If my memory serves me right this occurs from the natural 'wobbling' of the Earth on it's axis like a top. There's a 24,000 year cycle to this if I remember correctly.
Exactly, that is one component. I didn't get too technical, because any astrologer would not understand. Astrology is quackery from a scientific point of view. Especially when the actual data they are giving you(signs and associated slivers of time) are WRONG!!!!