opposition, in astronomy, alignment of two celestial bodies on opposite sides of the sky as viewed from earth. Opposition of the moon or planets is often determined in reference to the sun. Only the superior planets, whose orbits lie outside that of the earth, can be in opposition to the sun. When a planet is in opposition to the sun, its elongation is 180°, it exhibits retrograde motion, and its phase is full. This is a good time to observe a planet, since it rises when the sun sets and is visible throughout the night, setting as the sun rises.