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Let's say the magnetic North Pole is and has been located around 0, 0 (off the coast of Africa) for all of human history, and the magnetic South Pole 180 degrees to the east/west. Maybe the Earth's core has magically been thrown ajar in this reality.

Would current and medieval methods of navigation still work the same way? Could you still use a sextant to determine your location with the same accuracy? If not, what would the alternatives be?

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    $\begingroup$ A sextant will be trivially unaffected because it uses the positions of the sun, moon, and stars, rather than anything magnetic. (In fact there is a small deviation between "true north", as determined by Earth's rotation, and "magnetic north", as determined by its magnetic field.) $\endgroup$ – Cadence Oct 3 at 21:36
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    $\begingroup$ @Cadence the deviation between true and magnetic north (called magvar) is anything but small right now as we approach another pole reversal. It's wandering by up to hundreds of km per year. ;) $\endgroup$ – stix Oct 3 at 22:04
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    $\begingroup$ You use a sextant and clock to measure position on the globe, and those measures are unchanged. You use a compass to measure heading - that would change a bit. $\endgroup$ – user535733 Oct 4 at 3:27
  • $\begingroup$ Currently magnetic south isn't even inside the antarctic circle. $\endgroup$ – Separatrix Oct 4 at 7:11
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    $\begingroup$ They did not use magnetic compasses in the antiquity and this did not stop them from establishing a trade network linking the Roman empire with China. And magnetic declination was known since the very first magnetic compasses. In Magellan's time, magnetic declination in the strait which bears his name was some 30 degrees... (Plus I don't understand the link between magnetic poles and sextants. Sextans have nothing to do with magnets and do not even have magnetic properties, given that they were traditionally made of bronze.) $\endgroup$ – AlexP Oct 4 at 18:55
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Yes, but it depends on your level of timekeeping technology.

Magnetic North and True North are not the same, nor are they even close when you get near the poles. The magnetic field of the Earth varies over time, and will even undergo complete reversals (where the North magnetic pole moves to the South true pole and vice versa).

The difference between Magnetic and True north is known as magnetic variation (or MAGVAR for short), and must be accounted for in navigational charts, which are published periodically.

Right now, the Earth is likely (there is much scientific debate about it) approaching another rapid reversal in its poles, and it is possible (though unlikely) in the next 100 years the North and South magnetic poles could change position.

During a pole reversal, it is possible for the magnetic field to develop multiple north and south poles all over the surface, or even for the field to collapse entirely. We haven't seen this yet, but there are a lot of variations in the Earth's magnetic field, and even a possible new South Magnetic pole forming at what is known as the "South Atlantic Anomaly" off the coast of Argentina.

Similarly, the North Magnetic pole is wandering very rapidly, by up to hundreds of kilometers per year. Because of this, anywhere above roughly 60 degrees of latitude, navigating strictly by magnetic compass without knowing the current magvar for your location is extremely unreliable (luckily we rarely use magnetic compasses these days).

So it is entirely possible for the scenario you describe to occur. In this case, magnetic navigation would be completely unreliable, but all isn't lost, depending on your technology.

As some have mentioned, a sextant allows one to navigate very accurately by the stars, but there is a snag: It requires an extremely accurate clock to do so. Sextants work by measuring the horizon angle to a certain star at a certain time. This can give you both your latitude and longitude using charts. The British Royal Observatory in Greenwich was established partially for the purpose of creating these charts, and was invaluable to international navigation during Victorian times. In fact, the calculations for determining your latitude and longitude are based on the longitude of the Royal Observatory (defined as 0), and the charts published by the Observatory are essentially the time that one can expect a given star to rise on a given date at the Observatory's location. By doing a little trigonometry and comparing the local times at your location for the same star (using a sextant and clock), you can nail down your position to the accuracy of your clock.

Since the accuracy of your location is directly tied to the reliability of your clock, a great deal of effort was put into advancing timekeeping technology, and arguably, these increasingly accurate clocks are what enabled Europe to expand their seagoing empires and create some of the most powerful navies in the world.

Perhaps appropriately, today the most accurate atomic clocks that generate the standard time for the world are generally managed by the most powerful navies in the world. A prime example of which being the the US Naval Observatory which provides the "Master Time" for the United States from dozens of atomic clocks, which themselves are a part of the International Atomic Time standard (which gives rise to Coordinated Universal Time).

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    $\begingroup$ Actually in a world like this you can get your coordinates without clock - since you already have your latitude and 2 directions, one to polar star and one to magnetic pole. This way was tried in reality as well, but on Earth magnetic and true poles are too close. $\endgroup$ – Vashu Oct 4 at 5:22
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    $\begingroup$ "[Celestial navigation] requires an extremely accurate clock": An accurate clock is not strictly needed; it is convenient, but is can be replaced by an almanach giving the forecasted position of the Moon. See Lunar distances method for details. $\endgroup$ – AlexP Oct 4 at 18:58
  • $\begingroup$ @AlexP The Lunar Distances Method requires very accurate almanacs, which themselves require a very accurate clock to produce. The only thing LDM has done is move the problem from the ship to the shore, and ultimately can result in errors of up to 15 nautical miles if everything goes perfectly. $\endgroup$ – stix Oct 9 at 21:13
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    $\begingroup$ @stix: On the shore, in a fixed observatory, you can use the stars as an extremely accurate clock. This is what they did before the invention of the mechanical clock. The stars move in circles at constant angular speed, and you can safely use their culminations to determine local time. $\endgroup$ – AlexP Oct 10 at 4:24
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Leaving aside the very serious issues of physics and assuming that by some means the poles could be made to point to the equator in a stable fashion then it would still be possible to navigate by sailing east or west instead of north or south. However the rate of rotation of the pole (or lack of) would be an issue and navigation would be very much less reliable. As an example assuming a navigator was on the equator:

If the poles were angled to the equator and allowed to rotate with the earth once per day navigation east or west (towards the rotational poles) would reach a reliability maximum twice per day when the north or South Pole was at 90 degrees from the point where the measurement was going to be made. It would be impossible when either pole was pointing at the navigator.

If the poles were angled to the equator and then fixed in space with respect to the sun it would in effect rotate once per year with respect to the earth. In that case the problem with navigation would be a seasonal issue rather than a daily issue. Navigators at higher or lower latitudes would also suffer problems especially at the axial poles (as the case on Earth anyway).

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