Synastry is the branch of astrology concerned with comparing two birth charts to understand relationship dynamics. The word comes from Greek — syn (together) and astron (star) — and the practice involves overlaying one person's natal chart on another's to identify where their planetary positions reinforce, tension, or amplify each other. Unlike sun sign compatibility (the "are Scorpios and Geminis compatible?" type of analysis), synastry works with the full chart: all ten planets, the rising sign, the house system, and the specific angular relationships between planets across both charts. This produces a far more textured and specific picture of how two people are likely to experience each other.
The Basics of Synastry: What Gets Compared
In a synastry comparison, the practitioner looks at how the planets in one person's chart form aspects (angular relationships) to planets in the other person's chart. A few key elements:
- Personal planets — Sun, Moon, Mercury, Venus, Mars — are considered the most immediately relevant because they describe the core of personality and drive. Their inter-aspects are given the most weight in a synastry reading.
- Outer planets (Jupiter, Saturn, Uranus, Neptune, Pluto) form generational aspects that can add depth to a synastry picture, but because they move slowly, they describe patterns that apply to everyone born within a few years of each person, not uniquely individual dynamics.
- The Ascendant (rising sign) and the angles (midheaven, nadir, descendant) are highly sensitive points that can create powerful resonances when they land on another person's planets.
The aspects themselves carry different qualities. Conjunctions (planets at the same degree) intensify whatever the planets represent — for better or worse. Trines and sextiles (harmonious angles) indicate easy flow and natural affinity in those areas. Squares and oppositions indicate friction, challenge, and the kind of tension that can either generate growth or persistent difficulty depending on how it's handled.
The Moon Connection: Emotional Compatibility
In synastry, the Moon is considered one of the most significant indicators of emotional compatibility. The Moon in astrology represents your emotional needs, your instinctive reactions, your comfort requirements, and how you process feeling. When two people's Moons are well-aspected to each other (particularly in trine or conjunction), there tends to be an emotional ease — they feel comfortable together, their rhythms align, and they don't require extensive translation of each other's emotional states.
Moon-Sun aspects between charts are also highly significant. When one person's Sun conjuncts or trines the other's Moon, there's often a particular kind of complementarity — the Sun person tends to energise and vitalise the Moon person, who in turn provides the Sun person with a sense of being genuinely understood and cared for.
Challenging Moon aspects (square, opposition) in synastry don't make a relationship impossible — they make emotional attunement something that requires conscious effort. The partners may have fundamentally different emotional rhythms, comfort requirements, or ways of processing difficulty.
Venus and Mars: Attraction and Chemistry
Venus in the birth chart represents your aesthetic sensibility, your values in relationship, how you express affection, and what you find beautiful. Mars represents desire, drive, how you pursue what you want, and the quality of your assertiveness. In synastry, Venus-Mars inter-aspects are the classic chemistry indicators.
When one person's Venus conjuncts or trines the other's Mars, there's typically a strong mutual attraction. The Mars person tends to be drawn to pursue; the Venus person tends to be drawn to receive that attention. The energy can be compelling.
Venus-Venus aspects describe whether two people have compatible value systems and aesthetic sensibilities — the things they both appreciate and find worthwhile. Strong Venus-Venus harmony often describes relationships where people genuinely enjoy the same things and share a sense of beauty.
Mars-Mars aspects describe whether two people's drive energies work in harmony or at cross-purposes. Mars conjunctions can produce either shared purposefulness or competitive friction; Mars squares often indicate cycles of initiation and clash.
Saturn in Synastry: The Commitment and Constraint Indicator
Saturn's role in synastry is frequently misunderstood. Saturn aspects — particularly when one person's Saturn touches another's personal planets — are often associated with longevity and seriousness. Saturn adds weight, structure, and commitment. When Saturn conjuncts someone's Sun or Moon, the relationship tends to feel significant and weighty; there's often a sense that something important is at stake.
The difficulty is that Saturn also constrains. The Saturn person can feel inhibiting or heavy to the planet person; the planet person may feel restricted or judged by the Saturn person. Long-term relationships often have significant Saturn contacts — the stability comes with some friction — and the interpretation depends on whether the Saturn structure feels like support or like a cage.
Saturn-Saturn aspects between charts speak to whether two people are at compatible stages of development and whether their approach to responsibility, commitment, and long-term thinking aligns. A strong Saturn trine between charts often indicates mutual respect and shared seriousness of purpose.
House Overlays: Where You Activate Each Other
Beyond planetary aspects, synastry also considers house overlays — where each person's planets fall in the other person's chart. When someone's Sun falls in your 7th house (the house of partnership), they tend to feel like a quintessential partner; they activate your sense of what a significant other should be. When someone's Venus falls in your 4th house (home, roots, private life), they activate your desire for domestic comfort and belonging.
Significant planets in someone's 12th house (hidden life, unconscious patterns, dissolution) can create a complex overlay — both deep resonance and a quality of mystery or elusiveness. 8th house overlays (shared resources, transformation, the psychological underbelly) tend to create intensely bonded but often challenging dynamics.
If you want to explore your own natal chart and see which planetary placements define your relationship patterns, our free natal chart test generates your full chart with detailed interpretations.
Frequently Asked Questions
Is synastry?
No. Astrology, including synastry, has not been validated through controlled scientific studies. Studies that have tested astrological predictions — including the Shawn Carlson double-blind study published in Nature in 1985 — have not found evidence that astrological placements predict measurable outcomes. Synastry is a symbolic and interpretive framework that many people find psychologically useful; it isn't an predictive system.
Can synastry predict whether a relationship will work?
No synastry reading can predict that — and practitioners who claim otherwise are overstating the system. What synastry can describe are likely patterns of resonance, friction, and activation between two people. Many relationships with challenging synastry work beautifully; many with harmonious synastry still fail. Human choice, circumstances, maturity, and effort matter far more than planetary positions.
How important is sun sign compatibility compared to full synastry?
Sun sign compatibility is a tiny fragment of synastry. The sun sign represents one of many placements and is relevant primarily to how each person's core identity and life force express themselves. Full synastry adds the Moon, Venus, Mars, rising sign, Saturn, and all other placements — a vastly richer picture. Sun sign compatibility is so simplified as to be mostly entertainment; synastry has more texture and specificity, though still within the limits of the system as a whole.
What are the most important aspects to look for in synastry?
Most practitioners would prioritise: Sun-Moon aspects (general compatibility and complementarity); Venus-Mars aspects (attraction and desire); Moon-Moon aspects (emotional ease); and Saturn contacts (commitment and longevity patterns). Ascendant overlays are also significant for understanding first impressions and the felt experience of being with each other.
Does synastry apply to non-romantic relationships?
Yes — synastry can be applied to any significant two-person relationship: parent-child, business partners, close friends. The areas of emphasis shift (Venus-Mars chemistry is less central to a professional partnership than mutual Saturn contacts and Mercury aspects, which describe how two people think and communicate together), but the basic framework applies to any relationship dynamic worth understanding.