I’ve never fully understood how compasses work, and how the magnetic North Pole can affect them. Like how aren’t other things affected by it? And apparently the magnetic North Pole also moves? Please help me make it make sense!
In: Physics
Magnetism is a weird, twisty effect.
The general rule is that if you have something that has a magnetic field around it (like the needle in a compass), and you put it inside another magnetic field (from some other object, like the Earth), the thing will twist or shift so that its magnetic field lines up as much as possible with the other one.
You might have seen [diagrams like these](https://en.wikipedia.org/wiki/File:VFPt_cylindrical_tightly-wound_coil-and-bar-magnet-comparison_stacked.svg) for showing what magnetic fields look like; magnetic field lines go in closed loops around magnetic things (the lines continue through the magnet).
So you have some of these lines around your magnetic needle, and some around the Earth, and so the needle will shift to line up with the Earth’s field (as it is much easier for the needle to shift than for the Earth do to so!).
Now it is important to note that the needle *doesn’t point towards the magnetic North pole*! It lines up with the local magnetic field. The Earth’s magnetic field [is a giant complicated mess](https://en.wikipedia.org/wiki/File:Geodynamo_Between_Reversals.gif) and doesn’t flow smoothly. For example, if you tried to use a magnetic compass in Moscow, it would point about 15 degrees off true North. In Japan it is around 10 degrees off. Parts of Brazil get over 22 degrees out. Wikipedia has [this helpful graphic](https://upload.wikimedia.org/wikipedia/commons/a/a1/World_Magnetic_Declination_2020.pdf) (from 2020) showing the difference between the local magnetic field and actual North. On the green “0” line a compass will point towards true north. Everywhere else it will be out a bit.
The Earth’s magnetic field is caused by the materials inside the Earth, and they shift around a bit. Which means the Earth’s magnetic field also shifts. [This neat little animation](https://en.wikipedia.org/wiki/File:Earth_Magnetic_Field_Declination_from_1590_to_1990.gif) shows how the last diagram has changed from 1590 to 1990. Slight changes in the Earth’s internal structure lead to changes in the field.
Finally, let’s talk about a magnetic pole. A magnetic pole is the point on an object where the magnetic field goes straight down into the object or straight up out of the object. But remember, magnetic field lines go in loops; they don’t start or end. So all pole is is the point where the field happens to cross the boundary of the object, while going up (or down). So the Earth’s north magnetic pole (which is actually a magnetic south pole) is the point where the magnetic field goes straight down into the Earth. If you stood there with a compass the needle would try to point down.
As the Earth’s magnetic field shifts, that point will also shift.
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