human scale (years, days), only geological scale (thousands, millions of years).
Hence we can't know when and how it will happen and how long it will last.

Thoie

Slight reverse of that question - when people were originally drawing maps, were the magnetic poles at the poles on the maps initially, and they've moved that much since, or were early map drawers just a bit crazy?

Capt'n Midnight

Thoie wrote: »

Slight reverse of that question - when people were originally drawing maps, were the magnetic poles at the poles on the maps initially, and they've moved that much since, or were early map drawers just a bit crazy?

the last change was something like 680,000 years ago

maps with north on top are relatively recent

but north on top is kinda handy if you are using the Mercator projection. it means you can plan ship routes because straight lines on the map are straight headings on the globe and the bits of the map that go really weird are the poles where you can't go anyway

Thoie

Capt'n Midnight wrote: »

the last change was something like 680,000 years ago

maps with north on top are relatively recent

but north on top is kinda handy if you are using the Mercator projection. it means you can plan ship routes because straight lines on the map are straight headings on the globe and the bits of the map that go really weird are the poles where you can't go anyway

Yes, but if the North Pole had originally been set as magnetic north in the maps, the same would hold true - your longitude would just start from there. If we believe Wiki, the north pole is moving about 30 miles a year.

So is it that in the 1600s, the current north pole was also the magnetic north, or did they just pick a random spot for 90,0?

Capt'n Midnight

For those of us in the northern hemisphere True North is where shadows point when the sun is at it's highest point during the day.

you can also work it out relative to the stars, but the start used as the pole star changes due to precession , but that's over thousands of years

magnetic north is used because it's not that far out and works in fog.

depending on where you are it varies more or less from true north
http://ngdc.noaa.gov/wist/magfield.jsp - green is where magnetic north matches true north

https://en.wikipedia.org/w/index.php?title=File:World_Magnetic_Declination_2010.pdf&page=1 - clearer picture


If you ever want to use a sundial a thing called the equation of time is how you correct for earths orbit not being circular etc.

locum-motion

You seem to think that True North was just 'chosen' by early mapmakers by some sort of agreement between them.

The earth rotates. Imagine a line going through the earth, around which it rotates, like an axle. The two ends of this axle are the North and South Poles. They are close-ish to, but don't correspond exactly, to Magnetic North and South.

For most of us, most of the time, they're close enough together that it doesn't really matter that they don't match exactly. The Liffey flows E through Dublin, dividing it into Northside and Southside. Nobody's gonna say "Hang on a minute, is that the Magnetic Northside or the True Northside?"

On a long sea or air journey, though, the difference can matter greatly.

Thoie wrote: »

Slight reverse of that question - when people were originally drawing maps, were the magnetic poles at the poles on the maps initially, and they've moved that much since, or were early map drawers just a bit crazy?

Thoie wrote: »

Yes, but if the North Pole had originally been set as magnetic north in the maps, the same would hold true - your longitude would just start from there. If we believe Wiki, the north pole is moving about 30 miles a year.

So is it that in the 1600s, the current north pole was also the magnetic north, or did they just pick a random spot for 90,0?

Rubecula

Poulgorm wrote: »

Why are the magnetic poles not located at the north & south poles?

And why is magnetic north pole always moving ?

Perhaps an easy way to look at it is to imagine the earth as a ball, with a liquid centre.

The Earth's poles are it's axis of rotation. spin the ball and the same applies. But the centre is also spinning, not necessarily in exactly the same direction. The magnetic poles are produced by this centre. So they may not line up with the poles. The centre will slide around being fluid in nature and consequently so will it's (magnetic) poles.