( ( ( ( ( The ( vernal equinox ) ) ( -LRB- ( ( March ( ( 20 or ) 21 ) ) -RRB- ) ) ) and ) ( the autumnal ) ) ( ( equinox ( to ( ( represent ( the time ) ) ( when ( ( the sun ) ( ( ( is directly ) ( over ( the equator ) ) ) ( so ( that ( ( its rays ) ( strike ( at ( a ( 90 ( degree angle ) ) ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (NP (DT The) (JJ vernal) (NNS equinox)) (PRN (-LRB- -LRB-) (NP (NNP March) (QP (CD 20) (CC or) (CD 21))) (-RRB- -RRB-))) (CC and) (NP (DT the) (NN autumnal))) (VP (VBZ equinox) (S (VP (TO to) (VP (VB represent) (NP (DT the) (NN time)) (SBAR (WHADVP (WRB when)) (S (NP (DT the) (NN sun)) (VP (VBZ is) (ADVP (RB directly)) (PP (IN over) (NP (DT the) (NN equator))) (SBAR (RB so) (IN that) (S (NP (PRP$ its) (NNS rays)) (VP (VBP strike) (PP (IN at) (NP (DT a) (CD 90) (NN degree) (NN angle))))))))))))) (. .)))
The vernal equinox (March 20 or 21) and the autumnal equinox to represent the time when the sun is directly over the equator so that its rays strike at a 90 degree angle.
(ROOT (S (NP (DT The) (NN sun)) (VP (VBZ is) (ADVP (RB directly)) (PP (IN over) (NP (DT the) (NN equator))) (PP (IN during))) (. .)))