Composite Chart Calculator — and the Davison Relationship Chart, in One Tool

What composite and Davison charts are

Both techniques answer a different question than synastry does. Synastry compares two people; a relationship chart merges you into one chart that describes the partnership itself — its style, its purpose, the room it needs to grow.

The composite takes the mathematical midpoint of each pair of like planets (your Sun with theirs, your Moon with theirs, and so on) and also midpoints your Ascendants and Midheavens, producing one zodiac wheel for the couple.

The Davison (also called the composite of relationships or time-space chart) instead casts a completely ordinary natal chart — real ephemeris, real houses — for the couple's averaged moment and averaged location: the midpoint in time and space between your two births. This tool builds both from the same two birth charts, so you can see which lens speaks to you.

How we compute them

First we cast both natal charts independently, exactly as our natal calculator does — planetary positions from sub-arcsecond astronomy (VSOP87/ELP via astronomy-engine), Porphyry quadrant houses (whole-sign available on every chart). We approximate nothing in either chart for the merge.

For the composite, we take each planet-pair midpoint along the shortest arc (so a 350°/10° pair midpoints through 0°, not 180°). We then build the composite's house structure from the midpoint of the two Ascendants and Midheavens, using the same Porphyry division as a natal chart, so the relationship chart gets its own twelve houses.

For the Davison, we average the two birth instants and the two birth coordinates and cast a genuine chart at that event — for example, two births six years apart produce a chart for the moment halfway between them. Because it is a real chart-casting, the Davison's degrees are ephemeris outputs, not averages: a good sanity check is that its Sun usually sits near the composite Sun.

What you get

Once both charts are entered, in two switchable views:

• Composite Sun, Moon and Rising signs with their traditional readings and houses.
• The full composite placement table with houses recomputed from the composite angles.
• The composite's internal aspects — the relationship chart talking to itself — tightest first.
• The Davison chart: its exact midpoint instant and location, its own signed, housed placements.

How to read the two charts differently

Astrologers typically treat the composite as the more psychological chart — what the relationship feels like from inside as a single entity. A composite Sun in Pisces in the 9th, say, is read as a couple whose shared purpose runs toward compassionate, exploratory meaning-making, however unevenly the two individuals contribute to it.

The Davison is traditionally treated as the more event-oriented chart: because it has real transits and progressions running through it, practitioners use it for timing — when the relationship itself is entering a Saturn season or a solar-return year. Neither claim is a forecast; both are descriptive lenses. A sensible workflow is to read the composite for the relationship's character and keep the Davison for its calendar.

A worked example, with real numbers

Take a partner born 1 January 2000, 17:30 UTC in London (Sun 10.6° Capricorn, Ascendant 28.8° Cancer) and a second born 5 May 1990, 13:15 UTC in New York (Sun 14.8° Taurus, Ascendant in Cancer). Their composite Sun — the midpoint of Capricorn 10.6° and Taurus 14.8° — falls at 12.7° Pisces, measured the short way through Aquarius; the composite Moon lands at 22.0° Libra, and the midpoints of the two Cancer ascendants put the composite Rising at 18.8° Cancer.

The Davison of the same pair is cast for the averaged instant, 4 March 1995 at 15:22 UTC, at the averaged location of London and New York. Its Sun comes out at 13.6° Pisces in the 9th house — within about a degree of the composite Sun, as the geometry predicts — and its Ascendant at 18.9° Cancer, a near-clone of the composite Rising. Both charts describe the same couple; one is built from midpoints, the other from the sky itself.

What this tool does not claim

A relationship chart is a construct, not an observed thing: no birth actually happened at the midpoint moment, and there is no evidence that such charts predict how partnerships unfold. Controlled tests of astrology have found no such effect (see the Carlson study we cite below). Read both charts as projective symbolism — a structured way to talk about shared patterns — not as diagnostics.

Practically, both charts inherit the accuracy of their inputs: if a birth time is unknown, we estimate that person's Ascendant, which flows into the composite Rising and skews the Davison's angles too. Treat the houses as indicative in that case. All computation happens locally in your browser; neither person's data is transmitted.

Composite vs. Davison vs. synastry

Synastry keeps two people distinct and maps the aspects between them — best for 'how do we trigger each other?'. The composite fuses you into one chart of midpoints — best for 'what is this thing we are?'. The Davison casts a real chart at your averaged time and place — best for 'when is the relationship up to something?', since it supports standard transits and returns.

The three are complementary readings of one pair, and this tool gives you the latter two side by side; the cross-aspects live in the synastry calculator.

Our method in one line

Two independently natal-accurate charts (sub-arcsecond astronomy (VSOP87/ELP via astronomy-engine), Porphyry quadrant houses (whole-sign available on every chart)) combined by shortest-arc midpoints for the composite, and re-cast at the averaged instant and coordinates for the Davison. Reviewed by Dr. Amara Osei, Astronomy Reviewer.