What synastry is
It is built on two natal charts that are already cast, so both birth times and places have to be right. A single chart answers "what is this person’s structure"; a synastry chart answers "what happens when two structures meet" — the same Venus reading completely differently in someone’s 5th house than in their 12th, which is only visible once both charts are overlaid.
What the chart contains
House overlays
Which of A’s planets fall in which of B’s houses, and the other way round. This is the densest layer in synastry: the same planet in a different house describes a different everyday situation.
Inter-aspects
Aspects between A’s planets and B’s, with the real separation and the orb limit (conjunction/opposition 8°, trine/square 7°, sextile 4°, minor 1.5–2.5°, plus 1° with a luminary), and whether each one is applying or separating.
Composite chart
The midpoints of the two charts form a third chart that stands for the relationship itself. Ascendant and Midheaven are midpointed too, and houses are equal, because a composite Ascendant need not coincide with any house system’s cusp.
Davison chart
Midpoint the two birth instants and the two birthplaces, and you get a chart for a real moment at a real place. Its houses and aspects are self-consistent, unlike a pure midpoint chart.
Five themes
Emotional bond, communication, attraction and friction, stability and commitment, growth and space — each listing the inter-aspects that fall in it, counted as harmonious or tense. No score is produced.
How it is computed
1.Cast both charts
Each birth record is restored to a UT instant using its own historical timezone, then the ten planets, the Ascendant and the twelve houses are computed from the ephemeris.
2.Compare the two
Every planet in A is paired with every planet in B, tested against the orb table, and marked applying or separating from the pair’s relative speed.
3.Project the houses
Each chart’s own cusps are used to place the other person’s planets by house; houses 1, 5, 7 and 8 are called out as the relationship houses.
4.Build both midpoint charts
Short-arc midpoints of planets and angles form the composite; midpointing the instants and coordinates and re-running the same pipeline gives the Davison chart.
Chart timezone: each birth time is restored to a UT instant using its own birthplace timezone, including historical offsets and DST — exactly the same convention as the single natal chart. Western astrology does not apply true solar time.
Frequently asked
- What is a synastry chart good for?
- Questions about how the two of you interact: where emotions get met, where communication slips, where the pull comes from, where friction comes from, and what holds things up over time. It is not a tool for deciding whether to stay.
- How is this synastry computed?
- Each birth record is restored to a UT instant using its historical timezone and cast as a natal chart; the two sets of bodies are then paired for aspects against the orb table, projected by house onto the other chart, and combined into composite and Davison charts. All deterministic.
- What is on the chart?
- House overlays in both directions, inter-aspects with type, separation, orb and applying/separating state, the relationship houses, the composite chart, the Davison chart, and the aspects grouped under five themes.
- Can I make decisions from this chart?
- For reference only — not a substitute for medical, legal, or financial advice. Synastry describes how two structures interact; it decides nothing for either person and predicts no outcome.
- Why is there no compatibility score?
- Because a score is not computed — it is assigned. Different tables weight the same aspects differently and can produce opposite numbers for the same pair. This tool gives the checkable material: aspects, separations and house placements.
You can cast your own chart now: enter your birth details and get House overlays, Inter-aspects, Composite in one pass.
Open a chart