A magnetic compass does not point at the North Star. It aligns with Earth’s magnetic field, a field generated by molten iron moving in the outer core. The needle is a small magnet. Opposite poles attract, so the north-seeking end of the needle points toward Earth’s magnetic north, which is not exactly the geographic pole and which wanders over years. Maps for navigation include a correction called declination, the angle between true north and magnetic north, and that angle depends on where you stand and when you measure.

The field is not a simple bar magnet. It has irregularities, and it reverses polarity on geologic timescales, a fact recorded in cooling lava as iron minerals freeze in the direction of the field at that moment. Those reversals helped prove seafloor spreading. For a hiker, the practical issue is simpler: keep the compass away from speakers, phones, car hoods, and iron fences, all of which can bend the local field and lie.

At high latitudes the field lines dip steeply, and compasses become unreliable. Near the magnetic poles they may spin or point down. For most of the inhabited Earth, though, a cheap needle in oil is still a trustworthy partner to a map, a reminder that the planet itself is a weak, restless magnet wrapped around a spinning metal heart.
