Synastry: Astrological Compatibility Between Two People

Synastry is the branch of astrology dedicated to examining how two people's birth charts interact — where their planetary positions align, clash, or create something neither chart contains alone. It applies to romantic partners, but also to business associates, parents and children, and close friends. The mechanics are specific: one person's planets fall in degrees that aspect another person's planets, activating dynamics that can feel magnetic, abrasive, supportive, or destabilizing.

• Definition and scope
• Core mechanics or structure
• Causal relationships or drivers
• Classification boundaries
• Tradeoffs and tensions
• Common misconceptions
• How a synastry reading is structured
• Reference table: key synastry aspects and their dynamics

Definition and scope

Synastry sits at the intersection of natal chart interpretation and relational dynamics. The word itself is derived from Greek roots meaning "with the stars," but the practice as formalized in Western astrology draws on a tradition codified most systematically in the 20th century — notably through the work of Reinhold Ebertin, whose 1940 publication Kosmobiologie laid methodological groundwork for relational planetary analysis, and later through Robert Hand's Planets in Composite (1975), which remains a standard reference text.

The scope of a synastry reading is bounded by the two natal charts involved. It does not predict whether a relationship will succeed — that framing misunderstands what the tool does. Synastry maps the terrain: where energy flows easily, where friction is structural rather than accidental, and where one person's psychological patterns activate another's. Whether two people navigate that terrain well is a separate question entirely.

The natal chart basics page covers the individual chart foundations that synastry builds on — understanding what each chart contains is prerequisite to understanding how two interact.

Core mechanics or structure

The primary technical operation in synastry is overlaying two birth charts and identifying inter-aspects: angular relationships formed between planets belonging to different people. If Person A's Venus sits at 14° Scorpio and Person B's Mars sits at 12° Taurus, those two planets are in opposition — a 180° aspect, within a 2-degree orb. That aspect belongs to neither chart alone; it only exists in the relationship.

Astrologers working in the Western tradition typically apply orbs of 6–8 degrees for major aspects (conjunction, opposition, trine, square, sextile) and tighter orbs of 2–3 degrees for minor ones. The tighter the orb, the more pronounced the influence is considered to be.

Beyond inter-aspects, synastry also examines house overlays: where Person A's planets fall in Person B's house system. A planet landing in the 7th house of a partner's chart, for instance, colors that relationship zone in the particular tone of whatever planet is visiting. Saturn landing in someone's 7th often describes a relationship that feels dutiful or testing; Venus in the same position tends to feel aesthetically harmonious or romantically significant to the 7th-house person.

The planets most closely studied in romantic synastry are Venus (values, attraction, pleasure), Mars (drive, desire, approach to action), the Moon (emotional needs, instinctive responses), and the Sun (core identity, ego expression). The venus-mars compatibility analysis is often where practitioners begin, since those two planets describe the basic grammar of attraction and pursuit.

Causal relationships or drivers

What actually drives the felt experience of synastry dynamics? The interpretive logic rests on a few structural claims that practitioners make consistently.

When one person's Moon conjoins another's Sun — a classic "luminaries" contact — the Sun person is said to vitalize and illuminate the Moon person's emotional world, while the Moon person reflects and receives the Sun person's identity. The direction of the influence matters: which planet belongs to whom shifts the dynamic, even if the aspect degree is identical.

Saturn contacts between charts function differently than Venus contacts. Saturn aspecting another person's personal planets introduces themes of responsibility, limitation, and sometimes fear — but also stability and commitment. Research by astrologers like Liz Greene, whose Saturn: A New Look at an Old Devil (1976, Weiser Books) remains widely cited in psychological astrology circles, frames Saturn in synastry as the planet most correlated with long-term durability in relationships, precisely because it demands something real rather than just easy.

The outer planets — Uranus, Neptune, and Pluto — generate especially volatile synastry contacts. When Pluto conjoins another person's Venus, the Venus person may experience attraction that feels compulsive rather than chosen. Neptune on another's Moon can produce idealization that takes years to dissolve. These are not "bad" contacts, but they carry weight that lighter aspects don't.

For more on how aspects in astrology work mechanically within a single chart — before adding the relational layer — that page provides the foundational geometry.

Classification boundaries

Synastry is distinct from two related techniques that are sometimes conflated with it:

The composite chart creates a single "relationship chart" by calculating the mathematical midpoints between two people's planets — a 3rd chart that doesn't belong to either person but describes the relationship as its own entity. Composite chart analysis and synastry answer different questions: synastry shows how two people experience each other; composite shows what the relationship itself becomes.

Davison chart is a variation on composite that uses the midpoint in time and space between two births rather than midpoint degrees, producing a chart with an actual geographic location and a real date. It is less commonly used in mainstream Western practice than the composite.

Transit synastry — where a current planetary transit hits the synastry grid — is sometimes applied but falls outside strict synastry methodology, which by definition is static (two natal charts at fixed points in time).

Synastry also differs from compatibility profiling by astrological elements or astrological modalities alone. Those frameworks offer general tendencies (fire signs often find easy rapport with other fire signs), but synastry operates at the degree level and can describe a Virgo-Sagittarius pairing that works beautifully because their personal planets form trines — despite the signs being in a square relationship.

Tradeoffs and tensions

The field's practitioners disagree on several points that don't have clean answers.

Orb standards vary substantially. Traditional Hellenistic astrologers working from texts like Vettius Valens's Anthologies (2nd century CE) applied sign-based aspects without orbs at all — if two planets occupied the same sign or signs in a specific angular relationship, the aspect existed regardless of degree. Modern practitioners applying degree-based orbs of 8 or even 10 degrees for major aspects can find aspects that tighter orb standards would exclude. The difference isn't trivial: expanding the orb from 6° to 10° for a conjunction increases the zone of activation from a 12° arc to a 20° arc.

Weighting is another contested area. Should Venus-Mars contacts be prioritized over Moon-Sun contacts? Different schools weight these differently. Psychological astrologers following the tradition at the Centre for Psychological Astrology (founded by Liz Greene and Howard Sasportas in London in 1983) tend to emphasize lunar contacts as describing core emotional compatibility. Traditional astrologers often weight the luminaries and their essential dignities more heavily.

Symmetry vs. asymmetry in interpretation creates genuine complexity. If Person A's Saturn squares Person B's Moon, the Saturnian person and the Lunar person experience that aspect from opposite positions — one potentially feels constrained or judged, the other may feel they are simply being responsible. Reading the aspect as a single shared experience misses this.

Common misconceptions

"Sun sign compatibility is synastry." It is not. Matching Leos with Sagittarians based on elemental affinity is a much blunter instrument than synastry. A full synastry reading examines 10 or more planetary positions per chart, generating 100+ potential inter-aspects, most of which have nothing to do with Sun sign placement.

"Good synastry means a good relationship." Charts with predominantly harmonious inter-aspects (trines, sextiles) sometimes describe relationships that feel pleasant but generate no growth or commitment. Challenging aspects — squares, oppositions, even some Saturn contacts — often correlate with relationships that are intense, durable, and genuinely transformative. The absence of friction is not necessarily a sign of compatibility.

"Synastry can tell you if someone is 'the one.'" This is a category error. Synastry describes structural dynamics between two charts. Whether those dynamics are navigated well, ignored, or weaponized depends on the people involved, not the chart. The astrological authority home is clear on this framing: astrology describes tendencies, not destinations.

"Only romantic synastry matters." Parent-child synastry, business partnership synastry, and friendship synastry are equally valid applications. The same Saturn-Moon contact means something different in a parent-child context than in a romantic one — the relational category shapes interpretation.

How a synastry reading is structured

A practitioner approaching a synastry reading typically works through the following sequence:

1. Generate accurate natal charts for both people — birth date, exact time, and birth location are required for precise house positions.
2. Identify the major inter-aspects between personal planets (Sun, Moon, Mercury, Venus, Mars) first, before moving to outer planets.
3. Note the house overlays: where each person's planets land in the other's chart.
4. Identify any angular contacts — planets falling near the Ascendant, Descendant, Midheaven, or IC of the other chart, which tend to be especially activating.
5. Look for patterns: multiple contacts between the same two planetary energies amplify that theme.
6. Note any contacts involving Chiron, the asteroid associated with wounding and healing — Chiron contacts in synastry are often where relationships feel both painful and potentially restorative. More on this at Chiron in astrology.
7. Examine the nodal axis: contacts between one person's planets and the other's North Node/South Node are often cited as describing relationships that feel fated or karmically significant.
8. Synthesize — the reading emerges from the pattern of contacts, not from any single aspect in isolation.

Reference table: key synastry aspects and their dynamics

References

• Robert Hand, Planets in Composite (1975) — Whitford Press (publisher reference; no free government URL available)
• Liz Greene and Howard Sasportas — Centre for Psychological Astrology, London
• Reinhold Ebertin — Kosmobiologie / The Combination of Stellar Influences (Ebertin-Verlag, 1940/1972 English edition)
• Vettius Valens, Anthologies — translated by Mark Riley, publicly hosted at Sacramento State University
• Liz Greene, Saturn: A New Look at an Old Devil (1976, Weiser Books)