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A compass is an instrument used for navigation and orientation that shows direction relative to the geographic ''cardinal directions'', or "points". Usually, a diagram called a compass rose, shows the directions north, south, east, and west as abbreviated initials marked on the compass. When the compass is used, the rose can be aligned with the corresponding geographic directions, so, for example, the "N" mark on the rose really points to the north. Frequently, in addition to the rose or sometimes instead of it, angle markings in degrees are shown on the compass. North corresponds to zero degrees, and the angles increase clockwise, so east is 90 degrees, south is 180, and west is 270. These numbers allow the compass to show azimuths or bearings, which are commonly stated in this notation. The magnetic compass was first invented as a device for divination as early as the Chinese Han Dynasty (since about 206 BC),〔Li Shu-hua, p. 176〕〔 and later adopted for navigation by the Song Dynasty Chinese during the 11th century.〔〔Needham, p. 252〕〔Li Shu-hua, p. 182f.〕 The use of a compass is recorded in Western Europe and in Persia around the early 13th century.〔 == Magnetic compass == The magnetic compass is the most familiar compass type. It functions as a pointer to "magnetic north", the local magnetic meridian, because the magnetized 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 one end or ''pole'' of the needle approximately toward the Earth's North magnetic pole, and pulling the other toward the South magnetic pole.〔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.〕 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 Earth's magnetic fields are constantly changing which is referred to as geomagnetic secular variation. The effect of this means a map with the latest declination information should be used.〔(【引用サイトリンク】title=Declination Adjustment on a Compass – REI Expert Advice )〕 Some magnetic compasses include means to manually compensate for the magnetic declination, so that the compass shows true directions. == History == The first compasses in Han dynasty China were made of lodestone, a naturally magnetized ore of iron. The compass was later used for navigation by the Song Dynasty.〔 Later compasses were made of iron needles, magnetized by striking them with a lodestone. Dry compasses begin appearing around 1300 in Medieval Europe.〔 This was supplanted in the early 20th century by the liquid-filled magnetic compass.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「compass」の詳細全文を読む スポンサード リンク
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