☉Western astrology · 5 min
"Are we right for each other?" Examining relationships is one of astrology's most popular uses. There are two main tools: synastry and the composite chart. Both use the traditional symbolic language, and it is important to know what they can and cannot do.
Synastry
Synastry is the overlaying of two birth charts: the aspects between the planets of the two charts are examined, along with where one person's planets fall in the other's houses.
- Aspects between the two charts: for example, a harmonious angle between one person's Sun and the other's Moon is, in tradition, good rapport, the meeting of emotional need and identity. A Venus–Mars link is the traditional sign of attraction and drive.
- House overlays: if one person's Venus falls in the other's 7th house, the traditional reading stresses the theme of partnership.
- Saturn contacts: tradition regards Saturn's aspects as signs of durability but sometimes of heaviness.
The advantage of synastry is that it keeps both people's charts, but given the sheer number of possible aspects we almost always find both "good" and "difficult" links; the outcome depends on what we choose to emphasise.
The composite chart
The composite chart is a third chart calculated from the midpoints of the corresponding points of the two charts, the relationship's own "personality": the chart of "us". The midpoint of the two Suns, for instance, gives the composite Sun. A variant is the Davison chart, a real sky picture calculated for the midpoint of the two birth times and places.
According to tradition, the composite shows what the relationship becomes as a "third entity": its shared themes, tensions and aims.
What does the research say?
Scientific studies of "Sun sign compatibility" (for example analyses of marriage and divorce records) have not found a pattern that would confirm the traditional compatibility tables. According to relationship research, the success of a relationship is far better indicated by how people communicate and handle conflict, the similarity of their values and goals, trust, emotional stability (in Big Five terms, a lower level of neuroticism) and time spent together. A relationship chart is therefore not a sound way to decide whom you should be with.
What is it good for, then?
- A conversation starter: you can both read the description and discuss how much of it is true.
- A shared language: symbolic images (Venus, Mars, Saturn) can provide a playful frame for raising difficult topics.
- Self-knowledge: relationship themes can reveal your own patterns.
What not to use it for
- Do not decide whether to start or end a relationship on the basis of a chart.
- Do not treat a "tense" aspect as fate or a "harmonious" one as a guarantee.
- If you have difficulties in a relationship, professional help (for example couples counselling) is far more effective.
Synastry, then, is like a shared board game: it can be entertaining and thought-provoking, but it does not predict.