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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.
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 munk
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
External Links, Resources, and References
- Amir Aczel, The Riddle of the Compass: The Invention that Changed the World, ISBN 0156007533
- Science Friday, "The Riddle of the Compass " (interview with Amir Aczel,
first broadcast on NPR on May 31, 2002.
- Paul J. Gans, The Medieval Technology Pages: Compass
- Frederic Lane, "The Economic Meaning of the Invention of the Compass", American Historial Review, vol. 68, pp.
605-617 (1963)
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