Most people know their Sun sign compatibility — "Scorpios and Pisces get along well" or "Aries and Cancer clash." But Sun sign compatibility is a shortcut, not a full picture. Synastry is the fuller astrological comparison.
Synastry is the branch of astrology that compares two complete birth charts — not just Sun signs, but every planet, house, and angle — and reads the combinations as themes of ease and friction between two people. It is an interpretive practice; there is no scientific evidence that chart comparisons predict relationship outcomes.
How Synastry Is Different from Sun Sign Compatibility
Sun sign compatibility compares two zodiac signs in isolation. It's like judging a chess match by looking at only one piece on each board.
Synastry compares every planet in both charts. That means ten planets for Person A interacting with ten planets for Person B — up to 100 possible connections, each with its own meaning. A Scorpio Sun and Gemini Sun might seem incompatible on paper, but if their Moons are in trine and their Venus signs align, astrologers would read the combination as more harmonious than the Sun signs alone suggest.
In astrological practice the Moon sign is usually weighted more than the Sun for emotional compatibility, Venus and Mars are read for attraction and desire, and Saturn for commitment.
The Three Layers of Synastry
1. Planet Overlays
When you lay one person's chart over another's, each of your planets falls into one of their 12 houses. Astrologers read this as where each person's influence is felt in the other's life.
For example: if your Venus falls in your partner's 7th house (the house of partnerships), astrologers read that as a partnership theme: you may be associated, in their chart, with love and commitment. That is an interpretation, not a statement of how your partner actually feels; only they can tell you that. A Saturn in their 12th house is read as a theme of quiet discipline or restriction.
2. Cross-Chart Aspects
These are the angular relationships between planets in different charts — when Person A's Mars lines up at a significant angle with Person B's Sun, for example. These aspects are the heart of synastry, and they describe the quality of interaction between specific parts of each person's personality.
Traditionally, a conjunction (0°) is read as an intense blending, a trine (120°) as easy flow, a square (90°) as productive tension and an opposition (180°) as attraction through contrast. There is no inherently good or bad aspect — each has gifts and challenges.
3. House Overlays
Beyond where planets land in the other person's houses, you can also look at which houses are activated by the other person's chart overall. A cluster of someone's planets in your 5th house (creativity, romance, joy) is read as a theme of play and lightness; a concentration in your 8th house (shared resources, depth) as a theme of intensity. These are symbolic readings, not predictions of how the relationship will go.
The Most Important Synastry Planets
Astrologers do not weight every connection equally. The planets below are the ones usually given most weight.
Sun — Core identity, ego, life purpose. Sun-to-Sun and Sun-to-Moon connections are read for compatibility of character.
Moon — Emotions, instincts, inner needs. Many astrologers treat Moon connections as the most important for day-to-day emotional comfort, reading harmonious Moons as a theme of ease together.
Venus — Love, pleasure, values, what you find beautiful. Venus connections are read for romantic attraction and shared taste.
Mars — Desire, drive, how you take action. Mars connections are read for physical chemistry and how each person's drive meets the other's. Mars-Venus aspects are the classic attraction signature in astrological tradition.
Saturn — Structure, commitment, discipline, lessons. Astrologers often give Saturn aspects weight in long-term relationships, reading them as themes of commitment and of effort or testing. They do not determine whether a relationship lasts.
What Synastry Can and Can't Tell You
Read as symbolism, synastry can offer:
- Themes astrologers read as ease and flow
- Themes astrologers read as friction
- Areas the tradition associates with growth
- A symbolic picture of the connection's emotional tone
- Prompts for conversation between the two people
What it cannot do:
- Predict whether a relationship will last or "work"
- Override the choices, maturity, and effort both people bring
- Tell you whether to stay or leave
Two charts with many challenging aspects can describe a deeply meaningful, long-lasting relationship. Two charts with nothing but harmonious aspects can still fail if both people refuse to show up. Synastry is at most a set of prompts for conversation; the relationship is decided by the two people in it.
How to Use Synastry Practically
If you want to explore your synastry, you need accurate birth data for both people: date, time, and place. The time is especially important — it determines house positions and the Rising sign, which affect the overlay layer significantly.
Once you have both charts, look first at the major cross-chart aspects: Sun-Moon, Venus-Mars, Moon-Moon, and Saturn-to-personal planets. Astrologers treat these as the main themes for emotional compatibility, attraction and commitment.
AstroMystra's Oracle plan includes a full synastry reading — comparing both birth charts with AI interpretation of the cross-chart aspects, house overlays, and relationship dynamics. You get a written interpretation covering the traditional themes for emotional compatibility, communication and attraction. It is a symbolic reading for reflection, not a forecast of whether the relationship will last.