Opposition, in astronomy, the circumstance in which two celestial bodies appear in opposite directions in the sky. The Moon, when full, is said to be in opposition to the Sun; the Earth is then approximately between them. A superior planet (one with an orbit farther from the Sun than Earth’s) is in opposition when Earth passes between it and the Sun. The opposition of a planet is a good time to observe it, because the planet is then at its nearest point to the Earth and in its full phase.

A hard, strong, transparent substance, often used in windows and space helmets.

In positional astronomy, two astronomical objects are said to be in opposition when they are on opposite sides of the celestial sphere, as observed from a given body (usually Earth). A planet (or asteroid or comet) is said to be "in opposition" when it is in opposition to the Sun.

A hard, strong, transparent substance, often used in windows and space helmets.