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A compass, magnetic compass or mariner's compass is a navigational instrument for determining direction relative to the earth's magnetic poles. It consists of a magnetized pointer (usually marked on the North end) free to align itself with Earth's magnetic field. The face of the compass generally highlights the cardinal points of north, south, east and west. The compass greatly improved the safety and efficiency of travel, especially ocean travel. A compass can be used to calculate heading, used with a sextant to calculate latitude, and with a marine chronometer to calculate longitude. It thus provides a much improved navigational capability that has only been partially supplanted by modern devices such as the gyrocompass and the Global Positioning System (GPS).[1] A compass is any magnetically sensitive device able to indicate the direction of the magnetic north of a planet's magnetosphere. Often compasses are built as a stand alone sealed instrument with a magnetized bar or needle turning freely upon a pivot, or moving in a fluid, thus able to point in a northerly and southerly direction. An early compass was invented in ancient China before 1044. The dry compass was invented in medieval Europe around 1300.[2] This was supplanted in the early 20th century by the liquid-filled magnetic compass.[3] Other, more accurate, devices have been invented for determining north that do not depend on the Earth's magnetic field for operation (known in such cases as true north, as opposed to magnetic north). A gyrocompass or astrocompass can be used to find true north, while being unaffected by stray magnetic fields, nearby electrical power circuits or nearby masses of ferrous metals. A recent development is the electronic compass, or Fibre optic gyrocompass, which detects the magnetic directions without potentially fallible moving parts. This device frequently appears as an optional subsystem built into GPS receivers. Prior to the introduction of the compass, position, destination, and direction at sea was primarily determined by the sighting of landmarks, supplemented with the observation of the position of celestial bodies. Ancient mariners often kept within sight of land. The invention of the compass enabled the determination of heading when the sky was overcast or foggy. And, when the sun or other known celestial bodies could be observed, it enabled the calculation of latitude. This enabled mariners to navigate safely far from land, contributing to the Age of Discovery.
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