( ( A ( lunar eclipse ) ) ( ( occurs ( when ( ( the Moon ) ( passes ( through ( ( Earth 's ) shadow ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT A) (NN lunar) (NN eclipse)) (VP (VBZ occurs) (SBAR (WHADVP (WRB when)) (S (NP (DT the) (NNP Moon)) (VP (VBZ passes) (PP (IN through) (NP (NP (NNP Earth) (POS 's)) (NN shadow))))))) (. .)))
A lunar eclipse occurs when the Moon passes through Earth's shadow.
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (NP (NN earth) (POS 's)) (NN shadow)) (VP (VBZ falls) (PP (IN on) (NP (DT the) (NN moon)))))) (, ,) (NP (DT the) (NN shadow)) (VP (VBZ causes) (NP (DT a) (NN lunar) (NN eclipse))) (. .)))
When earth’s shadow falls on the moon, the shadow causes a lunar eclipse.