Rotation

The rotation of a body is the movement (or turning) of that body around a line, known as the axis, which runs through the body and is perpendicular to the direction of rotational motion.

In astronomy, rotation is a commonly observed phenomenon. Stars, planets and similar bodies, and galaxies all rotate around their central axes.

One consequence of the rotation of a planet is the phenomenon of precession. As precession applies to any spiining object, a rotating body such as a planet qualifies. This has the overall effect of introducing a long-term "wobble" in the movement of the axis of that planet. For example, the tilt of the Earth's axis to its orbital plane (obliquity of the ecliptic) is currently 66.5 degrees, but this angle has slowly changed over time due to the action of precession. One result of this motion is that the direction of the North Pole and South Pole with respect to the background stars has changed over time, such that the star currently appearing over the North Pole (Polaris) did not so appear ten thousand years ago, and will not ten thousand years from now.