Trine

A trine is the aspect of one hundred and twenty degrees, one of the five classical aspects and traditionally the most harmonious of them. It is also one of the four configurations Ptolemy counted as a genuine aspect in the strict sense of the Tetrabiblos, since bodies a third of the circle apart clearly behold one another, and among those four he treated it as the gentlest, the arc between the bodies being wide enough that their natures blend rather than collide. The angle corresponds to the interior structure of the equilateral triangle inscribed in the circle by dividing it into three equal parts, and the trine has always been read through that geometry: three equal arcs, three points of a single figure, a relationship of kind rather than of tension. Aspect doctrine of this sort was already established among Greek astrologers before Ptolemy wrote, and his contribution was less to invent the trine than to give its already recognised effects a causal explanation tied to the elements and the qualities, one that made the trine's favourable reputation part of a coherent theoretical system rather than a rule of thumb inherited from older practice.

Contemporaries and near contemporaries writing in the same Hellenistic milieu, such as Vettius Valens in his Anthology, worked with the same set of angular relationships in their case material, which is one reason historians of the subject now generally treat the trine and its companions as older than Ptolemy's own systematic account of them.