Classical synastry
Western · Love
Classical synastry is the foundational Western technique for comparing two people's birth charts, examining how each person's planets interact with the other's through aspects and house overlays to reveal the strengths, tensions, and potential of a relationship.
What it is
Classical synastry places both birth charts side by side — one as the 'inner' wheel and the other as the 'outer' wheel — and measures the angular distances (aspects) formed between one person's natal planets and the other's. The major aspects used are the conjunction (0°), opposition (180°), trine (120°), square (90°), and sextile (60°), each carrying a distinct quality of energy — harmonious, challenging, or dynamic. Minor aspects such as the quincunx (150°) and semisextile (30°) add further nuance.
Beyond aspects, synastry also examines house overlays: which house of Person B does each of Person A's planets fall into? A planet falling in someone's 7th house has a different flavour than the same planet in their 12th. This overlay technique reveals the experiential context — the arena of life — in which one partner's energy is most strongly felt by the other.
How it is calculated
Begin with two accurate birth charts — exact birth time, date, and place for both individuals — calculated in the tropical zodiac with the preferred house system (Placidus is most common in Western practice). The planets, Ascendant, Midheaven, and key points (Lunar Nodes, Chiron, Part of Fortune) of Chart A are then compared against all points in Chart B.
Aspects are calculated by measuring the ecliptic longitude difference between planets. Most synastry software and apps apply standard orbs: 8–10° for major aspects between luminaries (Sun, Moon), and 5–6° for other major aspects. Tighter orbs indicate more intense activation. The list of contacts is typically prioritised by orb tightness and the planets involved — Sun-Moon, Venus-Mars, and Saturn-Moon contacts are traditionally among the most significant.
What it reveals
Synastry reveals the interpersonal chemistry and long-term compatibility between two individuals with remarkable depth. Harmonious Venus-Jupiter contacts suggest ease, affection, and shared values; Sun-Moon conjunctions or oppositions point to a deep sense of recognition and complementarity. Hard Saturn contacts reveal where one person imposes structure, limitation, or responsibility on the other — which can build commitment or create resentment depending on the individuals.
The technique is most valuable for identifying where relationship energy flows naturally versus where it requires conscious work. Skilled synastry reading considers both the aspects between charts and each person's natal patterns — a Saturn square to someone's Venus may feel very different depending on whether their natal Venus is already strongly or weakly placed. Synastry does not determine outcomes; it maps the terrain and the energies available for both partners to work with.
Frequently asked questions
Which aspects are considered the most powerful in synastry?
Conjunctions are typically the most intense — the energies merge and are impossible to ignore. Sun-Moon, Venus-Mars, and Ascendant-Ascendant conjunctions are traditionally among the most relationship-defining. Oppositions create polarity and magnetic attraction. Tight square contacts indicate passion and friction that demands resolution.
Is synastry enough to judge long-term compatibility?
Synastry is a powerful starting point but works best alongside composite or Davison chart analysis, which shows the relationship as its own entity. Many astrologers also check whether each person's natal chart supports a stable relationship before evaluating the synastry contacts.
What does it mean when one person's planets fall in the other's 7th house?
Planets in a partner's 7th house carry strong marriage or partnership symbolism. The person whose 7th house is activated tends to perceive the planet-person as a natural partner archetype. The nature of the planet matters: Venus or Jupiter in the 7th is warm and attractive; Saturn in the 7th may feel stabilising but also demanding.
Classical sources
- Ptolemy, Tetrabiblos
- William Lilly, Christian Astrology
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