Composite or synastry: which chart answers which question

Two people, two birth charts, and two very different ways to compare them. Synastry and composite get mixed up constantly, sometimes even inside the same app. They answer different questions, and knowing which one you are asking saves you from reading the wrong chart.

Synastry: how you act on each other

Synastry keeps both birth charts intact and measures the angles between them: your Moon against their Sun, your Venus against their Mars, all ten bodies against all ten. Every angle that falls within its tolerance, the orb, is a contact.

Synastry answers the question: what happens between us? Where one person’s Saturn sits on the other’s Sun, structure and pressure enter the room together. Where a Moon trines a Venus, ease shows up without anyone working for it. The contacts describe the currents that flow in both directions, and they are not symmetrical: your planets land in the other person’s chart differently than theirs land in yours.

If you want to see these contacts for a real pair of charts, the free synastry calculator computes all of them in your browser, with real degrees and disclosed orbs.

Composite: what the relationship itself looks like

The composite takes a different step entirely. For every pair of matching points, it computes the midpoint: your Sun and their Sun become one composite Sun, your Moon and their Moon one composite Moon, and so on through the whole chart. The result is a third chart that belongs to neither person.

The composite answers the question: what is this relationship, as a thing of its own? A couple where both partners are fiery can still have a composite Moon in Capricorn: the relationship itself runs on reliability, whatever the individuals feel. People who know a couple often describe the composite better than either partner’s chart, because the composite is what shows in the room when both are present.

One technical point worth knowing, because implementations differ: midpoints must be taken on the shorter arc. Two Suns at 10° Aries and 10° Libra have two candidate midpoints, 10° Cancer and 10° Capricorn, and the convention decides which one your app picks. Astraleria uses the shorter-arc midpoint for every composite point and says so; a tool that does not disclose its rule can silently give you the opposite sign.

Which one should you look at first?

Start with synastry. It is the more concrete of the two: each contact names two planets and two owners, so you can check it against lived experience. Does closeness really carry a Saturn weight? Is the friction really a Mars square, or just a bad week?

The composite becomes interesting once a relationship exists as a shared reality: a couple, a long collaboration, a band, a company founded together. It has nothing to say about a first date, because there is no third entity yet to describe.

Two charts, one caution

Both methods compute real angles from real positions, and both are worth exactly as much as the input data. A composite built from a wrong birth time inherits the error twice. And neither chart is a verdict: a demanding synastry contact is a place where work sits, not a prognosis. The calculation tells you where to look, and that is all it does.