The earth has a slight nodding motion, over and above the 26000-year precession, which has a period of 18.6 years and an amplitude of 9.2 arc seconds. This nutation is caused by the 5 degree difference between the the moon and earth's orbital planes and hence a resultant gravitational tug on the earth's axis. (The orbits of the earth around the sun and the moon around the earth don't form a perfectly flat disc.)