Synastry: How Chart Comparison Actually Works

Sun-sign compatibility columns (“Aries and Libra: sparks!”) are to synastry what a horoscope is to a birth chart: the same vocabulary with the resolution turned down to zero. Synastry lays two complete charts over each other and asks a sharper question. Where exactly does my system plug into yours?

The core move

Take person A’s chart and place person B’s planets around the outside of it. Every meaningful contact is one of two kinds:

1. Planet-to-planet aspects. B’s Saturn squares A’s Moon; B’s Venus trines A’s Mars. Each contact is a measured angle, and the practice treats each one as a circuit with its own character.
2. Planets in houses. B’s planets land somewhere in A’s twelve houses, which says which department of A’s life the reading puts B in. Someone whose Sun falls in your 10th is read into the department of ambition and public standing; the same person’s Sun in your 12th is read into the private, unlit part of the chart instead.

Then you flip it and read the other direction, because synastry is asymmetrical. You can sit in my 7th house while I sit in your 3rd: a marriage on one side, a conversation on the other.

That asymmetry is the whole reason to run the overlay twice, and it is the step most compatibility writing skips.

The contacts worth checking first

The tradition puts a short list first. None of it is a finding; it is where astrologers have long agreed to look, and it is enough to keep a first pass from wandering.

• Sun–Moon contacts. The classic marriage signature in the literature: one chart’s engine landing on the other’s needs. Conjunctions, trines, and even squares here get read as relevance, comfortable or not.
• Moon–Moon. Do the two need-systems co-regulate or collide? Two Moons in square can love each other deeply and still soothe each other badly. (Deeper dive: Moon sign compatibility.)
• Venus–Mars. The attraction circuit, in both directions. Soft contacts are read as desire that is legible to both people; hard ones as desire that is urgent and still negotiable.
• Saturn contacts. The most underrated. Saturn on a partner’s personal planet is read as weight: commitment, structure, duration. The same contact is also the one read as criticism and control, and nothing in the geometry chooses between those two. You will see this stated more confidently, usually as the claim that Saturn synastry is the reason long relationships last. That is a claim about frequency, and there is no count behind it.
• Outer-planet contacts (Uranus, Neptune, Pluto). The slow, generational bodies landing on someone’s personal planets are what astrologers point at when a relationship gets called fated: electric, dissolving, or transformative, in that order. Read them as a description of a register, not as a forecast of one.

Orbs are stricter here

In synastry, tighten everything. A cross-chart aspect at 2° is where the practice says to look; at 6° it’s barely a hum. Judge that by the number and not by recognition. A contact you can talk yourself into at 6° is still a 6° contact, and personal resonance is a poor instrument for deciding which aspects count. (The grammar of aspects is in Aspects explained.)

What synastry is not

• It is not a verdict. There are no unworkable combinations, only circuits with different maintenance costs. A chart comparison describes the wiring; the two of you decide what gets built on it.
• It is not complete without both birth times. House overlays (half the technique) need real Ascendants on both sides. With one time missing you can still read planet-to-planet contacts honestly, and skip the rest. (Why the time matters so much.)
• It is not a search for your chart’s twin. Matching placements are not what the technique looks for, and the contacts astrologers find most interesting are the squares and the Saturn ones rather than a parade of trines. That is a statement about where the tradition’s attention goes. Whether the relationships built on those contacts turn out to be the important ones is your own evidence to weigh.

Doing it properly

Start with two accurate charts (get yours free, and have your person do theirs), then work the short list above in both directions. Write down what you find before either of you says it out loud. A contact you named in advance is much harder to argue into whatever mood the evening is already in.

Part of the Compatibility series, alongside Moon sign compatibility and Venus placements in love.