Editing Compass (section)

{{short description|Instrument used for navigation and orientation}}
{{About|the direction finding instrument used in navigation}}
{{pp|small=yes}}
[[File:2016 Busola.JPG|thumb|upright=1.5|alt=compass with a slit at cover and looking hole|A modern military compass, with included [[Sight (device)|sight device]] for aligning]]A '''compass''' is a device that shows the [[cardinal direction]]s used for [[navigation]] and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or [[compass rose]], which can pivot to align itself with [[North magnetic pole|magnetic north]]. Other methods may be used, including gyroscopes, [[magnetometer]]s, and [[GPS]] receivers.

Compasses often show angles in degrees: north corresponds to 0°, and the angles increase [[clockwise]], so east is 90°, south is 180°, and west is 270°. These numbers allow the compass to show [[azimuth]]s or [[bearing (angle)|bearings]] which are commonly stated in degrees. If local [[magnetic declination|variation]] between magnetic north and [[true north]] is known, then direction of magnetic north also gives direction of true north.

Among the [[Four Great Inventions]], the magnetic compass was first invented as a device for [[divination]] as early as the [[history of science and technology in China|Chinese]] [[Han dynasty]] (since c. 206 BC),<ref name="Li Shu-hua, p. 176">[[#Li|Li Shu-hua]], p. 176</ref><ref name="cambridge1" /> and later adopted for navigation by the [[Song dynasty]] Chinese during the 11th century.<ref name="Barbara M. Kreutz 367">[[#Kreutz|Kreutz]], p. 367</ref><ref name="needham volume 4 part 1 252"/><ref name="Li Shu-hua, p. 182f">[[#Li|Li Shu-hua]], p. 182f.</ref> The first usage of a compass recorded in [[Western Europe]] and the [[Islamic world]] occurred around 1190.<ref name="Barbara M. Kreutz 370">[[#Kreutz|Kreutz]], p. 370</ref><ref name=OEPST>{{Cite encyclopedia | publisher = Oxford University Press| isbn = 978-0-19-981257-8| editor= Ibrahim Kalin | last = Schmidl | first = Petra G. | title = Compass | encyclopedia = The Oxford Encyclopedia of Philosophy, Science, and Technology in Islam| date = 2014| pages=144–146}}</ref>

The magnetic compass is the most familiar compass type. It functions as a pointer to "[[Magnetic declination|magnetic north]]", the local magnetic meridian, because the [[magnetize]]d needle at its heart aligns itself with the horizontal component of the [[Earth's magnetic field]]. The [[magnetic field]] exerts a [[torque]] on the needle, pulling the North end or ''pole'' of the needle approximately toward the Earth's [[North magnetic pole]], and pulling the other toward the Earth's [[South magnetic pole]].<ref>The magnetic lines of force in the Earth's field do not accurately follow great circles around the planet, passing exactly over the magnetic poles. Therefore the needle of a compass only approximately points to the magnetic poles.</ref> The needle is mounted on a low-friction pivot point, in better compasses a [[jewel bearing]], so it can turn easily. When the compass is held level, the needle turns until, after a few seconds to allow oscillations to die out, it settles into its equilibrium orientation.

In navigation, directions on maps are usually expressed with reference to geographical or [[true north]], the direction toward the [[Geographical North Pole]], the rotation axis of the Earth. Depending on where the compass is located on the surface of the Earth the angle between [[true north]] and [[magnetic north]], called [[magnetic declination]] can vary widely with geographic location. The local magnetic declination is given on most maps, to allow the map to be oriented with a compass parallel to true north. The locations of the Earth's magnetic poles slowly change with time, which is referred to as [[geomagnetic secular variation]]. The effect of this means a map with the latest declination information should be used.<ref>{{cite web|url=http://www.rei.com/learn/expert-advice/compass-declination.html |title=Declination Adjustment on a Compass  |website=Rei.com |access-date=2015-06-06}}</ref> Some magnetic compasses include means to manually compensate for the magnetic declination, so that the compass shows true directions.