Synastry
Comparing two birth charts side by side.
Overview
Synastry is the astrological technique of comparing two complete birth charts to describe the dynamics tradition associates with a relationship. It is the tradition's own method for relationship analysis, and it is interpretive rather than predictive.
- Synastry examines aspects between one chart's planets and the other's.
- House overlays show where each person's planets fall in the other's chart.
- A composite chart is a separate construction, built from midpoints.
- Requires accurate birth times for both people to be done as the tradition prescribes.
Inter-chart aspects
The core of synastry is measuring the angular relationships between the planets in one chart and those in the other. One person's Venus forming a trine to the other's Mars would be described as a Venus–Mars trine in synastry, and read according to the traditional meanings of both bodies and the aspect.
The calculation is the same geometry used within a single chart, applied across two. What differs is the interpretive frame: aspects between charts are read as describing interaction rather than internal disposition.
House overlays
The second component places one person's planets into the other's house structure. If someone's Jupiter falls in their partner's seventh house, tradition reads that as Jupiter's significations operating in the domain that house governs.
Because houses depend on birth time and place, overlays are only meaningful when both birth times are accurately known. An unknown time makes this component of the technique unavailable, which practitioners generally acknowledge rather than working around.
Composite and Davison charts
A composite chart is constructed from the midpoints between each pair of corresponding planets in the two charts — a mathematical hybrid rather than a chart for any real moment. Tradition reads it as describing the relationship itself as an entity.
The Davison relationship chart takes a different approach: a real chart cast for the midpoint in time and space between the two births. Unlike a composite, it corresponds to an actual moment and place, so its planetary positions are genuine ephemeris values. Which construction to prefer is a matter of school.
How this is presented here
Synastry is documented on Asteria Star as a technique — what it does, what it requires, and how practitioners describe it. The interpretations are the tradition's, and they are not presented as findings about how relationships work.
The distinction is worth restating because relationship material is where interpretive claims most easily read as advice. Astrological compatibility analysis has not been shown to predict relationship outcomes, and this section describes a practice rather than recommending decisions based on it.
Continue in the data
Catalogues, hubs, and reference pages that hold the underlying records for this topic.
Frequently asked
- What is synastry?
- The astrological technique of comparing two complete birth charts to describe relationship dynamics. It examines aspects between one person's planets and the other's, and where each person's planets fall within the other's house structure. It is the tradition's own relationship method, as distinct from popular sun-sign compatibility.
- What is the difference between a synastry chart and a composite chart?
- Synastry compares two charts against each other, keeping them distinct. A composite chart merges them into one by taking the midpoints between corresponding planets, producing a hybrid that does not correspond to any actual moment. A Davison chart is a third option: a real chart for the midpoint in both time and place between the two births.
- Do I need both birth times for synastry?
- For the full technique, yes. Inter-chart aspects between planets can be calculated approximately from dates alone, but house overlays and the Ascendant relationship require accurate birth times for both people. Practitioners generally state this limitation rather than working around it.