( ( knows ( that ( gravity ( is ( ( the one ) ( of ( ( the forces ) ( that ( keeps ( planets ( ( arranged ( in ( ( ( orbits ( around ( the Sun ) ) ) and ) ( ( the Moon ) ( in orbit ) ) ) ) ) ( around ( the Earth ) ) ) ) ) ) ) ) ) ) ) ) ) . )
(ROOT (S (VP (VBZ knows) (SBAR (IN that) (S (NP (NN gravity)) (VP (VBZ is) (NP (NP (DT the) (NN one)) (PP (IN of) (NP (NP (DT the) (NNS forces)) (SBAR (WHNP (WDT that)) (S (VP (VBZ keeps) (SBAR (S (NP (NNS planets)) (VP (VBD arranged) (PP (IN in) (NP (NP (NP (NNS orbits)) (PP (IN around) (NP (DT the) (NNP Sun)))) (CC and) (NP (NP (DT the) (NN Moon)) (PP (IN in) (NP (NN orbit)))))) (PP (IN around) (NP (DT the) (NN Earth)))))))))))))))) (. .)))
knows that gravity is the one of the forces that keeps planets arranged in orbits around the Sun and the  Moon in orbit around the Earth.
(ROOT (S (NP (NNS Planets)) (VP (VBP are) (VP (VBN held) (PP (IN by) (NP (NP (DT the) (NN force)) (PP (IN of) (NP (NN gravity))))) (PP (IN in) (NP (NP (JJ elliptical) (NNS orbits)) (PP (IN around) (NP (NN sun))))))) (. .)))