How Does a Compass Find North?

A compass works because Earth acts as a giant (if weak) magnet, with a magnetic field generated by convection currents in its liquid iron outer core. A freely suspended magnetic needle experiences a torque from this field and rotates until it aligns with the local field lines — the north-seeking pole pointing toward Earth magnetic north pole. Magnetic north does not exactly coincide with geographic (true) north — the angular difference, called magnetic declination, varies by location and changes over time as the magnetic field shifts. Currently, magnetic north is in the Canadian Arctic, about 500 km from the geographic pole, and is moving toward Siberia at about 55 km per year. The earliest compasses were lodestone spoons floating on water, used in China by the 2nd century BCE for feng shui; their use for navigation at sea appeared by the 11th century. Modern navigation uses GPS, but magnetic compasses remain essential backups on ships and aircraft.