The compass began as a fortune-telling device, not a navigation tool. Lodestone, naturally magnetized magnetite, fascinated the ancient Greeks, who credited Thales of Miletus with noticing its pull on iron, and the word magnet likely comes from Magnesia in Anatolia. India’s Sushruta Samhita describes using lodestone to draw metal arrowheads out of wounds. Around the Han dynasty, Chinese geomancers carved lodestone into a south-pointing spoon that settled on a bronze divination plate. In the 11th century, during the Song dynasty, the scholar Shen Kuo described magnetizing an ordinary iron needle by striking it with lodestone, aligning its magnetic domains and making compasses cheap to produce. Dry pivoting compasses appeared in the Islamic world and medieval Europe between the late 1100s and 1300s.

The needle points to magnetic north, hundreds of miles from true north, so navigators correct for declination, which varies by place and drifts over time as the poles wander, making old maps’ corrections wrong. Near the poles, inclination drags the needle toward the ground. Liquid-filled capsules damp the needle and resist pressure for divers, while steel ships created deviation, fixed by swinging the ship in harbor and by binnacles fitted with magnets, Flinders bars, and two large iron spheres. Gyrocompasses and GPS compasses now find true north without magnets, yet ships still carry magnetic compasses as legally required backups because they need no power and cannot be jammed.

• Orienteering baseplate compasses use clear plastic and rare earth magnets so the needle settles in a fraction of a second.
• Military compasses glow with tritium, which has a half-life of about 12 years.
• The US M1950 lensatic compass drops the liquid and damps its card with eddy currents in a copper cup.
• Banking and accelerating an airplane tilt the compass card and produce false headings.
• MRI machines, car engines, and iron-rich rocks can all pull a needle away from north.