( ( The cause ) ( ( is ( ( ( ( the ( earth 's ) ) ( tilting axis ) ) , ) ( which ( exposes ( ( the hemisphere ) ( ( experiencing summer ) ( to ( ( more ( direct rays ) ) ( from ( the sun ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT The) (NN cause)) (VP (VBZ is) (NP (NP (NP (DT the) (NN earth) (POS 's)) (VBG tilting) (NNS axis)) (, ,) (SBAR (WHNP (WDT which)) (S (VP (VBZ exposes) (NP (NP (DT the) (NN hemisphere)) (VP (VBG experiencing) (NP (NN summer)) (PP (TO to) (NP (NP (JJR more) (JJ direct) (NNS rays)) (PP (IN from) (NP (DT the) (NN sun)))))))))))) (. .)))
The cause is the earth's tilting axis, which exposes the hemisphere experiencing summer to more direct rays from the sun.
(ROOT (S (NP (NP (DT The) (NN tilt)) (PP (IN of) (NP (NP (NN earth)) (PP (IN on) (NP (PRP$ its) (NNS axis)))))) (VP (VBZ causes) (NP (NN summer)) (S (VP (TO to) (VP (VB be) (VP (VBN warmer) (PP (IN than) (NP (NP (NN winter)) (PP (IN in) (NP (DT the) (JJ northern) (NN hemisphere))))) (SBAR (IN because) (S (NP (NP (DT the) (NNS rays)) (PP (IN of) (NP (DT the) (NN sun)))) (VP (VBP strike) (NP (DT the) (JJ northern) (NN hemisphere)) (ADVP (ADVP (RBR more) (RB directly)) (PP (IN in) (NP (DT the) (NN summer)))))))))))) (. .)))