Magnetic North

[petsim]

What is the difference between the north pole of a planet and the south pole. The axis of Uranus tilted at 94 degrees to the ecliptic and not 86 degrees, so it can't simply be that the north is on top..

[Loosehead]

Well there are 2 issues here. there's the magnetic pole and the Rotational Pole. On Earth what we know as the North Pole is the top of the axis about which the planet rotates. We Call it north because when the compass was discovered along with the magnetic field that is roughly where the needle pointed and maps generally have the North at the top by tradition. The actual magnetic north pole is not in the same place as the top of the axis although it is currently only about 6 degrees different but it does vary over time.

In the case of Uranus the planet has been knocked over as you say and rotates on it's side. I do not know where the magnetic poles of Uranus are but I do know that generally with other planets we refer to "the Poles" rather than a specific North or South and that is taken to mean the top and bottom of the axis about which a planet rotates. 

[jake-the-peg]

Good question! it's to do with it's direction of rotation.

As it happens there is some debate as to whether it's inclination is  97 degrees and it rotates directly or whether it's inclination is 83 degrees and it's rotation is retrograde.

I guess you'd have to know how it spun before it got tired and decided to have a lie down

[Loosehead]

There is some speculation that Uranus was originally upright or rather more upright but was knocked over in a collision. If you take the direction that it rotates (ie sun would rise in the east if it hadn't been knocked over) and assume that it rotates the same way as all the other planets (except Venus!) then it is 97 degrees off the vertical. That's is not conclusive though as we know that the direction of rotation is already not the same in all planets. Just thinking out loud!

[petsim]

Thanks. I thnik my question may go a bit deeper. Think of a simple bar magnet. Which is north and which is south and why?

[Clanad]

As to your last question, there really isn't a north or south wit the bar magnet.  These are labels to help understand the phenomena of magnetism.  The labels reflect our knowledge of magnetism, but they could just as easily be named Maude and John.  The labels reflect historical context of understanding and relates the bar magnet to our world view which is provided by our experiential matrix...

[Loosehead]

what Clanad said!

[fo3nix]

plus, a bar magnet always has a north and south. technically, this is because there is no magnetic monopole.

or at least, one has never been found. if you found one, you'd probably win a nobel prize.

i.e. if you take a bar magnet with north and south parts, and cut the north bit off, then the south bit becomes north and south.

[petsim]

So is there really no difference between N & S? A planet rotates clockwise (sun rises in the East) so North is at the top? Can't be! What about when the poles flip. What has our magnetic N got in common with the magnetic N on Mars or Uranus or a bar magnet. If I have a bar magnet how do I know which end is what we call North or John. i.e. is the flow of magnatism different (does it go from N to S or visa versa)?

[Clanad]

Ummm, no petsim, the Earth rotates counterclockwise...

[Marsh]

I'm open to correction on this but I think the logic goes as follows.

The terrestrial poles are the points on the earth's surface intersecting the axis around which the earth rotates.  The North Pole is the one away from the sun in Europe or the States.  Viewed from above the North Pole, the earth rotates anti-clockwise.  The North Pole of another planet is the one that gives that planet the same direction of rotation as the earth's.

[velcro]

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