Editing Gyrocompass (section)

== Operation ==
A [[gyroscope]], not to be confused with a gyrocompass, is a spinning wheel mounted on a set of [[gimbals]] so that its axis is free to orient itself in any way.<ref name=l /> 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 the Earth rotates, it appears to a stationary observer on Earth that a gyroscope's axis is completing a full rotation once every 24 hours.<ref group=note>Although the effect is not visible in the specific case when the gyroscope's axis is precisely parallel to the Earth's rotational axis.</ref> Such a rotating gyroscope is used for navigation in some cases, for example on aircraft, where it is known as  [[heading indicator]] or directional gyro, 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,<ref name=an /><ref name=l /> is some mechanism that results in an [[torque|application of torque]] whenever the compass's axis is not pointing north.

One method uses [[friction]] to apply the needed torque:<ref name="maritime.org">[http://www.maritime.org/fleetsub/elect/chap17.htm Gyrocompass, Auxiliary Gyrocompass, and Dead Reckoning Analyzing Indicator and Tracer Systems] {{webarchive|url=https://web.archive.org/web/20130601030840/http://www.maritime.org/fleetsub/elect/chap17.htm |date=2013-06-01 }}, San Francisco Maritime National Park Association.</ref> 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 (or true south) 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.<ref name=an /><ref name=l /> 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 [[Gyroscopic precession#Torque-induced|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 indicator|heading indicators or directional gyros]], which are not gyrocompasses and do not align themselves to north via precession, but are periodically aligned manually to magnetic north.<ref>NASA [http://asrs.arc.nasa.gov/publications/callback/cb_304.htm NASA Callback: Heading for Trouble] {{webarchive|url=https://web.archive.org/web/20110716165805/http://asrs.arc.nasa.gov/publications/callback/cb_304.htm |date=2011-07-16 }}, NASA Callback Safety Bulletin website, December 2005, No. 305. Retrieved August 29, 2010.</ref><ref>Bowditch, Nathaniel. [https://books.google.com/books?id=M54M8qjxLQMC&pg=PA93 American Practical Navigator] {{webarchive|url=https://web.archive.org/web/20170307022825/https://books.google.com/books?id=M54M8qjxLQMC&pg=PA93 |date=2017-03-07 }}, Paradise Cay Publications, 2002, pp.93-94, {{ISBN|978-0-939837-54-0}}.</ref>