Quick answer: Synastry compares two natal charts by overlaying them and reading the aspects, the angular relationships, between one person's planets and the other's. Each contact shows how a pair of energies meets: Venus trine Mars flows, Saturn square Moon steadies yet restricts. It maps relational tendencies, not a compatibility score or a fixed outcome.
Two people rarely feel the same thing from the same connection, and synastry is the technique that shows why. By placing one chart against another, it turns the meeting of two lives into a readable set of symbolic contacts you can actually work with.
What synastry is, and what it is not
Synastry is the branch of relationship astrology that measures the aspects formed between one person's planets and another's. The word itself comes from the Greek syn, meaning together, and astron, meaning star: a bringing-together of two skies. Traditional astrology, from the Hellenistic period through the Renaissance, read partnership mainly inside a single chart, through the seventh house and its ruler and natural significators such as Venus and the Moon. The systematic side-by-side scoring of two whole charts, with its emphasis on Sun-Moon and Venus-Mars, is largely a twentieth-century development. For the groundwork, see what synastry actually is. What synastry is not is a verdict. It describes how two people's energies tend to interact, read from both sides, never a compatibility percentage or a forecast of how things will end.
Which aspects flow and which create tension
The aspects come from harmonic divisions of the zodiac set out in Ptolemy's Tetrabiblos (c. 2nd century CE): opposition at 180 degrees, trine at 120, square at 90, and sextile at 60. Conjunction at 0 degrees is strictly co-presence in the same place rather than an aspect, though modern practice lists all five together. Sextile and trine are the harmonious contacts and tend to flow; square and opposition are the tense ones and create workable friction. Contacts weigh more the tighter they are: a synastry orb usually runs from about 1 to 8 degrees, with the luminaries allowed the widest reach and an exact aspect counting for far more than a loose one. Note that the semisextile (30 degrees) and quincunx (150 degrees) are not classical aspects at all; in the older tradition those signs stand in aversion and are said not to see one another. For the full set, see astrology aspects explained.
The core contacts
A handful of pairings carry most of the weight in a comparison. Sun-Moon blends core identity with emotional nature and is a classic marker of day-to-day rapport. Venus-Mars links affection to desire and describes romantic and sexual attraction. Moon-Moon shows how two emotional styles meet, and Moon-Venus adds tenderness and care. The rulerships behind these matter: Venus, ruler of Taurus and Libra, is the lesser benefic and the natural significator of love, while Mars rules Aries and Scorpio and supplies drive. This cheat-sheet gives the flavor of each contact by aspect type.
| Contact | What it governs | Flowing (conjunction/trine/sextile) | Hard (square/opposition) | Tradition | |---|---|---|---|---| | Sun-Moon | Core identity meets emotional nature | Natural rapport, feeling at home together | Different rhythms to reconcile | Traditional | | Sun-Venus | Affection and liking | Warmth, easy fondness | Vanity or mismatched tastes to navigate | Traditional | | Venus-Mars | Romantic and sexual attraction | Magnetic, easy chemistry | Charged friction, push and pull | Traditional | | Moon-Moon | Emotional styles meeting | Instinctive understanding | Moods that clash and need space | Traditional | | Moon-Venus | Tenderness and care | Gentle, nurturing ease | Over-giving or differing needs | Traditional | | Sun/Moon/Venus-Saturn | Commitment vs. restriction (greater malefic) | Steadying, durable, loyal | Weight, duty, or feeling constrained | Traditional | | Mars-Mars | Drive and initiative | Shared momentum | Competitiveness and friction | Traditional | | Venus-Neptune | Idealization | Romance, compassion, artistry | Illusion or disappointment to see clearly | Modern | | Sun/Moon-Pluto | Intensity and depth | Transformative closeness | Power struggles to disarm | Modern | | Moon-Uranus | Excitement and change | Refreshing spark | Instability and unpredictability | Modern |
Saturn and the outer planets
Saturn is the greater malefic, the planet of structure and limit, ruler of Capricorn and Aquarius. A Saturn contact to another person's Sun, Moon, or Venus reads as durable but heavy: it can commit and steady a bond, and it can also weigh on it. This is the point people most often get backwards, so hold it clearly: Saturn supplies commitment and restriction, Venus supplies ease and affection. The outer planets are modern additions, discovered long after the traditional system was complete: Uranus in 1781, Neptune in 1846, Pluto in 1930. Their synastry contacts, Uranus for excitement and instability, Neptune for idealization, Pluto for intensity and depth, were unavailable to older astrologers, who worked only with the seven visible planets. Treat them as texture layered over the classical core, not as the foundation.
Reading a contact both ways, and working with the friction
Every synastry contact is two-directional, because each person experiences it from their own side. With one partner's Saturn on the other's Moon, the Saturn person tends to feel responsible for, and sometimes controlling of, the Moon person, while the Moon person feels both stabilized and constrained. No single contact yields one shared conclusion. That is also why the hard aspects are not doom. A square or opposition marks a growth edge, a place where two energies pull differently and ask for adjustment, and repeated patterns across the whole chart matter far more than any one line. For how those patterns stack up, see chart aspect patterns. Tighter orbs and reinforcing themes strengthen a tendency, but it stays a tendency, shaped by two people's effort and free will.
Frequently Asked Questions
Does one difficult aspect mean we are incompatible?
No. A single square or Saturn contact never makes or breaks a bond. Synastry reads the whole picture: repeated themes, the balance of easy and challenging contacts, and how both people choose to work with what is there. One line is one thread, not the verdict.
How tight does an aspect need to be to matter?
The closer to exact, the stronger. Most astrologers work within roughly 1 to 8 degrees of orb, giving the Sun and Moon the widest allowance and the other planets a narrower one. A contact within a degree or two is felt keenly; a contact near the edge of orb is a quieter background note.
Are the outer planets part of traditional synastry?
No. Uranus, Neptune, and Pluto were discovered in 1781, 1846, and 1930, long after classical astrology was formed, so any outer-planet contact is a modern addition. Traditional practice used only the Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn.
Is synastry a prediction of whether a relationship will last?
No. Synastry describes tendencies and dynamics, the structure of how two people's energies meet, not outcomes or dated events. It shows where things flow and where they take work; what you do with that is a matter of choice, not fate.
Compare your two charts
Synastry is most useful when you read it as a map of tendencies to understand together, not a scorecard. Cast an accurate free chart for each of you, deepen your reading with a personality report, and explore more foundations on the blog.
