E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
In astrology, a planet distant from another one-third of the circle is said to be in trine; one-fourth, it is in square; one-sixth or two signs, it is in sextile; but when, one-half distant, it is said to be opposite.
In sextile, square, and trine, and opposite
Of noxious efficacy.
Milton: Paradise Lost, x. 659.
N.B. Planets distant from each other six signs or half a circle have opposite influences, and are therefore opposed to each other.