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A gyrocompass is a type of non-magnetic compass which is based on a fast-spinning disc and rotation of the Earth (or another planetary body if used elsewhere in the universe) to automatically find geographical direction. Although one important component of a gyrocompass is a gyroscope, these are not the same devices; a gyrocompass is built to use the effect of gyroscopic precession, which is a distinctive aspect of the general gyroscopic effect. Gyrocompasses are widely used for navigation on ships, because they have two significant advantages over magnetic compasses:〔 * they find true north as determined by Earth's rotation, which is different from, and navigationally more useful than, ''magnetic'' north, and * they are unaffected by ferromagnetic materials, such as ship's steel hull, which change the magnetic field == Operation == A gyroscope, not to be confused with gyrocompass, is a spinning wheel mounted on a gimbal so that the wheel's axis is free to orient itself in any way.〔 When it is spun up to speed with its axis pointing in some direction, due to the law of conservation of angular momentum, such a wheel will normally maintain its original orientation to a fixed point in outer space (not to a fixed point on Earth). Since our planet rotates, it appears to a stationary observer on Earth that a gyroscope's axis is completing a full rotation once every 24 hours.〔Although the effect is not visible in a specific case when the gyroscope's axis is precisely parallel to the Earth's rotational axis.〕 Such a rotating gyroscope is used for navigation in some cases, for example on aircraft, where it is known as heading indicator, but cannot ordinarily be used for long-term marine navigation. The crucial additional ingredient needed to turn a gyroscope into a gyrocompass, so it would automatically position to true north,〔〔 is some mechanism that results in an application of torque whenever the compass's axis is not pointing north. One method uses friction to apply the needed torque:〔(Gyrocompass, Auxiliary Gyrocompass, and Dead Reckoning Analyzing Indicator and Tracer Systems ), San Francisco Maritime National Park Association.〕 the gyroscope in a gyrocompass is not completely free to reorient itself; if for instance a device connected to the axis is immersed in a viscous fluid, then that fluid will resist reorientation of the axis. This friction force caused by the fluid results in a torque acting on the axis, causing the axis to turn in a direction orthogonal to the torque (that is, to precess) along a line of longitude. Once the axis points toward the celestial pole, it will appear to be stationary and won't experience any more frictional forces. This is because true north is the only direction for which the gyroscope can remain on the surface of the earth and not be required to change. This axis orientation is considered to be a point of minimum potential energy. Another, more practical, method is to use weights to force the axis of the compass to remain horizontal (perpendicular to the direction of the center of the Earth), but otherwise allow it to rotate freely within the horizontal plane.〔〔 In this case, gravity will apply a torque forcing the compass's axis toward true north. Because the weights will confine the compass's axis to be horizontal with respect to the Earth's surface, the axis can never align with the Earth's axis (except on the Equator) and must realign itself as the Earth rotates. But with respect to the Earth's surface, the compass will appear to be stationary and pointing along the Earth's surface toward the true North Pole. Since the gyrocompass's north-seeking function depends on the rotation around the axis of the Earth that causes torque-induced gyroscopic precession, it will not orient itself correctly to true north if it is moved very fast in an east to west direction, thus negating the Earth's rotation. However, aircraft commonly use heading indicators or directional gyros, which are not gyrocompasses and do not position themselves to north via precession, but are periodically aligned manually to magnetic north.〔NASA (NASA Callback: Heading for Trouble ), NASA Callback Safety Bulletin website, December 2005, No. 305. Retrieved August 29, 2010.〕〔Bowditch, Nathaniel. (American Practical Navigator ), Paradise Cay Publications, 2002, pp.93-94, ISBN 978-0-939837-54-0.〕 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「Gyrocompass」の詳細全文を読む スポンサード リンク
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