At first- and third-quarter phases (quadrature) of the moon, the gravitational attractions of the moon and sun upon the waters of the earth are exerted at right angles to each other.
(the gravitational attractions of the moon and sun upon the waters of the earth; are exerted; to each other; T:At first- and third-quarter phases)
the gravitational attractions of the moon and sun upon the waters of the earth
are exerted
to each other T:At first- and third-quarter phases