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Related to apparent drift: latitude nut
An error in the directional gyro resulting from the effect of the earth's rotation and longitudinal position. At the equator, the rotation of the earth causes no movement of the observer about the directional gyro vertical axis, and there is thus no drift. At the poles, the rotation of the earth causes a movement of the observer about the vertical axis of 360° in 24 h (i.e., 15°/h). At other latitudes, its value is equal to the product of the earth's angular velocity and the sine of the latitude. This drift is compensated for by applying a constant torque of proper magnitude (in computer-driven systems) or periodic corrections.