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In navigation or surveying, the chief device for direction finding on the Earth's surface. Compasses may operate on magnetic or gyroscopic (seegyroscope) principles or by determining the direction of the Sun or a star. The oldest and most familiar type is the magnetic compass, used in different forms in aircraft, ships, and land vehicles and by surveyors. Magnetic compasses work as they do because the Earth itself is a magnet with a north-south field (seegeomagnetic field) that causes freely moving magnets to align themselves with the field.
This entry comes from Encyclopædia Britannica Concise. For the full entry on compass, visit Britannica.com.