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

[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.

[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]

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

[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.

[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.