Sentence:  The Poynting-Robertson effect ( Robertson , 1937 ; Wyatt and Whipple , 1950 ) , which <WSD>is</WSD> a retardation of the orbital motion 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:  be identical to; be someone or something