( It ( ( is ( ( ( the rotation ) ( of ( ( the earth ) ( on ( its axis ) ) ) ) ) ( that ( determined ( a ( 24-hour day ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP It)) (VP (VBZ is) (NP (NP (DT the) (NN rotation)) (PP (IN of) (NP (NP (DT the) (NN earth)) (PP (IN on) (NP (PRP$ its) (NNS axis))))) (SBAR (WHNP (WDT that)) (S (VP (VBD determined) (NP (DT a) (JJ 24-hour) (NN day))))))) (. .)))
It is the rotation of the earth on its axis that determined a 24-hour day.
(ROOT (S (NP (PRP It)) (VP (VBZ takes) (NP (NP (CD 24) (NNS hours)) (PP (IN for) (NP (DT the) (NN earth) (S (VP (TO to) (VP (VB make) (NP (NP (DT a) (JJ complete) (NN rotation)) (PP (IN of) (NP (PRP$ its) (NNS axis))))))))))) (. .)))