Sentence:  The Poynting-Robertson effect ( Robertson , 1937 ; Wyatt and Whipple , 1950 ) , which is a retardation of the orbital <WSD>motion</WSD> of particles by the relativistic aberration of the repulsive force of the impinging solar radiation , causes the dust to spiral into the sun in times much shorter than the age of the Earth . 
Meaning of the word marked with <WSD> tags:  a natural event that involves a change in the position or location of something