@Genesis

Huh, I did not know that.

Now I do.

Kinda makes sense once I think about it.

Does this mean, even if I'm not switching regions, that a "global needle" compass will exhibit less of that annoying sticking behavior?

@Genesis blast from the past!

You can hold them at an angle. Two or three decades ago, all(?) the expensive ones worked globally to some extent without adjustment, though the really good ones are stationary globes like on ships.

@Genesis

Always nice to learn new things 😌

@Genesis That's a great thing to learn!

Only tangentially related, have you heard about the triple north alignment?

https://www.ordnancesurvey.co.uk/newsroom/news/three-norths-align-great-britain

@Genesis from my understanding, it's not the case that the magnetic field point through the earth to sun directly at the north pole, the fields are curved around the planet. It is true, however, that towards the poles, the vertical component of the fields are very strong. At the poles you have only a vertical component. But usually you won't find an issue until you're into the polar circles or closer to the poles. This, at least, is the case for aviation compasses.

Regarding the precision, a compass is stable over time but any given instant measurement can be off because of vibration by way more than a couple of degrees. In some areas, local geographic or geologic features can cause a disturbance in the local magnetic field. These can have up to 3 or 4° of variance, and they change over time. Just as the magnetic north shifts position over time as well.

That is in part why aviation maps are re printed every year. They not only allow changes to air routes and areas, but also local magnetic field alignments.

@u0421793 That's how I noticed that the storage company that had all my possessions for a couple of months earlier this year had reversed the magnetization of my compass needle - the weighting now doubles the dip instead of cancelling it.

(I have an ancestor called Robert Norman who lived at the same time as the Robert Norman who wrote _The Newe Attractive_, but no clarity as to whether they're the same person.)

@Genesis That seems like a poor explanation in that text. In high northern latitudes the north side of a compass needle is pulled downward so the south side of the needle is weighted to counteract it. In the far southern hemisphere the south side of the needle is pulled down (not the north side), so the north side is weighted.

A suspended, gimballed or floating compass can counteract this (usually found in ships or aircraft) but depending upon the mechanism it may still need a correction factor or adjustment at high (positive or negative) latitude. It also usually gets an offset during turns as well.

@Genesis Huh. There's presumably something a little more subtle than "red end points at magnetic North", because presumably "not red end points at magnetic South" is equally valid.

I suspect it's more that it tries to follow the Earth's magnetic field, that sticks out a bunch at the poles and TIL from the other replies also varies as you go round the Earth, too.

@Genesis

Batteries die magnetism doesn't

@Genesis 🤔 I am aware of the magnetic field being different around the world, and in fact constantly changing, which is why charts have magnetic divination for the area and year (f.e. add 10 + 1 per year after 2015) printed on them.
Additionally we need to adjust for vessel.

First time I read that the magnet in the compass itself also have to be different. I assume all the nautical ones will be global by default.

@Genesis Mind blown.

Back in the 90s I worked for a computer company. We got a huge batch of (CRT) monitors all with the same weird fault, the picture was askew. Like, rotated a few degrees off square.

Turned out, they were intended for the Southern hemisphere markets. The magnetic plates that deflect the beam had to be calibrated differently.

@Genesis yes, this is the "dip" problem. There is, somewhere, a model sphere with short magnetic needles in the outside, which John Dee was said to have used to demonstrate this to Queen Elizabeth. I append images from Gilbert's terrella 'Earth' magnet, published in 1600.

Those of us who worked in surveying used to use a dip circle, which is pivoted in the vertical plane.