( ( In astronomy ) ( , ( ( a tetrad ) ( ( is ( ( a set ) ( of ( ( four ( total ( lunar eclipses ) ) ) ( within ( two years ) ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN In) (NP (NN astronomy))) (, ,) (NP (DT a) (NN tetrad)) (VP (VBZ is) (NP (NP (DT a) (NN set)) (PP (IN of) (NP (NP (CD four) (JJ total) (NN lunar) (NNS eclipses)) (PP (IN within) (NP (CD two) (NNS years))))))) (. .)))
In astronomy, a tetrad is a set of four total lunar eclipses within two years.
(ROOT (S (NP (CD Two)) (VP (VBZ is) (NP (NP (DT the) (JJ minimum) (NN number)) (PP (IN of) (NP (NP (NNS times)) (SBAR (S (NP (DT a) (JJ partial) (NN lunar) (NN eclipse)) (VP (MD will) (VP (VB occur) (NP (DT each) (NN year)))))))))) (. .)))