Random question:

[conuly]

Is there any particular science-y reason having to do with rotation and whatnot why the magnetic poles of the Earth happen to be (more or less) North and South instead of East and West? Or did it just happen to shake out that way?

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Comments

[ancarett]

Regarding your question, yes, science. Particularly think of the Coriolis Effect, just as with prevailing winds or oceanic currents are affected by the spin on the axis. The liquid parts of the core flow in those directions due to rotation, essentially, and magnetism follows.

[conuly]

That makes sense. So you wouldn't find a natural planet where the poles went along the rotation?

[ancarett]

As long as it had some sort of metallic and semi-mobile core, this appears to be the thinking. There's some neat scholarship that's been done on Jupiter's magnetosphere and alignment - there the metal is hydrogen. The field is much stronger than earth's, and that's partly thought to be caused by the difference in rotation and core size.

Here's a decent write up - http://csep10.phys.utk.edu/astr161/lect/jupiter/magnetic.html

Way back when I was a geophysics major as an undergraduate, I was really interested in planetary magnetism. I still collect links and reports on the subject whenever I stumble across something of interest.

Edit to fix icon!

[conuly]

Thankies again :)

[elenbarathi.livejournal.com]

Yes. The compass points to Earth's North Magnetic Pole (https://en.wikipedia.org/wiki/North_Magnetic_Pole). Here's a pretty good explanation of magnetic declination (http://academic.brooklyn.cuny.edu/geology/leveson/core/linksa/magnetic.html).

[conuly]

Yes, I know that. I'm asking if there is a reason that earth's magnetic poles run n/s instead of e/w.

[elenbarathi.livejournal.com]

Okay; so did you actually read the links I posted there? Because if they don't explain the reason, I'm at a loss to know what your question means.

Picture a ball spinning around a stick stuck through the middle of it. The two ends of the stick are North and South. The directions perpendicular to the stick - the direction of the spinning and opposite to the spinning - are East and West.

That spinning is what creates Earth's magnetic field. There isn't really a stick, but there IS a humongous ball of molten iron spinning in one direction (counter-clockwise.) As with All That Spins, the center of the ball - the axis of rotation - is moving very slowly, while the outside is moving very fast.

It's like making a regular magnet, right? Say you want to magnetize a screwdriver: you take a magnet and you stroke the screwdriver with it over and over in one direction - handle to tip, handle to tip, handle to tip. Pretty soon the screwdriver will be a magnet itself, with a North and a South magnetic pole.

You can't magnetize a screwdriver (or anything) by stroking it in both directions (handle-tip-handle-tip) It's the going-one-direction that makes magnetic force line up the way it does. The Earth's magnetic field is lined up in the direction of the Earth's axis of rotation; the imaginary stick that the ball is spinning around.

Does that answer the question, or are you actually wondering about the linguistic origins of the words? I should think that as soon as people got enough language to talk with, they named the cardinal directions: sun-comes-up, sun-goes-down, sun-up-right-hand, sun-up-left-hand. The Big Dipper is a huge clue in the Northern hemisphere, since it's the biggest, brightest constellation and it's so close to the axis of rotation that it visibly swings around it. "Follow the Drinking-Gourd": however it swings, the last two stars of the cup point to Polaris, the North Star, that happens to be right 'over' the North pole.

Obviously, the directions had their names long before anybody had discovered magnetism. But the reason magnetism exists is the same reason we have cardinal directions at all: the Earth is spinning in one direction.

Hope that helps!

[oloriel.livejournal.com]

I think what she means is "is it physically impossible for the axis to point towards the Sun in a way that would make Sunrise appear to be in the direction of one of the poles, and Sunset in the direction of the other poles". The way Earth is set up, everything you say applies. It does NOT, however, answer whether it had to spin out that way.

I have no clue whether it does. I know that the axis of Uranus is (unscientifically speaking) pointing towards the sun rather than being sorta perpendicular to it. So Uranus would have an East pole and West pole, except that Earth humans have decided that the poles are always North and South, because in modern science, magnetism and rotation are more relevant than "where the sun rises". (Actually, it's even more effed up, because the poles change "identities" as related to the sun, depending on where Uranus is on its orbit: In Earth terms, sometimes the Sun rises in the direction of the "north" pole, and sometimes in the direction of the "south" pole.) But whether Uranus is different from Earth because of differences in size, mass or whatever, or whether it's all just coincidence.... I still don't know.

Linguistically, as you say, people defined the directions by looking at the sun long before magnetism was discovered, let alone understood. But if the axis were pointing another way and the sun would still be rising, setting etc. in the same direction, then technically we would have an East and West pole. But again, I have no clue whether with Earths specific size, mass, distance from the sun, number of moons or whatever, any other orientation would ever have been possible.

At any rate, the main problem is that two different concepts have mingled into one: the directions as observed by the "movement" of sun across the sky - and the rotational axis of the planet that just happens to be orientated - or rather, borealised - this particular way on Earth. The former came to define the latter, beyond our own planet. But planets can be running differently. I just don't know whether Earth theoretically could, or whether it had to pan out the way it does.

....wait, what?

[elenbarathi.livejournal.com]

"the main problem is that two different concepts have mingled into one: the directions as observed by the "movement" of sun across the sky - and the rotational axis of the planet that just happens to be orientated - or rather, borealised - this particular way on Earth."

Yes; you've hit it exactly. The magnetic poles of a planet are always called N and S, regardless of where they're pointing in relation to anything else.

Venus spins clockwise (http://scienceblogs.com/startswithabang/2010/10/07/counterclockwise-but-there-are/), and nobody knows why - there are theories, but no compelling evidence. In light of that fact, there's no reason (at this point) to think that whatever caused Venus and Uranus to have 'non-usual spin' couldn't have caused Earth to have it too.

If Earth had a 98% axial tilt like Uranus, the Sun would be rising and setting more-or-less over the equator each day, and the axis of rotation would still be sun-up-right-hand (N) and sun-up-left-hand (S).

I don't think it's possible for a planet's axis of rotation to point toward the sun. Consider: a stable orbit is a balance between the planet's angular momentum, that's trying to hurl it off into space, and the Sun's gravity that's trying to suck it back in. Suppose a planet got hit by something big enough to knock it cock-eyed, so that what had been the axis of rotation was pointing toward the Sun? It wouldn't stay that way; the axis of rotation would just shift, so two new places on the planet would then become the poles (which would still be called N and S by humans, regardless.)

EDIT:

"is it physically impossible for the axis to point towards the Sun in a way that would make Sunrise appear to be in the direction of one of the poles, and Sunset in the direction of the other poles".

... THAT one is a definitive 'yes, it is physically impossible', and here is how you can demonstrate that: take a ball and make two dots on it, opposite each other, to represent the poles, i.e. the axis of rotation. Turn on a lamp to represent the Sun, point the axis at it, and turn the ball as if it was spinning on that axis.

What do you see? There is no Sunrise or Sunset. One hemisphere is in permanent sunlight; the other in permanent darkness.

Re: ....wait, what?

[rachelkachel]

To your last point - depends on the season, unless the planet is tidally locked (the moon is; that's why we always see the same side). Uranus is not tidally locked, so sometimes it has normal days, and sometimes it has what you describe, when the axis of rotation points towards the sun. I think "east" and "west" (which direction the sun rises) would actually switch places over the course of the year.

I don't know how any of this affects the magnetic field, though Wikipedia tells me Uranus's is super weird.

Re: ....wait, what?

[elenbarathi.livejournal.com]

Hmmm, that's true. Here's what I found on this site (http://www.space.com/45-uranus-seventh-planet-in-earths-solar-system-was-first-discovered-planet.html) about it:

"Unlike the other planets of the solar system, Uranus is tilted so far (http://solarsystem.nasa.gov/planets/uranus/indepth) that it essentially orbits the sun on its side, with the axis of its spin nearly pointing at the star. This unusual orientation might be due to a collision with a planet-size body, or several small bodies, soon after it was formed.

This unusual tilt gives rise to extreme seasons (http://science.nasa.gov/science-news/science-at-nasa/2000/interplanetaryseasons/) roughly 20 years long, meaning that for nearly a quarter of the Uranian year, equal to 84 Earth-years, the sun shines directly over each pole, leaving the other half of the planet to experience a long, dark, cold winter.

The magnetic poles of most planets are typically lined up with the axis along which it rotates, but Uranus' magnetic field is tilted, with its magnetic axis tipped over nearly 60 degrees from the planet's axis of rotation. According to Norman F. Ness, et al, in an article in the journal Science, this leads to a strangely lopsided magnetic field (http://www.sciencemag.org/content/233/4759/85.abstract) for Uranus, with the strength of the field at the northern hemisphere's surface being up to more than 10 times that of the strength at the southern hemisphere's surface, affecting the formation of the auroras."

So, there is no Sunrise or Sunset when the axis of rotation is pointed toward the Sun - it's physically impossible to ever have the Sun rise over one pole and set over the other - but in the case of Uranus, the axis is not always pointed at the Sun, and when it's not, there are Sunrises and Sunsets.

You're right about the direction the Sun rises switching places over the course of its long 'year', too. From this site (http://solarsystem.nasa.gov/planets/uranus/indepth):

"Voyager 2, the only spacecraft to visit Uranus, imaged a bland-looking sphere in 1986. When Voyager flew by, the south pole of Uranus pointed almost directly at the sun because Uranus was near its southern summer solstice, with the southern hemisphere bathed in continuous sunlight and the northern hemisphere radiating heat into the blackness of space.

Uranus reached equinox in December 2007, when it was fully illuminated as the sun passed over the planet's equator. By 2028, the north pole will point directly at the sun, a reversal of the situation when Voyager flew by. Equinox also brings ring-plane crossing, when Uranus' rings appear to move more and more edge-on as seen from Earth."

Getting back to conuly's original question, I found this (http://cseligman.com/text/planets/uranusrot.htm):

"The axis of Uranus' rotation is tilted more relative to the pole of its orbit than that of any other planet, differing by 97.7 degrees if the rotational direction of the planet is used to define its north pole, or (as noted at Seasons on the Other Planets (http://cseligman.com/text/sky/otherseasons.htm)) by 82.2 degrees if the north side of our orbit is used to define the planet's north pole. According to the traditional definition based on the direction of the planet's rotation, Uranus' south pole faced the Sun in the 1980's, when Voyager 2 passed by it; but using the more modern definition based on the rotation of the Earth, Uranus' north pole faced the Sun at that time (this can obviously cause confusion in reading different discussions of the planet's rotation)."

... woohoo, jackpot! I just found an in-depth explanation, What makes one pole of a planet north vs south? (http://worldbuilding.stackexchange.com/questions/16774/what-makes-one-pole-of-a-planet-north-vs-south) Short answer: arbitrary human convention.

Re: ....wait, what?

[oloriel.livejournal.com]

Short answer: arbitrary human convention.

Excellent! That's what I was awkwardly trying to suggest in my comment up there: It's that way because we happened to name it that way. ^^

Re: ....wait, what?

[conuly]

Amazing, I disappear and the comments explode! Just like the good old days :)

This conversation is super informative and interesting, so thanks :)

[conuly]

I read - or skimmed, the layout was a little awkward - the second one, but not the first. I'm sorry I was so terse. I think the lack of daylight is beginning to hit me. I'm ever so much better once I *realize* that's the problem, but it always takes me a month or so to get to that point.

[elenbarathi.livejournal.com]

No worries, hon; it gets me too. We had 9 hrs. 32 min. of damp and cloudy daylight here today; by Dec. 8 we'll be down to 8 hrs. 30 min (and still falling) - gah.

I'd have a much easier time with all this if not for the stupid clock-changing.