Originally Posted by jact

(This is LadyM) The earth does not spin perfectly all the time. It has a slight wobble on it's axis as it performs it's orbit around the sun, sort of like a top that wobbles when it's not perfectly aligned (which the earth is not). It's a predictable wobble that has been noted and observed by astromoners since ancient times. That cycle is called precession (Known in astronomy as the Precession of the Equinoxes).. it's a roughly 21,000 year cycle that, depending on where the earth is in the precessional cycle, can dramatically affect climate (among other things).