Science Myths
The magnetic north pole is, in magnetic terms, a south pole
A compass needle points north because opposite poles attract, which means the thing it is attracted to must be the opposite kind. The naming convention was fixed centuries before anyone knew why it worked.
What follows is an argument about the polarity of the Earth's magnetic field, and about where the received version of it stops being true.
The argument in brief
- Magnetic poles were named for the direction they seek, not for their polarity.
- Opposite poles attract, so the Earth's northern magnetic pole is magnetically south.
- The magnetic pole is not fixed and has been moving measurably for decades.
The naming happened first
Compasses were in navigational use for centuries before anyone understood that the Earth itself behaves like an enormous magnet. The end of the needle that swung towards the pole star was therefore called the north pole of the magnet, purely as a description of its behaviour. That convention was locked in by practical use long before the underlying physics gave anyone a reason to reconsider it.
A treatise published at the very start of the seventeenth century argued that the Earth is a magnet, which explained the behaviour but arrived far too late to rename anything. Every magnet since has been labelled by the same rule, which means the labels record what sailors observed rather than what physicists later worked out.
Why that makes the Earth backwards
Like poles repel and opposite poles attract, which is the first thing anyone learns about magnets and is entirely reliable. If the north-seeking end of a needle is attracted towards the Arctic, the magnetic pole sitting up there must be of the opposite kind.
Tested properly, so the region conventionally called magnetic north is, in the language of magnetism, a south pole, and this is not a technicality anyone can dissolve. Textbooks handle it by simply not mentioning the contradiction, which works well until a student notices and asks. The alternative would be renaming every magnet in every laboratory and every compass on every ship, which nobody has proposed seriously.
Where the field comes from
The Earth's field is generated by motion in the liquid metal of the outer core, driven by heat escaping from below and by the planet's rotation. The result is approximately, but only approximately, the field of a bar magnet tilted at an angle to the axis of rotation.
That approximation is good enough for navigation and badly wrong for anything requiring precision, because the real field has substantial local structure. Rock formations, ore bodies and even the steel in a ship distort the local field enough to matter for accurate work. Navigators correct for these effects with published tables and with adjustments made to the instrument itself.
The pole does not stay put
The magnetic pole wanders, and its position has been tracked well enough over the past century to show substantial and accelerating movement. The rate has been fast enough in recent decades that the models used by navigation systems have had to be updated ahead of schedule. Anyone using a compass for accurate work needs the local difference between magnetic and true north, and that figure has a date attached to it.
Charts print the value along with the annual rate of change, precisely because the number goes out of date.
This is one of the few pieces of geography that has to be reissued rather than simply reprinted.
Reversals
The rock record shows that the field has repeatedly reversed its direction, with the two magnetic poles effectively swapping ends. The evidence sits in volcanic rock, where iron-bearing minerals lock in the direction of the field at the moment the rock cools.
Stripes of alternating direction on either side of mid-ocean ridges preserve this history symmetrically, which was one of the decisive pieces of evidence for plate tectonics. Reversals appear to happen at irregular intervals separated by long and unpredictable gaps, and the process itself takes a long time. What happens to the field during a reversal is not well understood, and confident claims in either direction should be treated carefully.
Where the earliest trace is a newspaper anecdote, treat the whole chain with suspicion.
Living with a bad convention
Science is full of terms that record an early guess rather than a later understanding, and most of them are too entrenched to fix. Positive and negative electric charge were assigned by convention, with the result that current is conventionally drawn flowing opposite to the electrons. Renaming would cost more than the confusion it removes, so the profession pays a small permanent tax in explanation instead.
The compass is a particularly good case because the wrong label is printed on an object in millions of pockets and rucksacks. Nothing about navigation is affected, which is why nobody has ever seriously proposed fixing it.
The takeaway
The compass was named by people watching what it did. The physics arrived later and had to live with the labels.
The satisfying version of a story is the one that travels, which is the whole problem.
Questions readers ask
Does this affect using a compass?
Not at all. The needle behaves exactly as expected. The oddity is purely in the labels, which were assigned before the mechanism was understood.
Is the pole really moving?
Yes, measurably, and fast enough in recent decades that the models used by navigation software have needed unscheduled updates.
