Every configuration on the previous pages — stelliums, T-squares, yods — is built from aspects. Here's what an aspect actually is.
An aspect is simply the angular distance between two planets on the 360° chart wheel, measured in degrees. Certain angles — 0° (conjunction), 60° (sextile), 90° (square), 120° (trine), and 180° (opposition) — recur across musical harmony, sacred geometry, and astrology alike, and are treated as points where the relationship between two planets becomes especially charged, whether harmonious or tense.
Aspects are rarely exact to the degree; astrologers work within an orb — an allowable margin of error — inside which the aspect is still considered active. Orbs vary by aspect (conjunctions and oppositions traditionally get the widest orbs, around 8-10°, while minor aspects like the quincunx are held to a tighter 2-3°) and by the planets involved (Sun and Moon aspects are traditionally given more leeway than aspects between the outer planets).
There is no single universally agreed orb system — different schools of astrology (traditional Hellenistic, modern psychological, Vedic) use meaningfully different orb widths, which is one of the more common sources of disagreement between astrologers reading the same chart.
An aspect is applying when the faster-moving planet is approaching exactness with the slower one — the aspect is still "building," and its effects are often described as more future-oriented and actively unfolding. An aspect is separating once the faster planet has passed exactness and is moving away — traditionally read as an influence that is fading, more connected to the past or to something already set in motion than to what's still arriving.
This distinction matters most in transit work: an applying transit is often felt as building anticipation, while a separating transit is felt as processing or integrating something that has already peaked.