What a Natal Aspect Actually Is (and Why the Angle Has to Be Almost Exact)
"Venus trine Mars" or "Saturn square the Moon" sound like fixed facts about a person, but underneath they're a single number: the angular distance between two planets' positions in the sky at a given moment, checked against five specific target angles. Synastri computes this from real orbital mechanics for every pair among the Sun, Moon, and five visible planets — not a lookup table.
Five aspects, five target angles
A conjunction is 0° apart, a sextile 60°, a square 90°, a trine 120°, an opposition 180°. Every pair of bodies has some angular separation; if that separation lands close enough to one of these five angles, the pair is "in aspect." Traditionally, conjunctions and oppositions read as intense (fused energy, or a pull toward balance), squares as dynamic friction, trines and sextiles as flowing or easy — a genuine spread from tense to harmonious, not five variations on the same theme.
"Close enough" has a real, disclosed margin — the orb
No angle lands on exactly 0°, 60°, 90°, 120°, or 180° in practice, so each aspect gets an allowed margin called an orb: 8° for a conjunction or opposition, 6° for a square or trine, 4° for the tighter sextile. Only the closest-matching aspect within its own orb counts per pair — Synastri checks all five angles and keeps the one with the smallest leftover orb, so a pair is never assigned two aspects at once.
Why this needs orbital mechanics, not guessing
Getting the angle right depends on knowing each planet's real ecliptic longitude on a given date, which is why this sits on top of the same low-precision Kepler-orbit calculations Synastri uses for sign placements — accurate to roughly an arcminute for the Sun and Moon, good enough that the aspect angles here are meaningfully real, not decorative. See your own chart's aspects as part of a `/daily` reading. For entertainment.