What is "North" on a planet?

[imaspacestation]

Was reading some articles about New Horizons and there was a pictured labeled with something about some geological details on the planet's "North Pole", when it struck me pretty curious how would you even know what is "North" when approaching a planet? I guess Pluto probably has a magnetic core so they could have used a sensor for that to measure it, but what about planets that don't? If the satellite does not know it's own orientation in space relative to the Sun, Earth and other bodies, and comes upon a planet that might have an inclined orbit around the Sun, how does one figure out what is North and what is South?

[HoloYolo]

The top and bottom of a body, well at least I think.

[ZetaX]

North and south can easily be defined by the axis of rotation. The axis meets the surface in two points, the poles, and we call the one where it rotates counterclockwise when looking from above "north", the other "south".

The top and bottom of a body, well at least I think.

There is no "top" or "bottom".

[HoloYolo]

North and south can easily be defined by the axis of rotation. The axis meets the surface in two points, the poles, and we call the one where it rotates counterclockwise when looking from above "north", the other "south".

There is no "top" or "bottom".

How? Top of a mountain? Bottom of the ocean?

[imaspacestation]

North and south can easily be defined by the axis of rotation. The axis meets the surface in two points, the poles, and we call the one where it rotates counterclockwise when looking from above "north", the other "south".

Ohhhh, thanks

[ZetaX]

How? Top of a mountain? Bottom of the ocean?

Isn't it quite obvious that we are talking about objects ins space here¿ You are using a distinguished direction. In your example on earth this is the direction of gravity. That one is obviously useless for defining poles as its direction depends on where you are.

Really, just try to write down an actual definition of "top" in space.

[LordFerret]

Heh... there is no 'bottom' to space, yet there are the 'depths' of space.

[ElWanderer]

Quoth wikipedia:

"Note that there are two standard methods of specifying [axial] tilt. The International Astronomical Union (IAU) defines the north pole of a planet as that which lies on the north side (defined by Earth's north pole) of the invariable plane of the Solar System; under this system Venus' tilt is 3°, it rotates retrograde, and the right hand rule does not apply. NASA defines the north pole with the right hand rule, as above; under this system, Venus is tilted 177° ("upside down") and rotates direct. The results are equivalent and neither system is more correct."

(the right-hand rule is as per ZetaX's description, as I understand it)

[vger]

It's semantics and depends on 'which' North Pole you're talking about. For instance, Earth's rotational axis and its magnetic pole are not in the same place.

[ZetaX]

(the right-hand rule is as per ZetaX's description, as I understand it)

Yes it is. But note that the other definition has the disadvantage of not satisfying the requirement in the first post (that we want to know north even if we have no clue where the sun or other things are).

[LordFerret]

'Right-hand rule'? Careful with that... PC and equality clowns will start a war over that.

[Accelerando]

'Right-hand rule'? Careful with that... PC and equality clowns will start a war over that.

Speaking as one of those fabled "equality" people, I tend to think it's comments like yours that start wars far more than the names of rules in physics. Cut it out.

[K^2]

'Right-hand rule'? Careful with that... PC and equality clowns will start a war over that.

Imagine if they hear about virtually all screw/bolt threads being right-handed. Although, this might be a great business opportunity. Sell left-handed screws for left-handed people.

But yeah. The only sensible definition for a planet is rotation based. I suppose, a planet with zero rotation can exist, but the odds are overwhelmingly against it. On the other hand, not all planets have magnetic poles, and some objects (planets among them?) can have large number of magnetic poles. Which makes any compass-based definition useless.

On that note, I wonder how feasible a Foucault compass would be. Naturally, it wouldn't be mechanical, but with optical gyros, you should be able to detect rotation precisely enough to tell which way the rotational north pole is.

[Camacha]

Quoth wikipedia:

"Note that there are two standard methods of specifying [axial] tilt. The International Astronomical Union (IAU) defines the north pole of a planet as that which lies on the north side (defined by Earth's north pole) of the invariable plane of the Solar System; under this system Venus' tilt is 3°, it rotates retrograde, and the right hand rule does not apply. NASA defines the north pole with the right hand rule, as above; under this system, Venus is tilted 177° ("upside down") and rotates direct. The results are equivalent and neither system is more correct."

(the right-hand rule is as per ZetaX's description, as I understand it)

Oh dear Jeb, can they not even agree on this? You would say that when both system are equally correct, one is easily discarded, but I assume local politics and chauvinism will not allow for that. We really need to get this kind of stuff sorted out.

[Hcube]

Oh dear Jeb, can they not even agree on this? You would say that when both system are equally correct, one is easily discarded, but I assume local politics and chauvinism will not allow for that. We really need to get this kind of stuff sorted out.

...Before you guys lose another mars climate orbiter