What is synastry in astrology?
Synastry is a branch of astrology that studies compatibility between two people by comparing their natal charts. The method is based on analyzing cross-chart aspects - angular relationships between one partner's planets and the other's. Synastry helps understand relationship dynamics: where harmony arises, where tension exists, and where there's potential for growth. This method has been used by professional astrologers for over a thousand years and remains one of the most popular tools for relationship analysis.
How does the compatibility calculation work?
Our synastry calculator uses Swiss Ephemeris - the most accurate astronomical library, providing arcsecond precision. A natal chart is built for each partner based on their date, time, and place of birth. Then all aspects between planets of both charts are analyzed: conjunctions, trines, sextiles, squares, and oppositions. The result is a compatibility index showing the ratio of harmonious to tense interactions, along with a detailed list of the most significant aspects.
Types of aspects in synastry
Trine (120°) and sextile (60°) are harmonious aspects indicating natural understanding and ease of communication. Square (90°) and opposition (180°) are tense aspects that create challenges but simultaneously stimulate growth and transformation. Conjunction (0°) is the most powerful aspect that amplifies qualities of both planets, and can be either harmonious or challenging depending on the planets involved. Minor aspects (quintile, semi-sextile, quincunx) add nuance and detail to the overall picture.
The role of planets in synastry
The Sun and Moon are the foundation of compatibility: their aspects reveal basic character harmony and emotional connection. Venus governs romantic attraction and shared values, Mars - physical chemistry and energetic dynamics. Jupiter aspects bring optimism and expansion, Saturn - stability and responsibility (but sometimes restrictions). Outer planets (Uranus, Neptune, Pluto) point to deep transformational processes in relationships.
What synastry shows
Synastry overlays two natal charts and calculates the aspects between the planets of different people. It is not a verdict on whether you match, but a map of how exactly the two affect each other: where the pull is, where the friction sits, and where the support comes from.
The calculator builds the full cross aspect table with orbs, shows which houses of one partner your planets fall into, and scores the balance of harmonious against challenging links. Both charts use the same ephemeris as the natal tool, so the numbers reconcile with professional software.
- •Cross aspects between every planet of both charts
- •Overlay of one partner's planets onto the other partner's houses
- •Balance of harmonious and challenging aspects as a percentage
Which contacts carry the most weight
Luminaries and angles outweigh everything else. The Sun, the Moon, the Ascendant and the Midheaven in aspect to a partner's planets produce the kind of influence you feel daily. Aspects between the outer planets of two people from the same generation repeat in almost every pair and say little about this particular one.
Venus and Mars govern attraction, Saturn governs duration and commitment, the Moon governs everyday compatibility. A couple with strong Saturn and weak Venus often lasts for years without ease; the reverse combination gives a bright start and burns out quickly.
- •Sun and Moon contacts describe how the basic rhythms of two people fit
- •Venus and Mars show the pull and what kind of pull it is
- •Saturn supplies structure but demands adulthood from both
- •Lunar nodes in synastry mark the sense that the meeting was not accidental
Hard aspects are not a verdict
Squares and oppositions between charts create tension, and that tension is usually what keeps the interest alive. Couples with nothing but trines and sextiles describe their relationship as calm and, often, static. The question is not whether tension exists but whether both people are willing to work with it.
Check which planets form the hard aspect. A Moon-Saturn square hurts in daily life and needs explicit agreements; a Mars-Pluto square turns into a fight for control. Those are different problems and they call for different answers.
Synastry, composite and Davison: the difference
Synastry answers what two people do to each other. A composite chart is built from the midpoints between the planets of both charts and describes the relationship itself as a separate entity: where it is heading, what it lives on, what its shared task is.
A Davison chart is calculated differently: it takes the midpoint in time and in space between the two births and builds an ordinary chart for that moment. Unlike the composite it has a real date, so transits apply to it. The working order is synastry first, then composite, then Davison for forecasting on the couple.
- •Synastry shows the interaction between two people
- •The composite describes the relationship as a third entity
- •Davison provides a real moment that transits can be applied to
When the partner's birth time is unknown
Without a time you lose the houses, the Ascendant and the exact degree of the Moon. Most aspects between planets still hold, since slow planets barely move in a day. The Moon is the exception: it covers up to 15 degrees a day, so its aspects in such a synastry need a separate check.
The practical approach is to run two versions, one for the morning and one for the evening, and see whether the picture changes fundamentally. If the key aspects survive both, you can work with them. If they do not, finding the real time beats building conclusions on guesswork.
Working with the result
Start with the cross aspect table and pick the five tightest orbs. That is the skeleton of the relationship; everything else adds detail. Then look at the house overlays: a partner's planet in the fourth house speaks about home and family, in the tenth about career and status.
Both charts can be saved to your account and revisited later, and the pair converts to a composite or a Davison chart in one click without re-entering anything.
