Sentence:  De Jager ( 1955 ) has calculated the times required for these particles to reach the atmosphere under the influence of the Poynting-Robertson effect , which in this case causes the orbits to become more and more eccentric without changing the semi <WSD>major</WSD> axis . 
Meaning of the word marked with <WSD> tags:  the longest axis of an ellipse or ellipsoid; passes through the two foci