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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From:

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.

Edited Date: 2015-11-07 10:07 pm (UTC)

From:

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.

From:

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.

Edited Date: 2015-11-08 05:39 am (UTC)

From:

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. ^^