North Pole vs. South Pole

[NanoByte]

The US Mars space probe is supposed to go witching and digging for water at the South Pole of that planet in December.

My ignorance of astronomy/geo-geometry leads me to ask the question: How is it determined which pole of a rotating body, at least in the solar system, is to be labeled 'North'/'South'? Is this another application of the Right-Hand Rule, argued at length here previously? Or does this determination relate to the orientation of the given body with respect to others, such as in the case of a planet in the more-or-less plane of all the planets?

Given that Santa Clauses are prone to hang out at *North* Poles, and that they probably require a certain amount of water, wouldn't it be most probable that there be water at Mars' *North* Pole?

Ray (disoriented from all the political spins)

[pluto]

Your speculation was correct. North/south on distant planets is based on north/south here on earth. Wrap your right hand around the planet at the equator, with your fingers pointing in the direction of rotation. Your thumb points north.

In fact, for most planets in the solar system their north poles do point more or less north as measured here on earth. Their orbits are (nearly) co-planar with earth's and their orientation with their orbits doesn't vary widely. Notable exceptions are Uranus, which is more or less sideways to the rest of the planets, and Pluto, which has a notably non-co-planar orbit.


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"non sunt multiplicanda entia praeter necessitatem"

[Strainger]

Pluto answered it, but dammit, I put the effort into searching and I'm gonna post the results! From http://www.seds.org/~spider/spider/ScholarX/coords.html :A reference plane containing the origin is fixed, or equivalently the axis through the origin and perpendicular to it (typically, an "equatorial" plane and a "polar" axis); elementarily, each of these uniquely determines the other. One can assign an orientation to the polar axis from "negative" to "positive", or "south" to "north", and simultaneously to the equatorial plane by assigning a positive sense of rotation to the equatorial plane; these orientations are, by convention, usually combined by the right hand rule: If the thumb of the right hand point to the positive (north) polar axis, the fingers show in the positive direction of rotation (and vice versa, so that a physical rotation defines a north direction).

[Undead Dude]

So then technically the "north" pole of Venus is opposite to that of the Earth and most planets?

[pluto]

Yeah, Undead, I guess you're right. I looked it up and the obliquity of Venus is 177.3 degrees.

I didn't know that.


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"non sunt multiplicanda entia praeter necessitatem"

[asrivkin]

For the record, there are actually
(let's see if i get this italics thing to work :) two competing definitions of "north" in planetary science:

1) The right hand rule definition, favored by almost everyone, which is geophysical in origin, I think. In this scheme, all planets rotate in a prograde direction.

2) The north pole of a planet is the one "above" the plane of the ecliptic. This is astronomical in origin. In this scheme, planets can have retrograde rotation, and Venus (and Uranus? Pluto? both?) and roughly half the asteroids do so. I think officially the IAU still prefers this definition.

-Andy

[MrKnowItAll]

I always assumed that the poles on the planets were determined by magnetic north. Are there any instances where magnetic north is opposite of "astronomical" (for want of a better term) north in the solar system?

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Carpe hoc!

[Undead Dude]

Seems to me that those could be two separate definitions. After all, as we know, rotational poles and magentic poles need not be aligned.

[NanoByte]

Well, I know the Right-Hand Rule relates an electric-current-produced magnetic north pole to the conventional direction of current, but how is a permanent ferromagnetic magnet's poles -- the case of astronomical bodies, I presume -- determined as to north and south?

Ray (transmitting from the West Pole of the US)

[NanoByte]

And then you have to admit that there's more than one North Pole in Gdansk.

Ray

[Undead Dude]

The north end of a magnet is simply the side of a magnet that will point towards magnetic north when free to do so, as in a compass.

Somewhat ironically, that would mean that if we treat the Earth as a magnet, the "magnetic north pole" of the Earth would be the south end of the magnet.

[asrivkin]

Defining poles magnetically doesn't work because periodically the polarity of the earth's field swaps sign-- one of the clinchers for many scientists as far as plate tectonics was seeing the evidence for field reversals on the ocean floor, though that's another story.

Furthermore, IIRC, there are at least two planets (Uranus and Neptune) whose magnetic axis does not intersect the spin axis (if that makes any sense)-- the line connecting the north and south magnetic poles does not go through the center of the planet!

I freely admit, though, that magnetohydrodynamics is not my strong suit.

-Andy

[Undead Dude]

because periodically the polarity of the earth's field swaps sign -- asrivkin
Which would be quite a boon to the compass industry...

there are at least two planets (Uranus and Neptune) whose magnetic axis does not intersect the spin axis (if that makes any sense)-- the line connecting the north and south magnetic poles does not go through the center of the planet! -- asrivkin
Do you know if there are any theories attached to this? Perhaps an iron core that "orbits" the center of the planet from deep inside?

[asrivkin]

Well, again, magnetohydrodynamics isn't my strong suit, but...

If I remember right, it's because in Uranus and Neptune, the magnetic fields are generated not in a metallic core, but in a conducting shell farther out from the center of the planet.

In Earth, the liquid part of the core generates the field. In Jupiter and Saturn (shameless guessing begins :) the pressures are high enough in their centers for metallic hydrogen to form, and again this metallic liquid generates the fields. (end of shameless guessing)

I suppose I should look this up for real.

-Andy

[asrivkin]

Hmmmph.

Seems like every time I want to enclose a colon in parentheses, I get a smiley. I'll have to watch that...

[typertrphy]

It seems that the planets closer to Ole Sol keep a fairly vertical axis ( even Venus, which is flipped ALMOST 180 degrees north v.s. south ).
SO.... this is one for my Dad, or Unc Cecil ( Dad was a science writer...). Does proximity to a star predicate the degree to which a planet will have a "vertical" polar alignment? If I am .....say....third rock from the sun, opposed to 18th, will I be any more or less straight up and down in terms of my polarity?
And, how does this relate do my magnetic personality??
Typer :)

[AWB]

Wouldn't it have been strange had the Earth's magnetic pole been in flux when the use of the compass had first been introduced.

All of the sudden, compasses would be pointing the opposite direction. Mankind might have given up on the magnetic compass as an unreliable navigation aid. Exploration of the globe may have been slowed by decades or even centuries.

Something to think about...

[cmkeller]

In fact, for most planets in the solar system their north poles do point more or less north as measured here on earth. Their orbits are (nearly) co-planar with earth's and their orientation with their orbits doesn't vary widely. Notable exceptions are Uranus, which is more or less sideways to the rest of the planets, and Pluto, which has a notably non-co-planar orbit.

Also, I'm led to believe that Venus's rotation is in the opposite direction of the other planets. If I'm correct about that, then Venus is also a planet whose North and South would be the opposite of Earth's if we measured that by the planet's own rotation rather than by using the plane of Earth's orbit.

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Chaim Mattis Keller
ckeller@schicktech.com

"Sherlock Holmes once said that once you have eliminated the
impossible, whatever remains, however improbable, must be
the answer. I, however, do not like to eliminate the impossible.
The impossible often has a kind of integrity to it that the merely improbable lacks."
-- Douglas Adams's Dirk Gently, Holistic Detective

[NanoByte]

Everyone knows that the earth rotates about Silicon Valley, so isn't that evidence of a West Pole?

Ray