What Is a Synastry Chart?
A synastry chart — sometimes called a compatibility chart or partner chart — overlays two natal charts to show how their planets interact. Where a natal chart maps you, a synastry chart maps the dynamic between two people.
Each person's planets sit on a different ring. Lines connect them when they form meaningful angles (aspects). Trines and sextiles are the easy connections — the places where your energies flow naturally together. Squares and oppositions are the friction points — not "bad," just where the work happens. Conjunctions amplify whatever planets meet there.
The Compatibility Score: What It Actually Measures
The score is computed from five classical synastry indicators that astrologers have used for centuries:
- Sun-Moon contacts — the bedrock blend of identity and emotion. The strongest single predictor.
- Sun-Ascendant contacts — how seen each person feels by the other.
- Moon-Ascendant contacts — emotional safety and recognition.
- Venus-Mars contacts — love and desire, the chemistry indicator.
- Destiny sign — both Sun signs sharing the same modality (cardinal, fixed, or mutable).
Each contributing factor is shown in your breakdown, so you can see exactly why the score is what it is. A high score means the relationship has classical "easy fit" markers. A low score doesn't mean the relationship can't work — it means the fit isn't classically obvious. Some of the most interesting relationships score low.
What the Cross-Aspects Mean
Cross-aspects describe how you affect each other. His Sun trine her Moon reads very differently from his Mars square her Venus. The first describes ease and recognition; the second describes attraction with friction.
The most important cross-aspects involve the personal planets (Sun, Moon, Mercury, Venus, Mars) and the angles (Ascendant, Midheaven). Outer planet aspects (Jupiter, Saturn, and beyond) shape long-term themes, but they're less personal.
Synastry vs Composite vs Compatibility Score
Synastry overlays two charts to show interactions — it's what this page calculates. Composite charts mathematically merge two charts into one chart that represents the relationship as its own entity (a feature for a future update). Compatibility scores distill those interactions into a single number; useful as a starting point, but the cross-aspects themselves tell the real story.
Frequently Asked Questions
What if I don't know one person's birth time?
You can check the "I don't know this birth time" box to default to noon. The Sun-Sun, Moon-Moon, and personal-planet cross-aspects will still be reliable. The Ascendant, Midheaven, and house-related cross-aspects will not be reliable without an exact time. Since the score weights Sun-Ascendant and Moon-Ascendant heavily, missing time data does affect the score.
Is a low score a relationship dealbreaker?
No. The score measures classical "easy fit" indicators — the markers that tend to make a relationship feel comfortable from day one. Plenty of strong, durable relationships score in the Minimal or Medium range and grow into something deep through tension and mutual growth. Plenty of high-scoring pairs flame out. The score is one data point, not a verdict.
Why are there so many cross-aspects?
Because every planet in Person A's chart can form an aspect to every planet in Person B's chart — potentially over 100 connections in total. The chart shows only the major aspects (conjunction, trine, sextile, square, opposition) by default to keep it readable. The "Strongest Cross-Aspects" section below the chart highlights the ones astrologers consider most significant.
Is this for romance only?
No. Synastry works for any pairing — romantic partners, friends, business partners, family. The same cross-aspects describe how any two charts interact. The Venus-Mars aspects matter most for romance specifically; the Sun-Moon, Sun-Ascendant, and Moon-Ascendant matter for any close relationship.
How is this different from your sun-sign compatibility pages?
Sun-sign compatibility pages (like our Western sun-sign hub) give you a fast read based only on each person's Sun sign — useful when you don't have full birth details. This calculator uses everything: every planet, every house cusp, every cross-aspect between the two charts. It's the difference between a book's blurb and the book itself.
How accurate are synastry calculations?
The math is precise — planet positions are calculated from NASA JPL data via Swiss Ephemeris and accurate to fractions of a degree. The interpretation is observational, not predictive. Use the chart for self-awareness about a relationship's dynamics, not as a verdict on whether it can work.