The Columbia Encyclopedia, Sixth Edition. 2001-07.
quadrature
in astronomy, arrangement of two celestial bodies at right angles to each other as viewed from a reference point. If the reference point is the earth and the sun is one of the bodies, a planet is in quadrature when its elongation is 90°. As viewed from the earth, the half-moon is in quadrature to the sun. The inferior planets (Mercury and Venus) never reach quadrature; the superior planets each have two points of quadrature in their orbits.
