What is the X in X-mas?
late 14c., an astrological term for the situation of two heavenly bodies exactly across from one another in the heavens, from Old French oposicion (12c.) or directly from Latin oppositionem (nominative oppositio) "act of opposing, a placing against," noun of action from past participle stem of opponere "set against" (see opponent). Meaning "that which is opposite something else" is from 1540s; meaning "contrast, antagonism" first attested 1580s; sense of "political party opposed to the one in power" is from 1704. Related: Oppositional.
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. The planets Venus and Mercury, whose orbits are smaller than Earth's, can never be in opposition to the Sun.