- Conjunction 0° · Orb 5-12°
- Sextile 60° · Orb 5-6.5°
- Square 90° · Orb 5-10°
- Trine 120° · Orb 5-12°
- Opposition 180° · Orb 5-12°
An aspect is an angle, not a symbol
An aspect is the angular distance between two points of the chart along the ecliptic. A trine is not a "good sign" but 120 degrees between planets; a square is 90. Everything else, the character and the strength, follows from that geometry: angles that divide the circle into quarters produce tension, angles that divide it into thirds produce ease.
The orb depends on the pair of planets
An exact angle is rare, so an aspect counts as active within a tolerance called the orb. A flat orb for every pair produces ghosts: in Monroe's chart Mars and Uranus would stand in conjunction with an 8 degree gap, yet no program shows that conjunction. We take the orb as the minimum of the two planets, following the ZET9 and astro.com matrix: the Sun up to 12 degrees, the Moon up to 10, every other body 5 in conjunction and opposition.
Why there are exactly five major aspects
The major aspects divide the circle into 1, 2, 3, 4 and 6 parts: 0, 180, 120, 90 and 60 degrees. They are the most visible both in the chart and in transits, which is why they are enabled by default in the calculations. The minor ones (30, 45, 135, 150 and others) work with a tolerance of about a degree and read as refinements rather than as the backbone of the picture.
Applying and separating
An aspect is applying while the planets are still closing on the exact angle, and separating once they are moving apart. In transits that is the difference between an event still arriving and one already spent: an applying aspect builds pressure, a separating one releases it. The tables of the service mark this next to the orb.