THE epic voyages of Captain James Cook and other mariners tell the story of the Earth’s magnetic field. It seems that the gradual weakening is a relatively recent phenomenon, so the Earth may not be on track for a reversal of the north and south poles after all.
It was common practice for captains to use the magnetic field to calibrate their ship’s compasses relative to true north. “Their lives depended on it,” says David Gubbins, an expert in geomagnetism at the University of Leeds, UK. Mariners also sometimes measured the steepness at which magnetic field lines entered the Earth’s…
