This precession is caused by the pull of the sun and the moon acting on the slightly nonspherical shape of the earth.    From Wordnik.com.  [TIME.com: Top Stories] Reference

Since a preferred direction of orientation can only exist for nonspherical, elongated objects, the self-organization of.    From Wordnik.com.  [Citizendium, the Citizens' Compendium - Recent changes [en]] Reference