= Gyroscope
{wiki}
A <gyroscope> is a rotating instrument whose rapidly spinning, nearly symmetric rotor provides a direction reference through its <angular momentum>. In an <inertial frame>, negligible external <torque> approximately preserves that direction by <conservation of angular momentum>. A transverse <torque> turns the <angular momentum> instead: the surprising sideways motion of a supported spinning wheel is <precession>. An ideal freely falling test gyroscope in <general relativity> has its spin undergo <parallel transport> along its <timelike geodesic>. Comparing that direction with distant reference axes measures <Lense-Thirring precession>; the direction need not remain fixed in arbitrary rotating coordinates.
A <gyroscope direction reference> is useful precisely because its direction can persist while its housing turns. A <spinning-wheel precession experiment> deliberately supplies gravitational <torque> to reveal how that persistence fails. The <gyroscope vector geometry> explains the direction, and <slow steady gyroscope precession> gives a numerical prediction with a check on its assumptions.
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