Synastry: Comparing Two Natal Charts for Relationship Compatibility

Synastry is the branch of astrology concerned with how two people's natal charts interact — specifically, what happens when one person's planetary positions are measured against another's. It's the technical backbone of astrological relationship analysis, used to assess compatibility, friction, attraction, and long-term staying power between two individuals. This page covers the mechanics of how synastry works, what it can and cannot reveal, where practitioners disagree, and what the reference literature says about its core components.

• Definition and scope
• Core mechanics or structure
• Causal relationships or drivers
• Classification boundaries
• Tradeoffs and tensions
• Common misconceptions
• Checklist or steps (non-advisory)
• Reference table or matrix

Definition and scope

Place two birth charts side by side — one for each person — and you're already doing synastry. The word itself comes from the Greek syn (together) and astron (star), but the practice predates modern Greek influence, appearing in Hellenistic astrological manuscripts dated to roughly the 1st century CE, including texts attributed to Dorotheus of Sidon.

Synastry is distinct from the composite chart, which mathematically averages the two charts into a single symbolic "relationship entity." Synastry preserves both individuals as separate systems and examines how one system affects the other. This means synastry is inherently bidirectional: Person A's Saturn falling on Person B's Venus tells a different story than Person B's Saturn falling on Person A's Venus — even though the degree contact is identical.

The scope of synastry covers romantic relationships, but practitioners also apply it to business partnerships, parent-child dynamics, and long-term friendships. The natal chart relationships page outlines how astrological tools are applied across these relationship categories more broadly.

Core mechanics or structure

The primary tool in synastry is the interaspect — the angular relationship formed between one person's planet and another person's planet. When Person A's Mars sits at 14° Aries and Person B's Moon sits at 16° Libra, they form an opposition (180°) with a 2° orb. That angular measurement is the raw data synastry analysis works with.

Astrologers use the same 5 major aspects in synastry as in natal work: conjunction (0°), sextile (60°), square (90°), trine (120°), and opposition (180°). Orbs — the tolerance bands around exact degrees — are typically kept tighter in synastry than in natal interpretation. Many practitioners use a 6° orb maximum for major aspects, and 2–3° for minor ones, though this varies by tradition.

Beyond interaspects, synastry analysis examines house overlays: which house in Person B's chart does Person A's Sun land in? A planet falling in another person's 7th house (the house of partnership) carries different weight than the same planet in their 12th (the house of isolation, hidden matters, and the unconscious). The natal chart houses page covers house meanings in depth.

Planets considered most significant in synastry, in rough order of interpretive weight:

• Sun and Moon — core identity and emotional needs
• Venus and Mars — attraction, desire, and relational style
• Saturn — commitment, restriction, and karmic-style patterns
• Jupiter — expansion, generosity, and mutual growth
• Ascendant/Descendant axis — the relational self and projected ideal partner
• North Node — often described in modern practice as a "fated" or growth-oriented contact

The natal chart aspects reference covers aspect mechanics as they function within a single chart, which directly informs how interaspects are read across two charts.

Causal relationships or drivers

Synastry operates on the premise that planetary symbolism generates predictable relational patterns — not causes in a deterministic sense, but signatures. A Saturn-Venus interaspect, for instance, is consistently associated in astrological literature with relationships that feel stabilizing but sometimes restrictive; the Venus person may feel limited, the Saturn person may feel burdened with responsibility. This pattern appears in texts ranging from Liz Greene's Saturn: A New Look at an Old Devil (1976, Weiser Books) to older traditional sources.

The interpretive logic is symbolic rather than mechanistic. Practitioners don't argue that Saturn's gravitational field is suppressing Venus's emotional expression. The claim is subtler: that two people born with those configurations seem to enact the archetypal dynamics the symbols represent. Whether that's meaningful correlation, cultural self-fulfilling prophecy, or something else entirely is a question the skepticism and natal charts page addresses without flinching.

What drives interpretation depth is the density of contacts. A single Venus-Mars trine across two charts is easy to overlook. But 4 or 5 reinforcing interaspects — Venus conjunct Mars, Sun trine Moon, Ascendant conjunct Jupiter — create what practitioners call a "stellium of synastry contacts," a clustering effect where the symbolic weight accumulates. The overall picture matters more than any single line.

Classification boundaries

Synastry belongs to a family of relational astrological methods that also includes the composite chart, the Davison chart, and the progressed synastry chart. These are not interchangeable.

Synastry is not the same as natal chart aspects within a single chart, though the aspect vocabulary is shared. The critical distinction: natal aspects describe one person's internal psychological dynamics; synastry interaspects describe the relational field between two people.

Tradeoffs and tensions

The central tension in synastry interpretation is between Venus-Mars attraction indicators and Saturn-based durability indicators. A chart comparison rich with Venus-Mars trines and Sun-Moon conjunctions reads as magnetically attractive — the kind of thing that produces instant chemistry. But practitioners in the psychological astrology tradition, notably Liz Greene and Howard Sasportas (The Luminaries, CPA Press, 1992), argue that attraction contacts alone don't predict longevity. Without Saturn contacts to provide structure, the relationship may feel electric but unstable.

The flip side: heavy Saturn contacts without softening Venus or Jupiter interaspects can describe a relationship that lasts but lacks joy. Saturn conjunct Venus, for instance, appears frequently in long-term partnerships — but it's also common in relationships described as duty-bound or emotionally cold.

A secondary tension exists around orbs. Traditional Hellenistic practice used tight orbs (1–3°); modern psychological astrology stretches them considerably. This produces different "maps" of the same two charts. Two practitioners analyzing the same synastry can disagree on which interaspects even register, let alone what they mean.

The natal charts and free will page engages a related question: whether synastry describes a relationship's potential or predetermines its outcome.

Common misconceptions

Misconception 1: Sun-sign compatibility is synastry.
Horoscope-column compatibility ("Scorpio and Capricorn are perfect together") operates purely on sun signs — one data point out of roughly 40–50 variables in a full synastry analysis. Practitioners universally consider this a reduction too far to carry interpretive weight. Two people with "incompatible" sun signs can have 8 strong Venus-Mars interaspects and a mutual Node contact.

Misconception 2: A bad synastry means the relationship won't work.
Difficult interaspects — squares, oppositions, hard Saturn contacts — are not disqualifiers. Traditional and modern astrologers agree that challenge contacts often produce the friction necessary for growth. A chart comparison with zero tension is sometimes described in the literature as producing pleasant but stagnant relationships.

Misconception 3: Synastry is only for romance.
The method applies to any two people whose charts can be compared. Business partnerships, sibling dynamics, and therapeutic relationships have all been analyzed through synastry by practitioners including Steven Forrest (The Changing Sky, ACS Publications) and Robert Hand (Planets in Composite, Para Research, 1975).

Misconception 4: The strongest contact determines the relationship's character.
Synastry is a systemic analysis, not a single-variable equation. Practitioners weigh the full picture — the density of contacts, house overlays, Node interactions — not one headline aspect.

Checklist or steps (non-advisory)

The following describes the sequence a synastry analysis typically follows:

1. Obtain both natal charts with accurate birth data — date, time (to the minute where possible), and location for each person. Birth time accuracy is explored at birth-time-accuracy-natal-chart.
2. Identify interaspects by overlaying the two charts and noting all angular relationships between planets, within standard orbs (typically ≤6° for majors).
3. Weight by planet significance — Sun, Moon, Venus, Mars, and Saturn interaspects are prioritized over outer planet contacts in most traditions.
4. Assess house overlays — determine which houses in each chart the other person's planets activate.
5. Examine Node contacts — North or South Node conjunctions or oppositions to personal planets are flagged as points of strong karmic or developmental significance in modern practice.
6. Identify the dominant theme — is the contact pattern primarily harmonious (trines, sextiles, conjunctions in compatible signs), challenging (squares, oppositions, hard Saturn), or mixed?
7. Note reinforcing patterns — a theme appearing across 3 or more interaspects carries substantially more interpretive weight than a single isolated contact.
8. Place findings alongside individual charts — each person's natal chart shapes how they experience and enact a synastry dynamic; a person with a strong natal Saturn may experience Saturn contacts differently than someone with no natal Saturn emphasis.

The natal chart reading process covers the single-chart interpretation sequence that precedes any synastry work.

Reference table or matrix

Major interaspect interpretive signatures (abbreviated)

This table reflects interpretive consensus across major English-language astrological reference works including Robert Hand's Planets in Transit (Para Research), Liz Greene's works published through CPA Press, and the Astrodienst (Astro.com) educational database maintained by Astrodienst AG in Zollikon, Switzerland.

The full framework for understanding any single planet's role in these contacts — how Venus in the natal chart or Mars in the natal chart shapes a person's relational style — directly informs how interaspects involving those planets are read. Synastry is not a standalone system but a relational layer built on top of each person's individual chart, a point that becomes obvious the moment two charts with strong but very differently natally-placed Venuses are compared.

For a foundational reference on what natal charts contain before any comparison is attempted, the natalchartsauthority.com home resource covers the full structure.

References

• Astrodienst (Astro.com) — Astrological Atlas and Educational Database
• Liz Greene & Howard Sasportas, The Luminaries — CPA Press (Centre for Psychological Astrology)
• Robert Hand, Planets in Composite — Para Research, 1975
• Dorotheus of Sidon, Carmen Astrologicum — Translated by David Pingree, Ascella Publications
• Steven Forrest, The Changing Sky — ACS Publications