( ( ( ( ( ( ( ( The position ) ( of ( these poles ) ) ) ( varies ( over time ) ) ) , ) and ) sometimes ) ( ( ( the positions ) ( of ( ( ( the north ) and ) ( south ( magnetic poles ) ) ) ) ) ( switch places ) ) ) . )
(ROOT (S (S (NP (NP (DT The) (NN position)) (PP (IN of) (NP (DT these) (NNS poles)))) (VP (VBZ varies) (PP (IN over) (NP (NN time))))) (, ,) (CC and) (ADVP (RB sometimes)) (S (NP (NP (DT the) (NNS positions)) (PP (IN of) (NP (NP (DT the) (NN north)) (CC and) (NP (JJ south) (JJ magnetic) (NNS poles))))) (VP (VBP switch) (NP (NNS places)))) (. .)))
The position of these poles varies over time, and sometimes the positions of the north and south magnetic poles switch places.
(ROOT (S (NP (NP (DT The) (NN earth) (POS 's)) (JJ magnetic) (NNS poles)) (VP (VBP have) (VP (VBN switched) (NP (NNS places)) (ADVP (RB repeatedly)) (PP (IN in) (NP (DT the) (NN past))))) (. .)))