( ( ( The ( rotation period ) ) ( of ( an ( astronomical object ) ) ) ) ( ( ( is ( the time ) ) ( that ( it ( takes ( to ( complete ( ( one revolution ) ( around ( ( ( its axis ) ( of rotation ) ) ( relative ( to ( the ( background stars ) ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN rotation) (NN period)) (PP (IN of) (NP (DT an) (JJ astronomical) (NN object)))) (VP (VBZ is) (NP (DT the) (NN time)) (SBAR (IN that) (S (NP (PRP it)) (VP (VBZ takes) (S (VP (TO to) (VP (VB complete) (NP (NP (CD one) (NN revolution)) (PP (IN around) (NP (NP (PRP$ its) (NNS axis)) (PP (IN of) (NP (NN rotation))) (ADVP (JJ relative) (PP (TO to) (NP (DT the) (NN background) (NNS stars)))))))))))))) (. .)))
The rotation period of an astronomical object is the time that it takes to complete one revolution around its axis of rotation relative to the background stars.
(ROOT (S (NP (PRP It)) (VP (VBZ takes) (NP (NN earth) (CD one) (NN week)) (S (VP (TO to) (VP (VB rotate) (PP (IN on) (NP (PRP$ its) (NNS axis))) (NP (CD seven) (NNS times)))))) (. .)))