Sentence:  On May 15 , a very large increase occurred with * * f of mass between * * f and * * f ; for the next two <WSD>days</WSD> , the impact rate was * * f ; and for the next nine days , the impact rate was less than * * f ( Nazarova , 1960 ) . 
Meaning of the word marked with <WSD> tags:  time for Earth to make a complete rotation on its axis