( ( A day ) ( ( is ( ( ( 86,400 seconds ) or ) ( ( ( 24 hours ) long ) ( which ( is ( ( the time ) ( it ( takes ( Earth ( to ( ( rotate once ) ( on ( its axis ) ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT A) (NN day)) (VP (VBZ is) (NP (NP (CD 86,400) (NNS seconds)) (CC or) (NP (NP (CD 24) (NNS hours)) (ADVP (RB long)) (SBAR (WHNP (WDT which)) (S (VP (VBZ is) (NP (NP (DT the) (NN time)) (SBAR (S (NP (PRP it)) (VP (VBZ takes) (S (NP (NNP Earth)) (VP (TO to) (VP (VB rotate) (ADVP (RB once)) (PP (IN on) (NP (PRP$ its) (NNS axis)))))))))))))))) (. .)))
A day is 86,400 seconds or 24 hours long which is the time it takes Earth to rotate once on its axis.
(ROOT (S (NP (NNP Earth)) (VP (VBZ rotates) (PP (PP (IN on) (NP (PRP$ its) (NNS axis)) (ADVP (RB once))) (CC times) (PP (IN in) (NP (CD one) (NN day))))) (. .)))