Question Time!

[such_heights]

Calling my geologically inclined friends: please excuse a little lazy-webbing and help me resolve a debate with my father. Is there a non-arbritrary way of distinguishing a planet's north from its south, one that doesn't simply rely on convention? And if not, does that make all sci-fi talk of an unhabitated planet's southern hemisphere etc completely nonsensical?

... These are the things that keep me up at night.

Comments

[kel_reiley]

that is a GOOD question! to which i do not know the answer, but i'd say no b/c north and south ARE human conventions and to atrribute them to an uninhabited planet... well, i suppose it would just be whatever the 1st human who arrived decides

[lefaym]

I think it's got something to do with the flow of electrons. You create a magnet (and the entire planet is a huge magnet) by making all (or most) of its electrons flow in the same direction, so I'm guessing it's something to do with that.

It can't be completely arbitrary, because the Earth's poles actually reverse every million years or so, which means all the compassess will point in the opposite direction.

However, the convention of representing the North as the "top" and the South as the "bottom" of the map is completely aribitrary.

[kel_reiley]

OH! although i *could* depend on the planet's axis and the respective poles, but i do not know how you'd determine north pole from south (assuming the planet only rotates on a single axis)

[kel_reiley]

However, the convention of representing the North as the "top" and the South as the "bottom" of the map is completely aribitrary.

^this, this is what i meant :)

[lotesse]

Magnetic north? Maybe?

[silveronthetree]

I'm an ex-geophysicist and I had NO idea what the answer to this question is! But some googling eventually brought up these pages on poles of astronomical bodies and geographic poles.

A geographical pole (also geographic pole) is either of the two points—the north pole and the south pole—on the surface of a rotating planet (or other rotating body) where the axis of rotation (or simply "axis") meets the surface of the body.

You have to be careful about the differences between Geographic Poles and Magnetic Poles. And I think that you are looking at Geographic poles here.

[lorannah]

My understanding as an archaeologist (and feel I have to point out, that archaeologists rarely understand anything properly) is that the planet is naturally a magnet of sorts - hence that on a compass the arrow points one way. Humans decided at some point, that the way that arrow points should be North. Though I'm not sure when they did this.

We're not really sure how the earth developed it's magnetic field, though there are theories - so we're not really sure if all planets have the same phenomenon.

Interestingly though in the past there have been points when the planets poles have switched, so then North sort of would have been what we now think of as south. And the poles always wander around a little every year, so north on a map is never entirely accurate. The magnetic fields also how animals migrate, and my personal favourite fact is that it apparently affects which direction cows stand (unless they're near power lines)

[sophinisba]

Ooh, neat. From that first link,

Another common definition uses the right-hand rule to define an object's north pole: it is then the pole around which the object rotates counterclockwise.

It's only one of several conventions but seems like one that could be applied across the universe, so our sci-fi southern hemispheres are safe!

[silveronthetree]

ANd to actually answer the questions the first link says this about the definition of a North Pole - if the planet is outside our solar system, the second paragraph applies.

The International Astronomical Union defines the geographic north pole of a planet or other object in the solar system as the planetary pole that is in the same ecliptic hemisphere as the Earth's North Pole. More accurately, "The north pole is that pole of rotation that lies on the north side of the invariable plane of the solar system".[1] This definition means that an object's axial tilt is always 90° or less, but its rotation period may be negative (retrograde rotation) – in other words, it rotates clockwise when viewed from above its north pole, rather than the "normal" counterclockwise direction exhibited by the Earth. Venus rotates in the opposite direction to the other planets, and Uranus has been knocked on its side and rotates almost perpendicular to the rest of the solar system.

Another common definition uses the right-hand rule to define an object's north pole: it is then the pole around which the object rotates counterclockwise.[2] With this definition, axial tilts may be greater than 90° but rotation periods are always positive.

[silveronthetree]

Yes, I just realised that I hadn't actually answered the question!

[elsane]

(hi! I'm mostly offline these days but just happened to log on in time for a question I am professionally qualified to answer, so!). Yes! It makes sense to have north and south poles unambiguously. It's determined by the direction of the planet's rotation. The angular momentum of a planet is a vector directed along its axis of rotation -- north means along the vector, south means against the vector.

Or in other words, sit on the equator facing the sunrise. This unambiguously determines east, and therefore north and south. (:

hope you are doing well, A.

[phaetonschariot.livejournal.com]

I do know that the sun can never, in fact, rise in the west even on other planets, because the definition of east is the place the sun rises and west is where it sets. Supposedly.

(Captcha is "codex 400,000". I feel this means something. I'm just not sure what.)

[skywaterblue]

All planets with active plate tectonics and cores have a magnetosphere, I believe. From there I would assume that north is the positive polarity end of the planet and south the negative polarity.

[zahrawithaz.livejournal.com]

It's also relatively recent and culturally specific. Maps drawn in Europe in the Middle Ages (usually called T-and-O maps, because they look like T inside a giant O) can point in any direction, but most often put East at the top--where the draftsmen thought Paradise was located--which meant that Europe was crammed into the lower left-hand corner.

[zahrawithaz.livejournal.com]

I'm only echoing others now, but yes, there are two types of pole--the axis on which the planet rotates, and the magnetic pole toward which the compass turns. The latter does reverse periodically, while the the former doesn't (though I suppose a big enough asteroid strike could jostle it).

I believe it's only happenstance that on Earth the two poles aren't that far off from each other, and that they could hypothetically be much farther apart on a planet with a different axial tilt; but someone may be able to correct me there.

[secondsilk]

I would use that definition: that counterclockwise around the northern pole one.

I lost marks on a science quiz once because I said the earth rotated West to East (which it does! (you just have to ignore place names that have east and west in them)), when the "correct" answer was supposedly counter-clockwise. Even though we were in Australia and closest pole is therefore the southern pole and clockwise rotation! I still hate that teacher.

[kel_reiley]

that makes sense

[avendya]

I think that between the answer about the plane of the ecliptic and elsane, you have it covered, but I thought I should point out magnetic poles and geographic poles can also switch (so this is a really bad method of determining north and south on geologic time scales).

(C.f. The Guardian.)

[such_heights]

Hello, you! How's everything going?