Gerardus Mercator’s interest in terrestrial magnetism made him a forerunner in this area. Noting that the magnetic north pole did not correspond to the geographic one, he posited that there might exist a Magnetic Island, Magnetu[m] insula, lying near the North Pole. He may have borrowed this idea from the Carta marina of the Swedish writer Olaus Magnus. Was he hoping to use it as a simplified solution to the longitude problem ?

According to historians it was between 1538 and 1541 that Mercator anticipated the crucial importance of terrestrial magnetism for maritime navigation. This realisation represented a huge step forward in this area and prompted him to situate the magnetic north pole on the Earth and leave the geographic north pole in the heavens, on the world’s rotational axis.

As we can see in the arctic region of the terrestrial globe’s gores, Mercator clearly placed a rocky magnetic island at a point on the calotte that is separate from the geographic north pole. This enabled him to allow for the angle of magnetic declination and thus explain why the north indicated by the stars and the direction given by a compass differed.