So, here you are in your starship, happily settling into orbit around an Earthlike world you intend to survey for colonization. You start mapping, and are immediately presented with a small but vexing question: which rotational pole should you designate as ‘North’?
There are a surprisingly large number of ways one could answer this question. I shall wander through them in this essay, which is really about the linguistic and emotive significance of compass-direction words as humans use them. Then I shall suggest a pragmatic resolution.
First and most obviously, there’s magnetic north. Our assumption ‘the planet is Earthlike’ entails a nice strong magnetic field to keep local carbon-based lifeforms from getting constantly mutated into B-movie monsters by incoming charged particles. Magnetic north is probably going to be much closer to one pole than the other; we could call that ‘North’.
Then there’s spin-axis north. This is the assignment that makes north relate to the planet’s rotation the same way it does on Earth – that is, it implies the sun setting in the west rather than the east. Not necessarily the same as magnetic north; I don’t know of any reason to think planetary magnetic fields have a preferred relationship to the spin axis.
Next, galactic north. Earth’s orbital plane is inclined about 26% from the rotational plane of the Milky Way, which defines the Galaxy’s spin-axis directions; these have been labeled ‘Galactic North” and “Galactic South” in accordance with the Earth rotational poles they most closely match. On our new planet we could flip this around and define planetary North so it matches Galactic North.
Finally there’s habitability north. This one is fuzzier. More than 3/4ths of earth’s population lives in places where north is colder and south is warmer. We might want to choose ‘North’ to preserve that relationship, which is embedded pretty deeply in the language and folklore of most of Earth’s cultures. Thus, ‘North’ should be the hemisphere with the most habitable land. (Or, if you’re taking a shorter-term view, the hemisphere in which you drop your first settlement. But let’s ignore that complication for now.)
If all four criteria coincide, happiness. But how likely is that? They’re probably distributed randomly with respect to each other, which means we’ll probably get perfect agreement on only one in every sixteen exoplanets.
But not all these criteria are equally important. Magnetic North really only matters to geophysicists and compass-makers. Galactic North is probably interesting only to stargazers.
I think we have a clear winner if spin-axis north coincides with habitability north. This choice will preserve continuity of language pretty well. If they’re opposite, and galactic north coincides with magnetic north, that’s a tiebreaker. If the tiebreakers don’t settle it, I’d go with spin-axis north.
But reasonable people could differ on this. Discuss; maybe we could submit a proposal to the IAU.