Pöörlemiskõikumine
Pöörlemiskõikumine, also known as nutation, is a slow, conical oscillation of the Earth's axis of rotation. It is similar to the wobbling motion of a spinning top as it slows down. This phenomenon is caused by the gravitational influence of the Moon and the Sun on the Earth's equatorial bulge. The Earth is not a perfect sphere; it bulges at the equator due to its rotation. The gravitational pull of the Moon and Sun on this bulge creates a torque that causes the Earth's axis to precess, a slow circular motion, and also to nod or wobble in a conical path.
The period of the principal component of nutation, known as the nearly 18.6-year nutation, is approximately