Sentence:  For example, suppose the Moon to be placed in 10° of any sign: suppose Saturn to pass over that spot and attain to the 15th degree of the same sign, and there become stationary; that his <WSD>retrocession</WSD> carries him back to the 8th degree, ... 
Meaning of the word marked with <WSD> tags:  The act of retroceding; a going back.