Opposition is a term used in positional astronomy and astrology to indicate when one celestial body is on the opposite side of the sky when viewed from a particular place (usually the Earth). In particular, two planets are in opposition to each other when their ecliptic longitudes differ by 180°. The symbol of opposition is ☍.

A planet (or asteroid or comet) is said to be "in opposition" when it is in opposition to the Sun as seen from the Earth. This is the best time to observe a planet because:

• it is visible almost all night, rising around sunset, culminating around midnight and setting around sunrise;
• its orbit brings it closest to the Earth, making it appear bigger and brighter.
• the opposition effect increases the reflected light from bodies with unobscured rough surfaces

Opposition occurs only in superior planets.

The Moon, which orbits the Earth rather than the Sun, is in opposition to the Sun at full moon. When it is exact in opposition, a lunar eclipse occurs.

Superior and inferior