Compass

A compass is a device that shows the <a href="/facts/Cardinal_direction/rbcPTkbJ">cardinal directions</a> used for <a href="/facts/Navigation/sGGAJ0AQ">navigation</a> and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or <a href="/facts/Compass_rose/alXJDuDj">compass rose</a>, which can pivot to align itself with <a href="/facts/North_magnetic_pole/hyFcL5yf">magnetic north</a>. Other methods may be used, including gyroscopes, <a href="/facts/Magnetometer/dqX9g1og">magnetometers</a>, and <a href="/facts/GPS/DKVLnzF3">GPS</a> receivers.
Compasses often show angles in degrees: north corresponds to 0°, and the angles increase <a href="/facts/Clockwise/3Npe4Swe">clockwise</a>, so east is 90°, south is 180°, and west is 270°. These numbers allow the compass to show <a href="/facts/Azimuth/kEyt9nJ3">azimuths</a> or <a href="/facts/Bearing_(angle)/rE84IorW">bearings</a> which are commonly stated in degrees. If local <a href="/facts/Magnetic_declination/GdB3stq3">variation</a> between magnetic north and <a href="/facts/True_north/276EMRXX">true north</a> is known, then direction of magnetic north also gives direction of true north.
Among the <a href="/facts/Four_Great_Inventions/gsYdBBDj">Four Great Inventions</a>, the magnetic compass was first invented as a device for <a href="/facts/Divination/b8gLkFfv">divination</a> as early as the <a href="/facts/History_of_science_and_technology_in_China/JlA3afbj">Chinese</a> <a href="/facts/Han_dynasty/hanfxS9D">Han dynasty</a> (since c. 206 BC), and later adopted for navigation by the <a href="/facts/Song_dynasty/FxV5bXS5">Song dynasty</a> Chinese during the 11th century. The first usage of a compass recorded in <a href="/facts/Western_Europe/YmWlV59w">Western Europe</a> and the <a href="/facts/Islamic_world/n9vVGogr">Islamic world</a> occurred around 1190.
The magnetic compass is the most familiar compass type. It functions as a pointer to "<a href="/facts/Magnetic_declination/GdB3stq3">magnetic north</a>", the local magnetic meridian, because the <a href="/facts/Magnetize/eo75vApl">magnetized</a> needle at its heart aligns itself with the horizontal component of the <a href="/facts/Earth%2527s_magnetic_field/73G36wmx">Earth's magnetic field</a>. The <a href="/facts/Magnetic_field/Ta8Ev588">magnetic field</a> exerts a <a href="/facts/Torque/KcrcvNtV">torque</a> on the needle, pulling the North end or pole of the needle approximately toward the Earth's <a href="/facts/North_magnetic_pole/hyFcL5yf">North magnetic pole</a>, and pulling the other toward the Earth's <a href="/facts/South_magnetic_pole/E5VHpUqE">South magnetic pole</a>. The needle is mounted on a low-friction pivot point, in better compasses a <a href="/facts/Jewel_bearing/jFFQse5C">jewel bearing</a>, 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 <a href="/facts/True_north/276EMRXX">true north</a>, the direction toward the <a href="/facts/Geographical_North_Pole/gjfItQJH">Geographical North Pole</a>, the rotation axis of the Earth. Depending on where the compass is located on the surface of the Earth the angle between <a href="/facts/True_north/276EMRXX">true north</a> and <a href="/facts/Magnetic_north/hyFcL5yf">magnetic north</a>, called <a href="/facts/Magnetic_declination/GdB3stq3">magnetic declination</a> 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 <a href="/facts/Geomagnetic_secular_variation/kr2IQPV7">geomagnetic secular variation</a>. The effect of this means a map with the latest declination information should be used. Some magnetic compasses include means to manually compensate for the magnetic declination, so that the compass shows true directions.

Compass open-in-new

Compass