Sentence:  This and other <WSD>fears</WSD> 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 motions . 
Meaning of the word marked with <WSD> tags:  an emotion experienced in anticipation of some specific pain or danger (usually accompanied by a desire to flee or fight)