Synastry vs composite chart

Synastry vs composite is two different relationship maps, not two names for one map. Synastry keeps both people. You calculate two natal charts, overlay the planets, and list aspects from person A to person B. A composite chart discards the two wheels as separate people and builds a third chart from midpoints. People search the comparison because apps use the words as if they were interchangeable. They are not.

SideraChart offers both as separate calculators. Each natal is tropical, Placidus by default, VSOP87 via astronomy-engine. Synastry aspects use the same orbs as a natal aspectarian: 8° conjunction and opposition, 6° square and trine, 4° sextile. Neither wheel is a verdict on whether to stay together, marry, or split assets. They are diagrams.

Synastry leaves both charts intact. On SideraChart, one set of glyphs is person A, the other is person B. The table lists inter-chart aspects, not the internal aspects of one natal. You are asking what happens when two skies share a kitchen: her Saturn on his Moon, his Mars on her Ascendant, a Sun–Sun trine.

Each person still needs their own birth date, time and place. If one time is unknown, that person’s Ascendant and houses drop out. You can still synastry planet-to-planet. You cannot honestly talk about “their planets in your seventh house” without that person’s rising sign. Unknown time = planets yes, houses no — per chart.

A composite takes each pair of natal planets (A’s Sun and B’s Sun, A’s Moon and B’s Moon, and so on) and finds the shorter-arc midpoint, then draws a new wheel. The result is the relationship as a third entity, not either person. It is not a Davison chart, which midpoints in time and space to a hypothetical birth of the relationship.

Composite houses still need a location convention and, in practice, a way to set angles. Read the composite as a separate instrument. Do not mix one person’s natal house meanings into composite midpoints without noticing you changed maps. SideraChart’s composite calculator is that third wheel, not a synastry overlay.

Garbage in, garbage out, twice. Wrong timezone on person B shifts their Moon and their Ascendant, which then either aspects person A wrongly in synastry or pulls every composite midpoint. Prefer certificates. If one clock is missing, say so and keep house-based synastry sentences off the table.

You do not average two rising signs to get a “couple rising” in synastry. That instinct belongs to composite midpoints, and even there it is a constructed angle, not a baby. Keep the methods in separate tabs and label which wheel you are reading.

Neither synastry nor composite tells you to commit, leave, have a child, or sign a contract. They describe contact patterns and a midpoint atmosphere. Easy Venus contacts are not a legal status. Saturn contacts are not a prison sentence. If you need advice of those kinds, you need a different professional.

Read natal charts first, separately. Two people with no relationship to their own Moons will not be rescued by a pretty synastry trine. The Big Three of each natal still come first. Then overlay. Then, if you want the third-entity view, composite.

Use the synastry calculator for overlays and inter-aspects. Use the composite calculator for midpoints. Same tropical engine, not sidereal, not Swiss Ephemeris. One free relationship run without an account; then sign in. Save individual natals if you also want daily transits in each cabinet at noon natal time — those transits are personal weather, not a couple forecast unless you build that separately.

When comparing with other sites, match tropical, Placidus, orbs, and whether they mean synastry or composite. A site that “combines charts” without saying midpoint vs overlay is the reason this article exists. Look at the wheel: two sets of planets means synastry; one set of mixed midpoints means composite.

House overlays in synastry need both clocks. If person B has no time, you still have planet-to-planet aspects with orbs 8° / 6° / 4°. You do not have “their Venus in your seventh.” Write that limit next to the table so the overlay does not grow houses it does not own.