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This article is about the navigational tool. For other meanings, see Compass (disambiguation)

A compass (or mariner's compass) is navigational instrument for finding directions. It consists of a magnetised pointer free to align itself accurately with Earth's magnetic field. A compass provides a known reference direction which is of great assistance in navigation. The cardinal points are north, south, east and west. A compass can be used in conjunction with a clock and a sextant to provide a very accurate navigation capability. This device greatly improved maritime trade by making travel safer and more efficient.

Navigational compass
mariner's compass

A compass can be any magnetic device using a needle to indicate the direction of the magnetic north of a planet's magnetosphere. Any instrument with a magnetized bar or needle turning freely upon a pivot and pointing in a northerly and southerly direction can be considered a compass. A compass dial is a small pocket compass with a sundial. A variation compass is a specific instrument of a delicate type of construction. It is used by observing variations of the needle. A gyrocompass can also be used to ascertain true North.

History of the navigational compass

Compasses were initially used in mysticism in ancient China. The first known use of Earth's magnetic field in this way occurred in ancient China as a spectacle. Arrows were cast similarly to dice. These magnetised arrows aligned themselves pointing north, impressing the audience. Curiously, it took some time for this trick to get used by the Chinese for naval navigation, but by the 11th or early 12th century it had become common.

Knowledge of the compass moved overland to Europe sometime later in the 12th century. Arab mariners apparently learned of it from the Europeans, adopting its use in the first half of the 13th century. About 1358, there is a story about a English monk under the name Nicholas of Lynne, who served as a navigator due to his competence and knowledge in the "magnetic compass". (See Inventio Fortunata.)

Prior to the introduction of the compass, wayfinding at sea was primarily done via celestial navigation, supplemented in some places by the use of soundings. Difficulties arose where the sea is too deep for soundings and conditions are continually overcast or foggy. Thus the compass was not of the same utility everywhere. For example, the Arabs could generally rely on clear skies in navigating the Persian Gulf and the Indian Ocean (as well as the predictable nature of the monsoons). This may explain in part their relatively late adoption of the compass. Mariners in the relatively shallow Baltic made extensive use of soundings.

In the Mediterranean, however, the practice from ancient times had been to curtail sea travel between October and April, due in part to the lack of dependable clear skies during the Mediterrean winter (and much the sea is too deep for soundings). With improvements in dead reckoning methods, and the development of better charts, this changed during the second half the 13th century. By around 1290 the sailing season could start in late January or February, and end in December. The additional few months were of considerable economic importance; it enabled Venetian convoys, for instance, to make 2 round trips a year to the eastern Mediterranean, instead of 1.

Around the same time traffic between the Mediterranean and northern Europe increased, and one factor may be that the compass made traversal of the Bay of Biscay safer and easier.

Construction of a simple compass

A magnetic rod is required. This can be created by aligning an iron or steel rod with Earth's magnetic field and then tempering or striking it. However, this method produces only a weak magnet so other methods are preferred. This magnetised rod (or magnetic needle) is then placed on a low friction surface to allow it to freely pivot to align itself with the magnetic field. It is then labeled so the user can distinguish the north-pointing from the south-pointing end; in modern convention the north end is typically marked in some way, often by being painted red.

Modern navigational compasses

Modern navigational compasses hold a magnetized needle inside a fluid-filled capsule; the fluid causes the needle to stop quickly rather than oscillate back and forth around magnetic north. Other features common on modern handheld compasses are a baseplate with rulings for measuring distances on maps, a rotating bezel for measuring bearings of distant objects, and a sighting mirror that lets the user see both the compass needle and a distant object at the same time. Many modern navigational compasses also include an adjustment for magnetic declination, the offset between magnetic north and true north, which varies from place to place on the Earth's surface.

Mariner's compasses can have two or more magnetic needles permanently attached to a compass card. These move freely on a pivot. A mariner reads this for a reference box mark that representd the ship's headings. The card is divided into thirty-two points (known as rhumbs). The glass-covered box (or bowl) contains a suspended gimbal within a binnacle. This preserves the horizontal position.

Points of the compass

Main article: Boxing the compass

The mariner's compass card is divided into thirty-two equally spaced points. Four of these - east, west, north, and south - are the cardinal points, and the names of the others are derived from these.

See also: Azimuth compass, Beam compass, coordinates, fluxgate compass, gyrocompass, Gyrosin compass, gyrostatic compass, inertial navigation system, radio compass, radio direction finder

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