Aspect, in full
Five angles make up the major set, called Ptolemaic after the second-century astronomer Claudius Ptolemy, who laid them out in the Tetrabiblos. Conjunction is 0 degrees, sextile 60, square 90, trine 120 and opposition 180. Each is a plain fraction of the circle: a half, a third, a quarter and a sixth. Ptolemy justified the selection by analogy with the whole-number ratios behind consonant musical intervals, and later writers added minor angles on the same logic.
Aspects do not have to be exact. The allowance on either side is called the orb, and its size is convention rather than rule. Many astrologers give a conjunction or opposition something like 8 degrees and a sextile 4 or fewer, and widen the allowance when the Sun or Moon is one of the two points. An aspect is called applying while the angle is still closing and separating once it has passed exactness.
Talking about aspects between whole signs, the way sign-to-sign compatibility writing does, is a coarse version of the same measurement. Two signs four apart have midpoints exactly 120 degrees apart, so any two planets inside them land somewhere between 90 and 150 degrees, within 30 degrees of an exact trine. A chart cast from a real birth time gives the actual figure, which often differs sharply from the sign-level angle.
- Astrology
- Nothing else in common use
A worked example
Put the Sun at 12 degrees of Taurus and Saturn at 15 degrees of Virgo. Taurus starts at 30 degrees of longitude, so the Sun sits at 42. Virgo starts at 150, so Saturn sits at 165. The separation is 165 - 42 = 123 degrees, three degrees off an exact trine and comfortably inside any orb an astrologer would allow. Both signs are earth, which is what a trine between whole signs guarantees.
Sources
Ptolemy sets out opposition at 180 degrees, trine at 120, quartile at 90 and sextile at 60 in the Tetrabiblos, deriving them from simple fractions of the circle.
Ptolemy — Tetrabiblos I.13, Of the Aspects of the Signs (LacusCurtius)
See also
Where it comes up on this site
Aspect: frequently asked
Conjunction at 0 degrees, sextile at 60, square at 90, trine at 120 and opposition at 180. Ptolemy set them out in the second century, choosing angles that cut the circle by simple fractions. Everything else is a minor aspect.
The margin allowed around an exact angle. Two planets 86 degrees apart count as a square if the astrologer works with a 6-degree orb, and do not if the orb is 3. Sizes are convention rather than rule, varying by school, by aspect and by which bodies are involved, which is why two readings of one chart can list different aspects entirely.