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 <WSD>force</WSD> that could account quantitatively for both terrestrial and celestial motions . 
Meaning of the word marked with <WSD> tags:  (physics) the influence that produces a change in a physical quantity