Aspects
The Ptolemaic aspects are Hellenistic; the minor aspects are Johannes Kepler's (17th c.)
The five classical aspects — conjunction, sextile, square, trine, opposition — are the angles that divide the circle by small whole numbers, and Ptolemy justified them with a mix of geometry and musical ratio (he ran the same argument in his Harmonics, probably the earlier work). Kepler, seeking a harmonic basis for astrology, added the quintile family in 1601 and several further aspects — the semi-sextile, quincunx, semi-square, decile and others — in Harmonice Mundi (1619). Even Manilius, writing before Ptolemy, warned that counting aspects by whole signs is not enough and that the actual degrees govern.
What it measures
The angular separation between two bodies along the ecliptic, and how close it sits to an exact aspect angle — the orb. An applying aspect is tightening as the faster body approaches; a separating one is loosening.
Where practitioners disagree
- How wide an orb, and by what rule?
A fixed orb per aspect (modern) — or 'moieties', each planet's own orb halved and combined (traditional)
Traditional practice gives the orb to the planet, not the aspect; modern practice gives it to the aspect.
This engine: A stated orb profile; the tightest, exact-to-1° aspects are drawn most strongly, and applying/separating is computed from the real motion.
Sources
- Claudius Ptolemy, Tetrabiblos; Harmonics
- Johannes Kepler, Harmonice Mundi (1619)
- Deborah Houlding, The Classical Origin and Traditional Use of Aspects (1995)