Which way is north on your new planet?
<p>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 &#8216;North&#8217;?</p>
<p>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.</p>
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<p>First and most obviously, there&#8217;s magnetic north.  Our assumption &#8216;the planet is Earthlike&#8217; 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 &#8216;North&#8217;.</p>
<p>Then there&#8217;s spin-axis north.  This is the assignment that makes north relate to the planet&#8217;s rotation the same way it does on Earth &#8211; that is, it implies the sun setting in the west rather than the east. Not necessarily the same as magnetic north; I don&#8217;t know of any reason to think planetary magnetic fields have a preferred relationship to the spin axis.</p>
<p>Next, galactic north. Earth&#8217;s orbital plane is inclined about 26% from the rotational plane of the Milky Way, which defines the Galaxy&#8217;s spin-axis directions; these have been labeled &#8216;Galactic North&#8221; and &#8220;Galactic South&#8221; 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.</p>
<p>Finally there&#8217;s habitability north.  This one is fuzzier.  More than 3/4ths of earth&#8217;s population lives in places where north is colder and south is warmer. We might want to choose &#8216;North&#8217; to preserve that relationship, which is embedded pretty deeply in the language and folklore of most of Earth&#8217;s cultures. Thus, &#8216;North&#8217; should be the hemisphere with the most habitable land.  (Or, if you&#8217;re taking a shorter-term view, the hemisphere in which you drop your first settlement.  But let&#8217;s ignore that complication for now.)</p>
<p>If all four criteria coincide, happiness.  But how likely is that?  They&#8217;re probably distributed randomly with respect to each other, which means we&#8217;ll probably get perfect agreement on only one in every sixteen exoplanets.</p>
<p>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.</p>
<p>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&#8217;re opposite, and galactic north coincides with magnetic north, that&#8217;s a tiebreaker.  If the tiebreakers don&#8217;t settle it, I&#8217;d go with spin-axis north.  </p>
<p>But reasonable people could differ on this.  Discuss; maybe we could submit a proposal to the IAU.</p>
