Opposition of Mars and the Earth, computer artwork. Two celestial bodies are said to be in opposition when they are on opposite sides of the sky, when viewed from a given place. A planet is said to be in opposition when it is in opposition to the Sun. Since most orbits in the Solar System are nearly coplanar, this occurs when the Sun, Earth, and the body are approximately in a straight line - the Earth and the body are in the same direction as seen from the Sun.