How to read a synastry chart
Start with the wheel. The first person’s planets sit on the inner ring in gold, the second person’s on the outer ring. Every line across the middle joins one planet from each chart. Green lines are trines and sextiles, angles that tend to feel easy. Red dashed lines are squares and oppositions, where the two of you push on each other. Gold lines are conjunctions, where a planet of yours sits right on top of one of theirs and the two are hard to separate.
Thicker lines are tighter aspects between more personal planets. A Moon to Moon trine within one degree matters far more than a loose Neptune sextile, so the table below the wheel is sorted by strength, not by planet.
The aspects that carry the most weight
Most synastry readings come back to a handful of contacts. Sun and Moon aspects describe whether one person’s sense of self sits comfortably with the other’s emotional needs. Venus and Mars describe attraction and how affection is shown. Mercury contacts show whether conversation flows or turns into debate. Saturn contacts are the glue and the weight of a relationship: they make things feel serious and lasting, and sometimes heavy.
A chart full of easy aspects is pleasant but can feel flat. A few tense aspects usually explain the spark, so don’t read red lines as a verdict. They describe where the two of you will have to negotiate.
House overlays: where each of you lands
When both birth times are known, the calculator also places each person’s planets in the other’s houses. If your Venus falls in their 7th house, they tend to experience you as a partner. Your Mercury in their 3rd house shows up as daily conversation. Overlays work in both directions and are rarely symmetrical, which is why one person can feel a relationship very differently from the other.
How this calculator works
Planet positions come from astronomy-engine, an open-source ephemeris accurate to well under an arcminute for the dates people are born. Birth places come from the GeoNames database, so the time zone and daylight saving rules are the ones that applied on the day. The score at the top weighs every aspect by orb and by how personal the planets are; it is a summary of the chart, not a prediction.
Questions people ask
What is a synastry chart?
Two birth charts drawn on one wheel: one person’s planets on the inner ring, the other’s on the outer ring. The lines between them are aspects, the angles one person’s planets make to the other’s. Astrologers read those lines to describe how two people affect each other.
Do I need an exact birth time?
No. Without a time we cast the chart for noon, which keeps the Sun and outer planets exact and leaves the Moon within about 7°. What you lose is the Ascendant and the house overlays, which depend on the time to within a few minutes.
Which house system and zodiac do you use?
The tropical zodiac, as most Western astrologers use. House overlays use Whole Sign houses, so each sign is one house counted from the Ascendant.
What orbs are allowed?
Conjunctions and oppositions up to 8°, squares and trines up to 7°, sextiles up to 4.5°, widened by 10% when the Sun or Moon is involved. Tighter aspects get more weight in the score.
Is a synastry chart the same as a composite chart?
No. Synastry keeps two charts and compares them; it shows what each of you triggers in the other. A composite chart merges them into one chart of midpoints that describes the relationship as its own thing.