Which way is north on your new planet?

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.

57 thoughts on “Which way is north on your new planet?

  1. Throwing some thoughts out, magnetic North changes from time to time, and completely flips on ocassion too. Unless North is allowed to change it is not a suitable criteria. Spin axis seems the only thing consistent. I suspect habitability is related, but not a foundation. Spin axis, consistent with galactic seems the most consistent way to provide a definition.

  2. I believe spin-axis north is standard-ish. That’s what they use for Uranus, whose “north” pole is 97.7 degrees from the sun’s north.

  3. Use the old physics trick – wrap your right hand around the predominant equatorial cloud movement in the direction of the movement, and stick your thumb up. That will be north.

  4. >wrap your right hand around the predominant equatorial cloud movement in the direction of the movement, and stick your thumb up.

    Yeah, if you think about how atmospheric circulation works a bit that should be spin-axis north.

  5. My understanding is that there’s a strong correlation between spin-axis north and galactic north. It’s a rotational-momentum thing, everything wants to spin on roughly the same axis. Could be wrong though.

  6. Another possible criterion is spin-axis north with respect to the planet’s orbit around its primary, rather than its rotation. This may be convolved with the spin-axis of its rotation, though, in which case it’s not significant. (Actually, I can’t really think of any way that immediately matters to Earthlings. The direction of the primary’s progression through the zodiac? Probably only interesting to astronomers, again.)

  7. >Another possible criterion is spin-axis north with respect to the planet’s orbit around its primary, rather than its rotation.

    I’d call this “orbital-axis north”. It probably is strongly correlated with spin-axis north, because it will be determined by the spin direction of the system’s early protostar cloud and that is in turn likely to set spin-axis north for most of the planets as well. (The Solar System has a couple of exceptions to this rule, however.)

  8. I vote spin-axis north, being a space-sim buff, but that’s mostly because I want to see this discussion unfold and subscribe to the comments. Bye!

  9. Astronomers, when discussing actual non-Earth planets, almost always use the spin axis to determine which way on a planet is “north”. This holds even for planets like Uranus, whose axial tilt is nearly parallel to the plane of the solar system (and perpendicular to “galactic north”). So I would nominate spin-axis north as correct.

  10. I see no reason to change the established meaning of spin-axis north — at least for a rotating planet.

    For a moon such as one of Jupiter’s, I’d still use spin-axis north, measured relative to the immediate primary (the planet) rather than the sun if it makes a difference.

    For Earth’s moon (which doesn’t rotate with respect to Earth) I’d use orbital-axis north.

    Quite a few other special cases are possible, especially for things like asteroids (where we’re likely to dig habitats out of the interior and not even use the surface) or man-made objects such as O’Neill cylinders. For those the builders will probably want to come up with their own “standard” coordinate grid and publish it.

  11. I’ve always taken spin-axis north as the obvious choice. Habitability is too fuzzy to be useful as a determinant, and the others are nearly irrelevant. Spin-axis is clear, unambiguous, and obvious to anyone who can see the sun rise or set on the planet.

  12. As Tom already mentioned there is orbit-around-primary north, which is unstable in that the orbit will be slowly decaying towards the primary (IIRC), and slowing it’s rotation.

    There is also likely to be significant tidal heating making these interesting worlds all around.

    – Foo Quuxman

  13. (Oddly, the first time I thought of this issue was playing Minecraft, where a mapping program we had defined north effectively randomly, at odds with “the sun rises in the east”, and got into a debate with a buddy about it)

  14. >For Earth’s moon (which doesn’t rotate with respect to Earth) I’d use orbital-axis north.

    There is an established “Lunar North”; it’s the pole with lower angular separation from Earth’s North. I think that’s a good rule for tide-locked satellites in general, but it need not coincide with the satellite’s orbital north.

  15. Given how often (geologically speaking) Earth’s magnetic field flips, I wouldn’t put any weight on magnetic north.

    I’d have spin-axis north outweigh all the other criteria, with the possible exception of orbital-axis north. And if habitability north disagrees with spin-axis north, we could name the planet “New Australia,” or possibly “Chalion.”

  16. ESR> I don’t know of any reason to think planetary magnetic fields have a preferred relationship to the spin axis.

    Since the evidence is that our own magnetic field spends roughly half its time pointing one way and roughly half the other way, I’d say no.

    I’m on the spin-axis team. The local star rises in the East just as it does here. There is no other definition of “North” that makes sense to me. Whether that happens to align with “galactic north” is irrelevant.

  17. In the literature right now, it’s spin-axis North, all the way down.

    The sample set we have of planets with observable magnetic fields indicates that spin axis and magnetic axis tend to be well aligned; only Earth’s has an offset of more than one degree.

  18. Curiously, one doesn’t even have to leave Earth to ask this question about what directional names really mean. At Amundsen-Scott South Pole Station, *every* direction is north – which makes it difficult to use our normal language to describe locations. In consequence, the direction that the predominant wind comes from is called “north” or “grid north”, with the other directions relative to that.

    If we settle a new planet, would we even use the same words – or would we invent new ones?

  19. Do you want an astronomical convention to determine something as fundamental to everyday life as the direction of north and its relationship to climate on a habitable, soon-to-be-colonized world? Eric is being very perceptive in nothing the south-is-warmer convention is deeply embedded in the language.

    This issue has come up most recently with the definition of a planet. The IAU decision to define a planet in a way that excluded Pluto has generated a small but noticeable social backlash against the “Pluto is not a planet” definition, because generations of people have been told that it was.

    Examples:
    http://www.youtube.com/watch?v=LnN8LYz-2Jo
    http://laurele.livejournal.com/1271.html

    So what is the relevance of this to the original question of north on a new planet? Only that we had better put what is culturally most important over the opinions of astronomers.

    Perhaps the lesser evil is to put south in the warmest direction (from the main landmasses), then define east and west so that the sun rises in the east. If that means that east is on my left when facing north, so be it…

    Cathy

  20. >If we settle a new planet, would we even use the same words – or would we invent new ones?

    While I was writing the OP I had the thought that a culture with a long enough history of knowing astronomy might use “spinward” and “antispinward” on planetary surfaces rather than arbitrary labels like “east” and “west”. North and South would be awkward, though: right- and left-hand of spinward? (I’m not sure I have those in the right order.)

    Of course, Traveller players will remember “spinward” and “antispinward” being used in conjunction with galactic north and south in that reference frame.

  21. >Do you want an astronomical convention to determine something as fundamental to everyday life as the direction of north and its relationship to climate on a habitable, soon-to-be-colonized world?

    That is in fact one of the questions I was trying to open up.

  22. For Uranus, the axis is at a right angle. Remember that the sun doesn’t rise or set above the arctic (or below the antarctic) circle on the solstice. If the axis is on the ecliptic, the sun isn’t going to rise or set at all.

    Then there’s precession…

    “Habitible” is subjective. Most of the land mass is in the Northern hemisphere, but ‘first landing” or whatever else is subjective.

    The planet may not rotate (like our moon or Mercury), or not have a Magnetic Field.

  23. When you get to this planet.
    Pull into the first rest stop you see and flush the toilet.
    If the water spins clockwise, you’re on the northern hemisphere.

  24. I was going to argue for spin-axis north, but it seems that everyone has already come to that conclusion.

    So, new question: what happens if you are (un?)fortunate enough to find a planet that isn’t rotating, or equivalently, rotating too slowly to measure? (The closest thing our solar system has is Venus whose “day” I think clocks in at just shy of one of our years. But maybe there’s a planet out there with effectively 0 angular velocity?)

    My instinct says orbital-axis north, but I’m not exactly sure why.

  25. As for magnetic, is there any guarentee it will align with the rotational poles at all? I realize Earth’s usually does, as does Jupiter’s, but neither perfectly, and that’s only two datapoints.

  26. IIRC, Lunar East and West are reversed from the point of view of Earth (the left side as seen from Earth is East). Because of this, Mare Orientale (The Eastern Sea), named before the convention on East and West, is on the far West of the Near Side of the Moon.

  27. @Joshua: The intuitive reason for spin-axis north is to maintain the sunrise=east, sunset=west equivalence. Thus, if a planet actually has zero angular momentum itself, but orbits its primary, it’s effectively counterrotating in the same plane as its orbit, with a period of rotation equal to the year. (This strikes me as a really unlikely scenario; tidally locked would be much more stable, intuitively at least.) Thus, you can use that reference frame for spin-axis.

    It’s actually more ambiguous in the case of a tidally locked planet. There, I guess you might as well just use orbit-axis north, since it’ll be the same as spin-axis in the solar system’s reference frame. But there, more of the rationale for keeping our words ‘north’ &c. will be missing.

  28. >It’s actually more ambiguous in the case of a tidally locked planet. There, I guess you might as well just use orbit-axis north, since it’ll be the same as spin-axis in the solar system’s reference frame.

    I concur with this. For a moon, there’s a good argument for using the primary’s spin-axis north even if it’s opposite from orbital north – but, er, there are unlikely to be observers on the surface of a star and thus in a position to benefit from that convenience.

  29. From a pragmatic perspective, ignoring spin-axis north is going to require editing or footnoting every physics textbook (the right-hand rule, noted above). Worse, any way you do it is going to cause problems — either the physics book is out of sync with the way reality is labeled (locally) or the physics book is out of sync with the way physics is taught back on Earth.

    Ignoring magnetic north has similar issues, plus navigational concerns that could well lead to loss of life or property (maybe a lot of it). Granted, there may be no choice if spin-axis and magnetic don’t coincide as they (very roughly) do on Earth, but why borrow trouble?

    Ignoring sociocultural north would have the fewest real consequences, IMO. Yeah, lots of books make those assumptions, but I haven’t noted that it’s caused any special psychological trauma for Australians.

  30. Besides RSO’s obscure reference, there’s also the classic “The Road Less Taken” by Harry Turtledove. “Always some water-lovers every trip.”

  31. “So, new question: what happens if you are (un?)fortunate enough to find a planet that isn’t rotating, or equivalently, rotating too slowly to measure?”

    I doubt such a planet would be habitable, so we can leave that question to the astronomers.

  32. “Ignoring magnetic north has similar issues, plus navigational concerns that could well lead to loss of life or property (maybe a lot of it).”

    Thought experiment: suppose that over the course of the next decade, the magnetic axis on Earth flips. Would you change the definition of magnetic north? How do you get rid of all the existing compasses that point the “wrong” way?

    How many people have even thought about the fact that it’s the south magnetic pole of the Earth that is located near the north geographic pole?

  33. esr: “For a moon, there’s a good argument for using the primary’s spin-axis north even if it’s opposite from orbital north…”

    Which could get interesting if the moon is habitable (perfectly possible), and raises all the cultural questions in the OP.

  34. Imagine several planets, all spinning and orbiting a star. Each planet has its spin axis, that is, the axis around which the planet is rotating anticlockwise. Imagining it on a solar system level, it is simpler to always call this axis the north pole, rather than sometimes the north pole and other times the south pole. In other words, the speed of rotation of a planet around the north pole will always be positive, which is easier to imagine.

    If settlers ever did do it the other way round, it would cause great annoyance and it would be generally seen as “clunk” that we wish we could get rid of, but can’t for “historical reasons”.

    “Frozen wilds of the north” (or similar expressions) does sound more familiar than “frozen wilds of the south” – but we do have Antarctica on this planet – and other exceptions to the rule that the further south you go, the warmer it is.

  35. Anne McCaffrey had fun with this in her Pern series. She landed everyone in the “northern” hemisphere, and then unveiled a surprise when the “southern” hemisphere turns out to have far more area that is suitable for settling. Part of why it works is that her readers, being predominantly in the anglosphere, have a subconscious connection of “north” being where people live.

    Anyway, I vote for spinward north. It applies to a wider variety of bodies: you don’t have to assume habitability, a star it is rotating around, or a magnetic field.

    Also, while we are fixing these things, electrons ought to have positive charge, and magnetic north ought to be whichever way is spinward north if you spin an electron in a circle.

  36. While it would be wise to use the spin axis to define “north pole”, that is not what the IAU did. They said (wikipedia):
    The International Astronomical Union (IAU) defines the geographic north pole of a planet or any of its satellites in the solar system as the planetary pole that is in the same celestial hemisphere relative to the invariable plane of the solar system as Earth’s North pole.
    This has the immediate disadvantage that for a planet with an inclination very near 90 degrees, small perturbations could cause the “north pole” and “south pole” to switch places. But that’s what the IAU did. And so, in our solar system, the “north pole” of Uranus does not match the spin convention.

  37. >The International Astronomical Union (IAU) defines the geographic north pole of a planet or any of its satellites in the solar system as the planetary pole that is in the same celestial hemisphere relative to the invariable plane of the solar system as Earth’s North pole.

    Huh. That’s galactic north, then.

    Lots of people, here and on G+, have been asserting authoritatively that the astronomical standard is spin-axis north – and I admit that would have been my guess at the standard. Have there been different fiats from different sources?

  38. I’d definitely go for spin-axis north, above all the other alternatives. The “sun”, or however you call the central star of the system, should rise in the east and set in the west, period. :-)

  39. On Mars, a day is called a “sol.” Consequently, on a non-Earth planet, we could choose an entirely different terminology for the polar extremis. For example, Polar Prime.

  40. might use “spinward” and “antispinward” on planetary surfaces

    I’m a flat-earther, so I’ll go with rimward and hubward

  41. On a related note, once you have settled on where north is, how do you decide where to zero your lines of latitude? Original landing zone?

  42. You zero your lines of latitude midway between the poles. Do you mean longitude?

    Original landing spot sounds reasonable for that.

  43. Yes…sorry…having a ‘senior moment’ over here. I did, of course, mean “longitude”

  44. I vote for spin-axis.

    One of the most fundamental aspects of the environment on Earth is the day/night cycle. It sure as hell trumps compasses.

    This doesn’t work for odd-ball cases like Neptune, so arbitrarily assign one spin-axis pole to be North. It sucks to be Neptune.

    As to which pole is North and which South, I don’t really care that much. If the majority of folks want the sun to rise in the East, that’s fine with me.

  45. I seem to have left out the reason for my position. Folks here can of course see it, but I would like to make the point.

    I figure the directions in which the sun rises and sets are the most fundamental directions; North and South are the other two orthoganal directions when standing on the surface.

    I have read that in the past, maps used to be have East on the top – hence the phrase “orienting a map” – the Orient being at the top.

    So: day/night means spin-axis if North/South poles are going to be non-arbitrary points.

  46. “IIRC, Lunar East and West are reversed from the point of view of Earth (the left side as seen from Earth is East). Because of this, Mare Orientale (The Eastern Sea), named before the convention on East and West, is on the far West of the Near Side of the Moon.”

    A little history here:

    When the first lunar maps were created, it was decided that it would be more convenient if the “east” side of the Moon was the side closer to the eastern horizon of Earth, and vice versa for the West. For the same reason, maps were usually drawn with south at the top. Astronomical telescopes of the day inverted the image, so when looking in an eyepiece, south was usually more or less “up”. It was during this period that Mare Orientale (“Eastern Sea”) was named.

    When the Space Age began, it was pointed out that the astronomical conventions would mean that as astronaut on the lunar surface would see the Sun rise in the west and set in the east. Furthermore, the rocket engineers were more comfortable with north at the top of the map. So the IAU reversed the convention, and all lunar maps since then have north at the top and Mare Orientale on the western side of the Moon.

  47. OT:

    Eric, I have a comment to make on your Slashdot interview, regarding bugs in router firmware.

    The modem/router combo units that AT&T uses as CPE for its U-verse service have some really annoying DNS bugs: the model we currently have has one where DNS and DHCP get out of sync if a machine changes hostnames, or if a NIC is moved between machines, and any attempt to fix things gets you into more of a tangle. Our previous router (an older model) had the same bug, plus one where the router’s DNS returned 127.0.0.1 as the IP address for the most recent device to connect. We got the new router when we threatened to leave because AT&T’s tech gave us zero help with the issue. We probably should still leave, but we’re frankly not sure who we can trust.

  48. While I’m inclined to go with spin-axis to be the logical choice for determining north and south, I’m also inclined to say that we don’t get to decide, one way or the other. This is a decision that would ultimately be made by the settlers of the planet–and it might not even be made by the original settlers! It’s something that will likely be determined by trial and error, taking into account spin-axis, position relative to the primary star (and perhaps to the planet it orbits, if it’s a moon), magnetic north if the planet/moon has one, and geography/navigation/exploration issues that can only be discovered by being on the planet itself.

    For that matter, it might even be determined by something as unexpected as wind patterns: I have seen a proposal for the settling of Venus, where colonies would float in the atmosphere at the level that it’s about the same density as Earth’s; at that level, both the temperature and the air pressure (of course) would be a close match to what we are used to, and Venusian winds would blow those colonies around Venus at a rate of about once every four Earth days, which, while not quite the same as our day/night schedule, it’s certainly a lot better than Venus’s day, which is about 290-ish of ours. If the winds blew opposite of the direction of rotation, then “wind-north” might be the way to go on such a planet!

  49. @ESR

    I am not entirely sure if this post is meant borderline ironically, or with a hidden meaning, or people who are interested in the natural sciences really think so differently from people who are interested in the humanities?

    Because on Earth, North matters so much because our maps have North as the “up” side, and “up” is generally considered by most human cultures as “better” – for example we call despicable behavior “lowly” (the opposite of “up”, “high”, “tall”), or if a man becomes more powerful we say he rises (up) in the ranks. Thus, when we call a place southern from us “down there” it has hidden pejorative tones. This is why this actually matters so much, this is why we stick to this terminology in space exploration instead of just saying Pole A and Pole B. (Which, too, implies ranking.)

    And North is “up” largely because of the cultures who advanced the sciences and the discovery of the world, generally considered themselves northern from the less advanced people. The people they looked _down_ on were largely southwards from them.

    So, you proposed four different version of a Scientific North and entirely ignored the concept of the Cultural North that matters for people who are oriented towards humanities, like myself. I don’t know if this is accidental, or purposeful and ironical?

    So, this means, on another planet with many different cultures, the culture that is the most advanced in science and technology, and thus determines the narrative for other cultures, will be the Cultural North – even if it means the Scientific Southwest on that planet. Maps will be oriented that way, to make their place show on top, literally.

    On planets lacking intelligent life, we may as well import our own Cultural North. Which we can determine pretty much the same ways as your methods for determining Scientific North – “the place most similar in location to Northern America and Europe”.

    This is not a criticism. Rather an expression of surprise how minds oriented towards science and humanities work so different – and the expression of a suspicion, that you, who are well versed in humanities, meant this too obvious blindness to their approach consciously, as a form of a joke or irony.

  50. (Adding to my previous post – this may have sounded too postmodernist, too Frankfurt-School-social-justice like. Actually, the reason 90% of the humanities oriented people are like that is that they consider this kind of culturally determined ranking at some level intrinsically wrong and unethical. When they analyse the concept of a Cultural North they generally mean if it is not rooted in 100 % objective science, if it is cultural and has something to do with relationships of power, then there should not be a Cultural North. I find it entirely normal – I see nothing intrinsically wrong with relationships of power – they usually come from differences of efficiency. And we need a lot of efficiency to have anything to do with exoplanets.)

  51. >So, you proposed four different version of a Scientific North and entirely ignored the concept of the Cultural North that matters for people who are oriented towards humanities, like myself. I don’t know if this is accidental, or purposeful and ironical?

    Your “Cultural North” can’t have a meaning on a planet that isn’t settled yet.

  52. I agree with your criteria exactly, but contend that they should be designated “south” on any planet but Earth?

    I bet you can guess why, but I’ll spell it out. By making the criteria for “north” on Earth “south” everwhere else, the special status of Earth as the origin planet is forever encoded.

  53. In fact, Pern was settled in the southern hemisphere, which turned out to be unsafe for settlement, so the colonists moved to the harsher but safer north.

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