Sentence:  Conjunctions of I and II [Io and Europa] occur when they are near <WSD>perigee</WSD> and apogee respectively; conjunctions of II and III [Europa and Ganymede] occur when II [Europa] is near <WSD>perigee</WSD>. 
Meaning of the word marked with <WSD> tags:  The point, in an orbit about any planet, that is closest to the planet: the periapsis of any satellite.