Why a compass points north

A compass points north because its magnetized needle lines up with the Earth's magnetic field. The needle is free to turn, and the field twists the north-seeking end toward magnetic north.

It is not aiming at the geographic North Pole, and it is not pointing at the North Star. Those are different targets, and a chart has to allow for the angle between magnetic north and true north.

The mechanism is more particular than the slogan. The Earth carries a large, messy magnetic field, often pictured as a bar magnet buried in the planet, with lines that leave one polar region and enter the other. A small magnet placed in that field feels a twist until it lies along the local line.

Follow it one step further. The end of the needle we call north is simply the end that seeks magnetic north. In the language of poles, that means the magnetic north of the Earth behaves as the opposite kind of pole, because opposite poles attract.

The case worth keeping in mind is specific. A hiker takes a bearing off the needle and then turns the map until map north agrees with it. Where the declination is large, that bearing is not the same line as a meridian of longitude drawn on the map.

That case has a boundary. The bar-magnet picture fails where the field plunges steeply into the ground, close to the magnetic poles. A needle allowed only to spin flat is then trying to follow a line that is mostly vertical, and it turns sluggish or wrong.