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Comment by tialaramex | Hacker News Reader

So far as I understand it the existence of a magnetic "North Pole" is a weird temporary coincidence, like if you discovered a friend's phone number is the same as yours but in the opposite order of digits. It doesn't mean anything and it might change also for no particular reason, so you should probably avoid depending on it if possible.

Magnets are more or less bound to point somewhere on a planet containing a core of molten iron, but my understanding is that it could easily have been towards Australia, or Mexico or anywhere.

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No, it's almost always roughly aligned with the axis of Earth's rotation. I believe that's because it's related to the rotation of the internal molten iron core.

Though every few million years, the north and south magnetic poles swap, for unclear reasons.

We are also about 350,000 years overdue for a magnetic pole swap. [1]

Also, it cycles at 300,000-400,000 years not millions.

[1] https://futurism.com/earths-magnetic-poles-overdue-switch

Looking at a map someone else linked to, the pole has shifted drastically in the last 50 years. Could that be an indication that it is in the process of shifting? It's been moving North in a straight line for the past 100 years, and now it's actually heading South towards Siberia.

https://maps.ngdc.noaa.gov/viewers/historical_declination/

> Could that be an indication that it is in the process of shifting?

It could be. Since we've never seen a shift, we don't really know how it looks.

And a shift takes a few thousand years, so we can probably be centuries into it before it's clear what's happening.

Ah, too bad, it would have been great drama if it flipped instantly! Or even better, oscillated a few times over the course of a few years before settling in.

I mean, for all we know it might oscillate like that.

I doubt the geological record is fine grained enough to tell fast oscillations apart from a slow drift.

Thanks for the timing correction!

I hoped "every few million years" was vague enough that I didn't have to look it up, but sometimes you gamble and lose :)

Also, the swap apparently takes ages and during that time you have loads of magnetic poles floating about and the ionosphere dips by 80% or so. We currently have a little north pole floating around the south Atlantic. https://en.wikipedia.org/wiki/South_Atlantic_Anomaly

Is it really a "little north pole"? I don't know anything about it beyond what's in the WP article, but that doesn't sound like an accurate description.

I reckon it isn't that inaccurate, especially given the reported behaviour of compasses.

"From these models and extrapolating down into the Earth, it is known that regions of reversed flux at the core-mantle boundary have grown over time. In these regions the compass points in the opposite direction, in or out of the core, compared to that of surrounding areas. It is the growth in area of such a reversed flux patch under the south Atlantic that is primarily responsible for the decay in the main dipolar field. This reverse patch is also responsible for the minimum in field strength called the South Atlantic Anomaly, now centred over south America. In this region energetic particles can approach Earth more closely, causing increased radiation risk to low Earth orbit satellites."

http://www.geomag.bgs.ac.uk/education/reversals.html

Does anybody have a link about that first part?

"Almost always" is a term of art in probability theory, where it means p = 1 despite the existence of possible outcomes that defy it. For example if you keep tossing a (fair) coin it will /almost always/ eventually come up "Heads". But I guess here you mean something else and I'm not sure what.

It isn't just a theoretical supposition, I believe we measure alignment of magnetic fields in igneous rocks and the 'almost always' comes from that, the current field of the earth being fixed in the rock when it solidifies.

> "Almost always" is a term of art in probability theory, where it means p = 1 despite the existence of possible outcomes that defy it.

That's a somewhat misleading term! I much prefer "probability one".

> But I guess here you mean something else and I'm not sure what.

Read it literally. The everyday definition of "almost". In other words, "the vast majority of the time".

I agree this should be read as “the vast majority of the time”, but if one wants to be precise, it is unavoidable to deviate from everyday definitions (is 99% the vast majority of the time, or does that need to be 99,9%, 99,99%,…?)

“probability one” is misleading, too, as laymen would assume the reverse cannot happen.

The interpretation of “almost always” as ”p = 1 despite the existence of possible outcomes that defy it” likely is based on http://mathworld.wolfram.com/AlmostEverywhere.html (which has “almost surely” as the mathematical expression, but I would immediately read “almost always” with the same meaning)

> “probability one” is misleading, too, as laymen would assume the reverse cannot happen.

In the real world it cannot happen. It can only happen in an unbelievably unlikely mathematical abstraction. So I would call that layman understanding closer to the truth.

I would assume that "almost always" is a common English phrase that has been adopted by probability.

Sure, but the more accurate English phrase for that meaning is more like a simple "always".

"almost always" is more like a 95% threshold, sometimes higher but with an implication that failure is realistic.

Perhaps. However - "the magnetic axis of Uranus, for example, is inclined by as much as 60°"

While that's true, when using something to measure you're not trying to find a universal truth, just something constant and approximate enough for your purposes that you can utilize it.

If magnetic north was located in Mexico it would still be of use, assuming it was relatively constant.

Is it not roughly perpendicular to the plane of rotation because of the relationship between electromagnetism and applied force (see the right-hand-grip-rule we all learnt in highschool physics)?

However it's still currently good enough to get an average person to their destination if their GPS enabled device has run out of battery and they happen to have a map and compass on them.

Unfortunately for most people nowadays, the compass is the same electronic device (their smartphone) that does GPS.

...and the map is also the same device.

Most smartphones that are out of a battery can give you magnetic north if you also have a big rock (smash phone apart), and make a compass leaf out of a piece of metal in the phone, something like a leaf and a patch of still water. The phone most likely has a magnet (e.g. in the speaker) to magnetize the metal.

I can't think of any ferrous parts in a modern phone.

Enclosures are all either plastic, aluminum or magnesium. Circuit board will be copper. Maybe some internal structural elements or RF shield cans are made of steel?

Microphones are MEMS these days, so no magnets there. Speaker magnets are tiny and also tend to have weird magnetization patterns that seem useless for making a compass (e.g. disc with a circular channel for the coil where N-S are inside of the channel)

Note that PCBs are made of dielectric (non-conductive) composite materials such as fiberclass-resin laminate. The conductive copper film covering the substrate is on the order of 100 micrometers thick.

I believe most if not all smartphones also have a magnetometer in them, apart from the GPS or the metal components in general.

True but that's going to be a MEMS[1] device, so not much use without the rest of the phone operating.

[1]: https://en.wikipedia.org/wiki/MEMS_magnetic_field_sensor

I doubt this is true for most people who can and might realistically need to be able to navigate with a paper map and analog compass.

In other words, the skill is about as rare or rarer than the equipment.

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