Nutation

[2] If a top is set at a tilt on a horizontal surface and spun rapidly, its rotational axis starts precessing about the vertical.

After a short interval, the top settles into a motion in which each point on its rotation axis follows a circular path.

[5] If the top has mass M and its center of mass is at a distance l from the pivot point, its gravitational potential relative to the plane of the support is In a coordinate system where the z axis is the axis of symmetry, the top has angular velocities ω1, ω2, ω3 and moments of inertia I1, I2, I3 about the x, y, and z axes.

The largest component of Earth's nutation has a period of 18.6 years, the same as that of the precession of the Moon's orbital nodes.

[citation needed] In the theory, parameters are adjusted in a more or less ad hoc method to obtain the best fit to data.

Simple rigid body dynamics do not give the best theory; one has to account for deformations of the Earth, including mantle inelasticity and changes in the core–mantle boundary.

[7] The principal term of nutation is due to the regression of the Moon's nodal line and has the same period of 6798 days (18.61 years).

In Star Trek: The Next Generation, rapidly 'cycling' or 'changing' the 'shield nutation' is frequently mentioned as a means by which to delay the antagonist in their efforts to break through the defences and pillage the Enterprise or other spacecraft.