( ( The ( autumn equinox ) ) ( ( is ( when ( ( the sun ) ( ( ( is directly ) overhead ) ( at ( noon ( on equator ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT The) (NN autumn) (NNS equinox)) (VP (VBZ is) (SBAR (WHADVP (WRB when)) (S (NP (DT the) (NN sun)) (VP (VBZ is) (ADVP (RB directly)) (ADVP (RB overhead)) (PP (IN at) (NP (NP (NN noon)) (PP (IN on) (NP (NN equator))))))))) (. .)))
The autumn equinox is when the sun is directly overhead at noon on equator.
(ROOT (S (NP (DT The) (NN sun)) (VP (VBZ is) (ADVP (RB directly)) (PP (IN over) (NP (DT the) (NN equator))) (PP (IN during))) (. .)))