Compasses, at their simplest, are magnets suspended in such a way that their north pole points toward Earth's north pole. Larger compasses were made of a single piece of lodestone hung on a thin rope. Smaller ones are magnetized iron or steel, either balanced on a small point or floated in oil or water.
Compasses are most reliable near the Earth's equator. Earth's magnetic field is not perfectly aligned with the planet, and geomagnetic north is located about 10 degrees from the physical north pole. Local differences in the strength and direction of the magnetic field can also cause issues with navigation.
11th century to modern times.
- Compass at the New World Encyclopedia.
- GIS Geography on the difference between Magnetic North and Geographic North
- A Telltale Compass properly mounted on the ceiling, from the Smithsonian.
- Pages on Compasses and Earth's Magnetic Field on Wikipedia.