In positional astronomy, two celestial bodies are said to be in opposition when they are on opposite sides of the sky, viewed from a given place (usually the Earth).

A planet (or asteroid or comet) 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, that is, the Earth and the body are in the same direction as seen from the Sun. The instant of opposition is defined as that when the apparent geocentric celestial longitude of the body differs by 180° from the apparent geocentric longitude of the Sun.