Relationship Astrology
The Synastry Chart, Explained
A synastry chart lays two birth charts over one another to read the chemistry between two people — the easy currents, the friction, and the deep gravitational pulls. Here's how it works, which aspects matter, and how to cast your own from the real sky.
Two full charts overlaid — measured aspects, not Sun-sign shortcuts
Build Your Free Synastry Chart →What Is a Synastry Chart?
Two complete charts, compared planet by planet — the oldest and most detailed technique in relationship astrology.
A natal chart is a snapshot of the sky at one person's birth. Synastry (from the Greek for “star alignment”) takes two of these snapshots and overlays them, measuring the angles — the inter-aspects — that one person's planets make to the other's. When your Venus sits close to their Moon, or your Mars squares their Saturn, those contacts describe a specific, repeatable dynamic between you.
Where a single horoscope generalises across millions of people who share a Sun sign, synastry is about the space between two specific charts. It doesn't predict whether a relationship will “work” — no honest astrology does — but it maps where connection comes easily, where you'll have to do conscious work, and what keeps pulling you back toward each other.
The result is far richer than “a Leo and a Scorpio.” It's a portrait of how two whole skies meet: your communication styles, your emotional needs, your drives, and your long-term wiring, all measured against each other.
The Planets That Carry a Relationship
Every planet plays a role, but four do the heavy lifting in romantic synastry.
Sun — Identity
How your core selves recognise and energise one another. Strong Sun contacts bring mutual respect, visibility, and a sense of being seen for who you really are.
Moon — Emotional Safety
The single most important factor for lasting closeness. Moon contacts govern comfort, instinctive understanding, and whether you feel at home with each other.
Venus — Affection
Love languages, tenderness, taste, and what you each find beautiful. Venus contacts are the warmth and the romance — the pleasure of simply being together.
Mars — Drive & Spark
Desire, initiative, and the friction that becomes either passion or conflict. Mars contacts supply the heat; the aspect decides whether it warms or burns.
Reading the Inter-Aspects
An aspect is the angle between two planets. The angle — not just the planets — sets the tone.
Conjunction · 0°
Two planets fused together. Intense and unmistakable — their energies merge for better or worse, depending on which planets meet.
Trine · 120° & Sextile · 60°
The flowing aspects. Ease, support, and natural harmony — gifts that can be taken for granted precisely because they feel effortless.
Square · 90°
Dynamic tension. Friction that demands growth — often the source of both the strongest attraction and the most repeated arguments.
Opposition · 180°
Polarity and magnetism. You see in each other what you lack — a balancing act that can feel like completion or like a tug-of-war.
How to Build Your Synastry Chart
AstroPrecise computes both charts and their inter-aspects from real planetary positions — free, and entirely in your browser.
Open the Compatibility tool and enter the first person's birth date, time, and place. Exact times unlock the Moon, Ascendant, and house overlays.
Enter the second person's details the same way. Each chart is calculated independently from the same precise astronomy engine.
Read the overlay: the strongest inter-aspects, where the two charts harmonise, and where they challenge each other — then share the result with a private link.
Synastry FAQ
What is a synastry chart?
A synastry chart overlays two people's birth charts to study the angles their planets make to each other. These inter-aspects describe the chemistry, friction, and rhythm between the two people across love, communication, drive, and commitment.
What is the difference between synastry and a composite chart?
Synastry compares two separate charts to each other, planet by planet. A composite chart instead merges the two charts into a single new chart that represents the relationship itself as a third entity. Both are valid lenses; synastry is the more common starting point.
Do I need both people's birth times for synastry?
Birth times sharpen the picture — they fix the Moon precisely and unlock the Ascendant, house overlays, and angle contacts. Synastry can still be read from dates alone using the slower-moving planets, but exact times make it far more specific.
Which synastry aspects matter most?
Sun, Moon, Venus, and Mars contacts are the heart of romantic synastry. Conjunctions, trines, and sextiles tend to flow easily; squares and oppositions create tension that can be either charged attraction or recurring friction depending on the planets involved.
See Your Two Charts Meet
Cast a full synastry chart from real planetary positions — two complete charts, their strongest inter-aspects, and a shareable compatibility reading. Free, with nothing to sign up for.
Build Your Free Synastry Chart →No account · No tracking · Computed on your device