Magnetism of the Earth

William Gilbert's 1600 treatise De Magnete established that Earth itself is a giant magnet—explaining why compass needles point north and why magnetic declination varies with location. This was the first significant advance in magnetic understanding since the Chinese had discovered compass needles aligned with north over a thousand years earlier.

Gilbert worked experimentally, creating small spherical magnets called terrellas (little Earths) that reproduced compass behavior on a miniature scale. When he moved a compass needle across the terrella's surface, it dipped and deviated exactly as compasses did on Earth's surface. The correspondence was too precise to be coincidence; Earth must possess the same magnetic properties as his lodestone spheres.

The treatise demolished centuries of misconception. Medieval scholars had attributed compass behavior to celestial influences, magnetic mountains near the pole, or various mystical causes. Gilbert showed that terrestrial magnetism was a property of Earth itself, arising from the planet's interior.

The discovery of magnetic declination—the angle between true north and magnetic north—had practical consequences for navigation. Columbus had noted that his compass readings varied during his 1492 voyage. Gilbert's work explained why: Earth's magnetic field did not align perfectly with geographic poles, and the misalignment varied by location.

De Magnete also influenced broader scientific thinking. Gilbert proposed that magnetic force could act at a distance without material contact—an idea that would inform Newton's gravitational theory decades later. The suggestion that invisible forces could operate across empty space was philosophically radical.

Modern geophysics has confirmed Gilbert's basic insight while revealing complexities he could not have imagined. Earth's magnetic field arises from convection currents in its liquid iron core, reverses polarity irregularly over geological time, and protects the surface from solar radiation. Gilbert's terrellas could not model these dynamics, but his central claim—Earth is a magnet—was correct.