( ( ( ( The north ) and ) ( south ( magnetic poles ) ) ) ( ( ( have repeatedly ) ( ( changed places ) ( while ( ( the axis ) ( has ( stayed ( in ( the ( same position ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN north)) (CC and) (NP (JJ south) (JJ magnetic) (NNS poles))) (VP (VBP have) (ADVP (RB repeatedly)) (VP (VBN changed) (NP (NNS places)) (SBAR (IN while) (S (NP (DT the) (NNS axis)) (VP (VBZ has) (VP (VBN stayed) (PP (IN in) (NP (DT the) (JJ same) (NN position))))))))) (. .)))
The north and south magnetic poles have repeatedly changed places while the axis has stayed in the same position.
(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))))) (. .)))