Aspect

The doctrine of aspects originates with Ptolemy, who in Tetrabiblos Book I, Chapter 13 established five major angular relationships derived from musical harmony ratios: conjunction (0°), sextile (60°), square (90°), trine (120°), and opposition (180°). Ptolemy grounded each aspect in the arithmetic ratios of the octave, fifth, fourth, and unison — the same intervals that underpin Greek musical theory. Signs in trine share an element and 'regard' one another harmoniously; signs in square share neither element nor modality and produce friction. Signs in opposition stand at maximum polarity. This five-aspect canon remained essentially unchanged through William Lilly's Christian Astrology (1647) and into modern practice. Johannes Kepler (1571–1630) later expanded the system with the quintile (72°), biquintile (144°), and semi-sextile (30°), arguing that any harmonically resonant division of the circle could produce an aspect. Modern practice (Robert Hand, Horoscope Symbols, 1981) treats the five Ptolemaic aspects as primary and the Keplerian minor aspects as supplementary refinements. The interpretive framework is consistent: harmonious aspects (trine, sextile) show ease and natural cooperation between planetary functions; tense aspects (square, opposition) show friction that demands conscious integration.