Astrolium's synastry guide covers the comparison of two natal charts: inter-aspects, house overlays, the composite, and how to time a couple's year. The technique is older than its modern romantic framing — Greek and medieval astrologers used synastry for business partners, political allies, parent-child pairings. Anyone whose life is structurally entangled with yours has a chart that is worth comparing.
For a free preview, run the synastry calculator. For the full feature stack — biwheel, composite, Davison, AI scoring — see synastry charts in Astrolium. For $29 per month Pro pricing, see pricing.
What synastry actually is
Synastry is the comparison of two natal charts to read the relationship between them. Each chart describes a person's interior weather. When two charts overlay, the planets of one fall into the houses of the other and aspect the other's planets, and those crossings are what produce the felt sense of connection, friction, recognition, irritation. The reading method moves from the Sun-Moon overlay (the basic temperament fit) to the personal-planet aspect signature (Mercury, Venus, Mars between the two charts), then to outer-planet contacts (the karmic or generational themes), house overlays (whose planets land in whose 7th, 5th, 10th), and finally the composite or Davison chart that treats the relationship as a third entity. Ptolemy treats synastry in Tetrabiblos Book IV; the technique is older than its romantic framing and extends to business, parent-child, and creative pairings. Astrolium runs the full grid, both house overlays, the composite, and the Davison free, in under 60 seconds.
The principle: each natal chart describes a person's interior weather. When two charts overlay, the planets of one fall into the houses of the other and aspect the other's planets. Those crossings produce the felt sense of connection. Synastry catalogues them.
What synastry can and cannot tell you: it can describe the structural dynamics — what gets activated, what frictions are built in, what years are likely to be turbulent. It cannot tell you whether to stay or leave. That decision belongs to the people involved.
A short history
Synastry as a technique is implicit in Ptolemy's Tetrabiblos (2nd century CE) — Book IV discusses friendship, marriage, and the comparison of nativities. The medieval Persian and Arabic astrologers — Abu Ma'shar notably — formalized the rules: which planets indicate marriage, which indicate enmity, how to read the lord of the 7th in one chart against the planets of the other.
Modern synastry as most practitioners use it crystallized in the 20th century, particularly through Stephen Arroyo's and Liz Greene's writing in the 1970s and 80s. They added the depth-psychological reading — Saturn aspects as commitment versus burden, Pluto aspects as transformation versus obsession — that became standard.
The four ingredients
A complete synastry reading combines 4 kinds of information. Beginners often work with only 1; the result is shallow. The four:
- Inter-aspects. What angle does Person A's Venus make to Person B's Mars? To Person B's Saturn? Each of A's planets to each of B's. The classical orbs are tighter than for a single chart — usually 3 to 5 degrees for the personal planets, 1 to 2 degrees for outers.
- House overlays. When you place A's planets in B's chart, which houses do they fall into? Person A's Sun in Person B's 4th house has a different felt quality than the same Sun in B's 10th, regardless of aspects.
- The composite chart. A third chart computed by midpointing the two natal charts. It describes the relationship as its own entity — what it wants, what it is for, what it is like to be inside it.
- Timing — synastry transits. Transits affect both charts simultaneously. When transit Saturn squares both partners' Venuses in the same 6 months, the relationship is in a Saturn-on-Venus year on both sides.
Sun, Moon, Ascendant — the spine
The luminaries and the rising sign carry the structural weight of any synastry. Aspects between them describe whether the daily fit is easy or effortful. The classic indicators:
Sun-Moon contacts
Trines and sextiles: the famous "easy" relationship — A's identity meets B's emotional life with native compatibility. Squares and oppositions: structural friction, often productive over time but tiring in the short term. Conjunction: deep recognition, but if too tight it can feel like sharing one nervous system.
Sun-Ascendant contacts
Person A's Sun aspecting Person B's Ascendant means A's identity strikes directly at B's body and presentation. Conjunctions are often the "you walked in and I knew" recognition. Squares and oppositions describe a relationship where one partner's existence feels like a comment on the other's self-presentation.
Moon-Moon contacts
Often underweighted by beginners. Two Moons in compatible signs share the same emotional dialect. Two Moons squared describe 2 people whose default emotional reactions are at right angles to each other — manageable, but it is labor.
Venus, Mars, and chemistry
If the luminaries describe the daily fit, Venus and Mars describe the chemistry. Astrologers of all schools agree on this even when they disagree on much else.
Venus to Venus describes shared aesthetic and shared idea of pleasure. Two Venuses in the same element rarely struggle to enjoy the same things. Venus to Mars is the classic erotic indicator — A's Venus conjunct B's Mars is the configuration most often cited as "instant attraction." Trines and sextiles are smooth chemistry; squares and oppositions are charged chemistry, the kind that produces both heat and conflict.
Mars to Mars describes how the couple fights. Two Marses in compatible signs argue with the same vocabulary. Two Marses squared describes a couple whose conflicts escalate quickly because each partner's natural fight-style triggers the other's.
One caution: chemistry indicators are not predictions of longevity. Many of the most charged Venus-Mars connections produce relationships that burn brilliantly and end. Pair chemistry with the structural indicators — luminaries, Saturn — to read the long-term.
Saturn — commitment and friction
Saturn in synastry is the planet that asks "is this serious?" Saturn contacts between two charts describe weight, structure, and obligation. They are the indicators that distinguish a fling from a relationship.
Saturn-Sun, Saturn-Moon, Saturn-Venus: these are the classic commitment markers. They indicate that one partner provides structure, weight, or limit-setting for the other's identity, emotion, or affection. Trines and sextiles read as stabilizing. Conjunctions, squares, and oppositions can read as binding or constricting depending on the maturity of both parties.
Saturn synastry in the hands of an inexperienced reader becomes catastrophizing. The traditional reading is more nuanced: Saturn aspects ask both people to do work. They are the configuration most likely to produce a relationship that lasts decades, and the configuration most likely to produce one that crushes the partners. Which it becomes depends on whether the people involved take Saturn's invitation to build seriously. The Saturn return guide covers the structural side of Saturn at length.
Outer planets — the karmic vocabulary
Uranus, Neptune, and Pluto in synastry describe the relationship's deep vocabulary. They move slowly enough that 2 people of similar age share generational placements; what makes them personal in synastry is when they hit a partner's personal planets.
- Uranus contacts describe shock, surprise, freedom, and the unexpected. A's Uranus conjunct B's Venus is the "love hits like lightning" pattern. Beautiful when fresh; demanding to live with long-term.
- Neptune contacts describe dissolving boundaries and shared imagination. A's Neptune on B's Sun can produce art, devotion, and idealization — sometimes all 3 at once. The risk is projection: not seeing the actual person.
- Pluto contacts describe transformation and intensity. A's Pluto on B's Venus is the "this person changed me" connection. Powerful and binding; needs maturity to handle without becoming controlling.
House overlays
Each of A's planets sits somewhere in B's chart by house. The house tells you the area of B's life that A activates, regardless of aspect.
- Planets in the 1st house — A profoundly affects B's identity and self-presentation.
- Planets in the 4th — A becomes part of B's home, family, and inner foundation.
- Planets in the 5th — romance, creativity, children, pleasure — the classic overlay for romantic synastry.
- Planets in the 7th — A is a partner-figure for B in the literal sense.
- Planets in the 8th — depth, shared resources, intimacy, the parts of life that do not easily come up in conversation.
- Planets in the 10th — A becomes part of B's career, public life, or vocational sense of self.
- Planets in the 12th — A activates B's hidden, unconscious, or solitary parts; sometimes feels fated, sometimes feels invisible.
Read both directions. A's overlay into B's chart and B's overlay into A's chart are different. Many imbalanced relationships show up here: A is in B's 10th but B is in A's 12th — A is publicly important to B, but B is privately important to A. That asymmetry is worth surfacing.
Composite vs synastry
The composite chart is a third chart computed by midpointing the two natal charts. The midpoint Sun, midpoint Moon, midpoint Ascendant become the composite Sun, Moon, Ascendant. The chart describes the relationship as a third entity — what it wants, what its rhythms are, where it sits in the world.
The composite chart is the chart you transit. When you ask "what is coming up for the couple?" you read transits to the composite chart, not to either natal chart. A composite Saturn return (the relationship hitting its 29th year, or the composite Saturn at its return point) is its own structural moment, distinct from either partner's individual Saturn returns.
Synastry and composite answer different questions. Synastry: how does each of these people feel inside this connection? Composite: what is the relationship as its own thing? Read both.
Timing a relationship
The most actionable use of synastry is timing. Three layers stack:
- Synastry transits. Current transits to either partner's natal chart, especially to the planets activated by the other's chart. Astrolium tracks transit Saturn, Jupiter, and the lunar nodes against both natals.
- Composite transits. Current transits to the composite chart, especially to its angles and luminaries. A composite Saturn return is a relationship's structural moment.
- Profections of the composite. The relationship has its own annual time-lord, profected from the composite Ascendant. See the profections guide for the year-lord method.
When transit Saturn squares the composite Sun, the year is structurally heavy for the couple. When transit Jupiter trines both partners' Venuses, the year is generous to the connection. When the composite is in a 7th-house profection year ruled by a planet that is being transited hard, the year is dominated by partnership questions.
Reading synastry ethically
Synastry is the most ethically loaded technique an astrologer practices. People bring synastry questions in moments of vulnerability: should I marry this person? Should I leave? Why does this hurt? The reading shapes choices the astrologer does not have to live with.
A few principles working astrologers tend to converge on:
- Read with both charts present and informed consent from both parties when possible. Reading one person's chart against another's without that other person's awareness is a grey-zone move.
- Avoid verdicts. "You will never be happy with this person" is outside what astrology can support, even when the chart looks rough. State what you see; let the client decide what it means.
- Name strengths first. Most synastries contain real connection. Lead with it. The frictions land better when the foundation has been seen.
- Hold ambivalence. Hard synastry is not doomed; easy synastry is not safe. Both contain risk and possibility. Say so.
Common mistakes
- Reading aspects without orbs. A 9 degree "square" is a coincidence, not a contact. Use 3 to 5 degrees for personal planets in synastry.
- Sun-sign compatibility. "We are both Tauruses!" is not synastry. Sun signs are 8 percent of the chart.
- Treating Saturn as fatal. Saturn synastry is the architecture of long relationships. Heavy, not bad.
- Forgetting the composite. Reading synastry without the composite is reading 2 people who happen to be in the same room. The composite tells you about the room.
- Missing the asymmetry. A's chart on B's is not the same as B's on A's. Read both directions.
What to read next
Synastry is a static reading. To time the year, run profections on each partner's natal chart and on the composite. To read the era, run zodiacal releasing on both partners — relationships often pivot at the major L1 changes. And Saturn returns arriving at different ages for each partner produce some of the most predictable relationship pivots there are.
To compute synastry on your own charts, the free synastry calculator takes 30 seconds. To run synastry on every couple in your client roster with the timing layer attached, see pricing for the $29 per month Pro plan — Astrolium spots which couples are in a heavy Saturn year before they call you.
