Sentence:  This and other fears of the solar system have disappeared gradually , first , with the Ptolemaic system and its built-in concept of periodicity and then , more firmly , with the Newtonian innovation of an universal force that could account quantitatively for both terrestrial and celestial <WSD>motions</WSD> . 
Meaning of the word marked with <WSD> tags:  a natural event that involves a change in the position or location of something