Source: http://code7700.com/gyroscopes.htm
Apparent Precession. Because of the gyroscopic property of rigidity in space the spin axis of a universally mounted gyro points in a fixed direction. However, the earth is rotating, turns under the gyro and the spin axis appears to tilt. For example, imagine a gyroscope at the equator with the spin axis horizontal to the earth as shown [in the figure]. The earth turns in the direction of the arrow (counterclockwise) with an angular velocity of one revolution every 24 hours. To an observer on the earth, the spin axis appears to gradually tilt or drift. At the end of 3 hours, the spins axis has tilted 45掳 and after 6 hours the spin axis has tilted 90掳 to a vertical position. At the end of 12 hours the spin axis is again horizontal, and at the end of 24 hours it is back in its original position.
