How to Read a Synastry Chart: Relationship Astrology Explained
Synastry is the comparison of two birth charts to understand a relationship. Instead of one chart, you read the conversation between two — which planets touch, where each person’s planets land in the other’s houses, and what that says about chemistry, friction, and longevity. Here is how to do it in order.
Step by step
- Cast both charts accurately. Calculate each person’s birth chart with exact times. Synastry leans heavily on the Moon, Ascendant, and houses, all of which need the birth time.
- Overlay the two charts. Place one chart inside the other to create a biwheel. Now you can see person A’s planets falling into person B’s houses and vice versa.
- Read the inter-aspects. Look at the aspects between the two charts. Venus–Mars contacts signal attraction; Sun–Moon contacts signal ease; Saturn contacts signal commitment or weight.
- Check the house overlays. Note which houses each person’s planets land in. A partner’s Sun in your 7th house feels like a natural partner; in your 12th, more hidden or karmic.
- Weigh the key pairs. Prioritize the “relationship planets”: Moon (emotional fit), Venus (affection), Mars (desire), Mercury (communication), Saturn (durability).
- Read the composite (optional). For the relationship as its own entity, generate a composite chart from the midpoints of both charts and read it like a single chart.
The aspects that matter most
Not all contacts carry equal weight. In practice these are the ones that decide the feel of a relationship:
- Venus–Mars: physical chemistry and attraction.
- Sun–Moon: a sense of natural fit and being “at home”.
- Moon–Moon and Moon–Venus: emotional safety and tenderness.
- Mercury–Mercury: how easily you talk and think together.
- Saturn contacts: commitment, duty, and sometimes restriction.
House overlays, briefly
When your partner’s planet lands in one of your houses, they activate that area of your life. Planets in the 1st, 5th, and 7th houses tend to feel romantic and visible; the 8th deepens intimacy and intensity; the 12th can feel fated or elusive. Overlays add the “where” to the aspects’ “what.”
Reading direction matters
Synastry contacts are not always mutual. If your Saturn aspects their Sun, you are the one bringing structure or limitation to their identity — not the reverse. Always note who is the source of a contact; it changes who feels what.
Frequently asked questions
What is the difference between synastry and a composite chart?
Synastry compares two separate charts and reads the aspects between them. A composite chart merges both into a single chart of the relationship itself. Most readings use synastry first, then a composite for the relationship’s overall character.
Which synastry aspect is the most important for attraction?
Venus–Mars contacts are the classic markers of physical chemistry. Sun–Moon and Moon–Venus contacts matter more for long-term emotional fit.
Do both people need exact birth times for synastry?
For the most accurate reading, yes — the Moon, Ascendant, and house overlays all depend on birth time. You can still read planet-to-planet aspects without it, but the overlays become unreliable.