Chronometer, a timepiece that is exceptionally accurate. Traditionally, the term refers to the marine chronometer, a rugged mechanical instrument used at sea to keep time for navigational purposes. By measuring the position of a celestial body (with a device called a sextant), and by knowing the exact time of the measurement, a navigator can determine a ship's longitude from published tables. The typical marine chronometer is mounted on suspension devices called gimbals to keep it in a horizontal position and to reduce the effect of vibrations from the ship.

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