The opposition is an aspect of one hundred and eighty degrees, formed when two planets stand directly across the circle from one another, half its span apart. It is one of the five major, or Ptolemaic, aspects, and with the square one of the two hard aspects. One hundred and eighty degrees sets planets in signs of the same mode and complementary element face to face, and the tradition reads the opposition as confrontation, polarity, and the tension of two forces pulling in contrary directions, often projected onto other people and relationships, where the opposed energies are met in another. Like the full moon, which is an opposition of Moon and Sun, it carries a quality of culmination and awareness. It is given a wide orb. These are the interpretive conventions of the astrological tradition for this angular relation between planets.
Book I.13 of Ptolemy's Tetrabiblos, compiled in Alexandria in the second century CE, names the opposition alongside the trine, quartile and sextile as one of only four figures the earliest systematic source recognises, the conjunction being treated separately as no aspect at all. Hellenistic astrologers read it through the same whole-sign lens Vettius Valens applies throughout his Anthology of around 175 CE, as two planets set at the full stretch of the circle from one another, each looking the other directly in the face with no intervening sign to soften the contact. The now-common psychological reading, that the querent identifies with one end of the axis and meets the other in the people around them, is a reinterpretation dating from the twentieth century; the older texts describe the same geometry as straightforward enmity and confrontation and offer no theory of projection at all.