There is a third, more subtle cycle that takes place over about 25,800 years. It concerns the way the Earth's rotational axis wobbles. This is not the same thing as the way its obliquity changes—the two motions are at right angles to each other.
Earth behaves like a spinning top, and a spinning top has certain properties. As long as it maintains a high enough angular velocity, or rotational speed, and it experiences no outside force, the top is stable. It will remain upright, spinning on its pointed end. If something nudges it, however, the top will begin to wobble. If you imagine its rotational axis extended upward from the top, this will mark out a cone.
This is a property of gyroscopes, and the top is a gyroscope. You can see the effect more clearly with a toy gyroscope. It happens because the mass of the spinning gyroscope or top is concentrated around the edge, or equator. Earth also has more mass around its equator, so it is not quite spherical. The bulge is due to the gravitational attraction of the Moon and Sun and it means that the Earth behaves like a gyroscope.
Gyroscopes experience precession. This means that if a force is applied to a gyroscope, the gyroscope will move not in the direction of the applied force, but at right angles to it in the direction of rotation. Gravitational forces exerted by the Moon and Sun—the same forces that produce the tides—act at right angles to the rotational axis and the Earth responds like a gyroscope. It moves at right angles to the applied force and this makes it wobble like a top.
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