Sphere rotating around one of its diameters
Polar motion in arc-seconds as function of time in days (0.1 arcsec ≈ 3 meters).
An example of rotation. Each part of the worm drive—both the worm and the worm gear—is rotating on its own axis.
Figure 2. Rotation vector m of the annual component of polar motion as function of year. Numbers and tick marks indicate the beginning of each calendar month. The dash-dotted line is in the direction of the major axis. The line in the direction of the minor axis is the location of the excitation function vs. time of year. ( on the Earth's surface at the poles)

Special case of rotational motion.

- Rotation around a fixed axis

If the earth were perfectly symmetrical and rigid, M would remain aligned with its axis of symmetry, which would also be its axis of rotation.

- Polar motion
Sphere rotating around one of its diameters

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