Synastry is the branch of astrology that compares two birth charts to describe the gravity of a relationship. Each person's natal chart is calculated normally — Sun, Moon, Ascendant, the seven other planets, and the houses they fall in. The two charts are then overlaid. The geometric angles (aspects) between planets in chart A and planets in chart B describe how the two people interact: what is instinctively easy, what is structurally difficult, what themes the relationship keeps returning to.
Synastry is one of the oldest applications of astrology. Hellenistic astrologers were comparing charts to evaluate political marriages two thousand years ago; Indian Vedic astrology has used compatibility analysis for arranged-marriage decisions for longer than that. The modern Western form of synastry — with named aspects, orbs, and detailed contact analysis — was systematized in the 20th century by astrologers like Stephen Arroyo and Robert Hand.
Synastry is descriptive, not predictive. It accurately describes the raw material a relationship is made of. It does not predict whether the relationship will be chosen, endure, or be happy. Those are decisions, not coordinates.
Run your synastry: Our free Synastry Calculator overlays two charts and shows the key aspects between them. Pair it with the Cosmic Circle group reading for friendships and chosen-family dynamics.
Among the many possible inter-chart aspects, a handful carry disproportionate weight. These are the contacts that show up repeatedly in long-running couples, in defining friendships, in relationships that mark a life.
Sun-Moon contacts. One person's Sun in close aspect to the other's Moon. This is the classic compatibility signature in traditional astrology. The Sun person's identity meshes naturally with the Moon person's emotional climate. Sun-Moon conjunctions and trines feel like coming home; oppositions feel like discovering you are reading two halves of the same book.
Venus-Mars contacts. Aesthetic and erotic attraction. Venus is what you find beautiful and how you love; Mars is how you pursue and what you desire. When one person's Mars lands on the other's Venus, the chemistry is usually immediate and physical. Venus-Mars conjunctions are the single most common "instant attraction" signature in synastry.
Moon-Moon contacts. Shared emotional weather. When two Moons sit in compatible signs or close aspect, the two people relax around each other without trying. Daily life is easy. Moon-Moon contacts are underrated — they are what makes long-term cohabitation actually bearable.
Saturn contacts. When one person's Saturn lands on the other's personal planet — Sun, Moon, Venus, Mars — the relationship carries the gravity of commitment. The Saturn person is felt as authority, structure, sometimes parent-figure, sometimes brake. Saturn contacts are common in marriages because they describe the actual energy of "this is serious and durable." They can also feel restrictive if the Saturn person uses the contact to control rather than steady.
Synastry shows the gravity. The relationship is what gets built inside it.
Ascendant contacts. Planets aspecting the partner's Ascendant create instant recognition. The Ascendant is the front door of the chart; a planet landing on it announces itself before words. Sun-on-Ascendant tends to produce mutual fascination. Saturn-on-Ascendant produces a feeling of being seriously seen. Mars-on-Ascendant produces immediate physical charge.
Node contacts. The lunar nodes (North Node and South Node) describe karmic direction. When one person's nodes land on the other's personal planets, the relationship feels like it is moving you somewhere. North Node contacts especially tend to produce relationships that develop you in the direction your chart is asking you to grow. South Node contacts feel comfortable, familiar, sometimes a return to a pattern that has run its course.
Synastry uses the same aspect grammar as natal astrology. The major aspects, ordered roughly by intensity:
Conjunction (0°): Two planets fused. Maximum intensity, but the quality depends on the planets involved. Sun-Moon conjunction is harmonic; Mars-Saturn conjunction is friction.
Opposition (180°): Direct facing. Often feels magnetic, sometimes confrontational. Opposition aspects in synastry produce strong polarity: each person sees in the other what they have less of in themselves.
Square (90°): Friction. Square aspects in synastry create the most growth — they make you uncomfortable in ways that change you. Squares are not "bad" aspects; they are the engine of relational evolution.
Trine (120°): Easy flow. Trines feel natural and effortless. They are the most pleasant aspects, but they can also be the most easily overlooked — the things that feel easy often go unappreciated.
Sextile (60°): Cooperative. Sextiles offer opportunity without demand. They are the aspects of mutual support and shared interest.
Three things synastry is genuinely bad at predicting:
Whether a relationship will last. Many long-term marriages have difficult synastry; many short-lived romances have brilliant synastry. Duration is a function of choice, circumstance, and willingness to do the work, not of chart contacts.
Whether a relationship will be 'good.' Good is a moral and lived judgment, not an astrological one. A relationship with difficult synastry can be deeply good — it can be the relationship that develops both people the most. A relationship with easy synastry can be shallow if neither person grows.
Whether you should be in the relationship. This is the most common question asked of synastry and the one it answers least well. Synastry can describe what is naturally present and what will require attention. It cannot make the choice for you.
Western synastry is not the only system that compares two charts. Bazi compatibility in Chinese metaphysics examines the interaction of two Four-Pillars charts, looking at how the Day Masters relate, whether the Useful Gods of each chart conflict or support, and whether the two charts' elemental balances complement or destabilize each other.
The Tree of Life adds another layer: when two people share contacts on certain sephiroth (especially Tiphareth — the heart center — or Yesod — the subconscious), the relationship has an unusually strong center of gravity. The full six-system birth chart reading can be run for both people and the cross-system contacts mapped.
Hermetic Alchemy describes the conscious union of opposites as the coniunctio — the marriage of Sun and Moon, of King and Queen. Synastry, in alchemical terms, is the description of which two principles a particular relationship is asking to integrate.
The most productive way to use synastry is descriptively, not predictively. Two practices:
Name the gravity, then choose. Run the synastry. Identify the three or four strongest contacts. Name what they describe (easy emotional fit, intense erotic charge, Saturn weight, node direction). Then notice: which of these patterns are you actually living? Which are you ignoring? Which are running the relationship without your conscious participation?
Treat difficult aspects as the assignment. If your charts have a tight Mars-Saturn square between them, that aspect will show up as friction around action and authority. Repressing it does not make it go away. Naming it — "we have a tendency to fight about who gets to act first" — lets the two of you address the pattern directly rather than getting blindsided by it again and again.
Synastry is not a verdict. It is a map of the territory. The work of the relationship is to walk the territory consciously.
Synastry overlays two charts and tells you what the gravity between two people is made of. It is descriptive, not predictive. It can name what is easy, what is hard, what is unusually present, what is unusually absent. It cannot tell you whether to be in the relationship, whether it will last, or whether it will be good. Those are decisions, made daily, by the two people involved. Synastry tells you what you are choosing between. The choosing is yours.