At least three astronomic cycles affect climate on Earth: the varying eccentricity of Earth's orbit around the Sun, the varying obliquity of the Earth's axis of rotation, and the precession of the equinoxes. The sum of these waveforms are known as the Milankovitch cycles, which is a theory about the correlation of glacial/interglacial periods with those astronomic cycles. I'm not sure what the effect of these cycles would have been before the current ice age, but in this ice age it really does seem like glacial and interglacial periods correlate with the Milankovitch cycles. Specifically, interglacials correlate with maximum insolation during the summer in the Northern hemisphere, specifically around the 65th parallel, while glacial periods coincide with minimum insolation in the summer of the Northern hemisphere -- not every maximum/minimum leads to a state transition, of course.