By the late 1500s, ships’ captains already relied on magnetic compasses to maintain their course across the oceans. Yet no one knew how they worked. Some thought the compass needle was attracted to the North Star, others that it was drawn to magnetic mountains in the Arctic. It was English physician William Gilbert who discovered that Earth itself is magnetic.

Stronger reasons are obtained from sure experiments and demonstrated arguments than from probable conjectures (推测) and the opinions of philosophical speculators.

Gilbert’s breakthrough came not from a flash of inspiration, but from 17 years of careful experiment. He learned all he could from ships’ captains and compass makers, and then he made a model globe, or “terrella,” out of the magnetic rock lodestone and tested compass needles against it. The needles reacted around the terrella just as ships’ compasses did on a larger scale—showing the same patterns of declination (pointing slightly away from true north at the geographic pole, which differs from magnetic north) and inclination (tilting down from the horizontal toward the globe).

Gilbert concluded, rightly, that the entire planet is a magnet and has a core of iron. He published his ideas in the book De Magnete (On the Magnet) in 1600, causing a sensation. Johannes Kepler and Galileo, in particular, were inspired by his suggestion that Earth is not fixed to rotating celestial spheres, as most people still thought, but is made to spin by the invisible force of its own magnetism.