( ( that is ) ( , ( ( they ( can ( ( rotate ( around ( only ( one axis ) ) ) ) ( at ( a time ) ) ) ) ) . ) ) )
(ROOT (S (INTJ (DT that) (VBZ is)) (, ,) (S (NP (PRP they)) (VP (MD can) (VP (VB rotate) (ADVP (IN around) (NP (RB only) (CD one) (NNS axis))) (PP (IN at) (NP (DT a) (NN time)))))) (. .)))
that is, they can rotate around only one axis at a time.
(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)))))) (. .)))