What is an obliquity cycle?
The axis of the Earth is currently 23.5 degrees. Over a 41,000 year cycle the axis angle of the Earth varies from 22.5-24.5 (don't quote me on those decimals). This is due to the fact that the sun is not the only large body exerting gravity on the Earth. The other planets all have an effect, and the current orbit around the sun is the end result, but all those forces applies torque to the Earth that gives it it's oscillation.