Orb

Exact aspects are mathematical instants — two planets stand at precisely 90° apart, to the arcsecond, only for a moment. To make aspects usable in practice, astrology defines a tolerance zone around the exact angle: the orb. The smaller the actual distance from the exact angle, the "tighter," and so the stronger, the aspect is considered to be.

How large the orb is allowed to be is a matter of convention and varies by school: for natal aspects, 6–10° is common (more for the Sun and Moon, less for minor aspects), for transits noticeably less — often just 1–2°, or something would constantly be "in effect" somewhere. This is exactly where careful astrology parts ways with sloppy astrology: with generous orbs, almost any claim can be backed up by some aspect or other.

The orb isn't a physical quantity and doesn't measure a force; it's a methodological choice about where to draw the line between "aspect" and "no aspect." Careful calculations state their orb rules openly.