Relationship astrology guide
Synastry charts, inter-chart aspects, and overlays
Synastry compares two sets of calculated chart positions and then interprets the relationships between them.
Relationship Astrology · 9 minute read
Written and reviewed by ReadAstrology Editorial & Calculation Team. Published 2026-07-18T00:00:00-05:00. Modified 2026-07-18T00:00:00-05:00.
Key takeaways
- Synastry requires two separately calculated natal charts.
- Inter-chart aspects need exact separation and orb evidence.
- Compatibility should not be reduced to a deterministic score.
Methodology: ReadAstrology links compatibility themes to exact inter-chart aspects and carries birth-time uncertainty into confidence.
Astrology and numerology are cultural and symbolic traditions, not scientifically validated methods for predicting events, diagnosing conditions, or guaranteeing outcomes.
Sources and calculation references
- Swiss Ephemeris General Documentation — Astrodienst AG. Planetary, lunar, house, aspect, and coordinate calculation background.
- IANA Time Zone Database — Internet Assigned Numbers Authority. Historical civil-time offsets and daylight-saving rule provenance.
Calculate both natal charts first
Each person's birth data must be converted into a complete chart using the same zodiac and house settings. Time uncertainty should be carried into the comparison.
Comparing only Sun signs removes the Moon, angles, houses, and most inter-chart aspects that make synastry specific.
Measure inter-chart aspects
Synastry aspects are angular relationships between a planet or point in one chart and a planet or point in the other. Each aspect should record exact separation and orb.
The same aspect can be interpreted differently depending on the planets involved and the wider pattern of the relationship chart.
Use house overlays carefully
A house overlay places one person's planets into the other person's houses. Reliable overlays require accurate birth times, especially for the receiving chart.
If either time is unknown, house-based claims should be omitted or marked low confidence.
Compatibility is not a single score
A useful reading shows supportive patterns, friction, communication dynamics, attraction, boundaries, and growth themes without declaring a relationship destined or impossible.
Scores can summarize evidence, but they should link back to the aspects and assumptions that produced them.
Frequently asked questions
What is a synastry chart?
It is a comparison of two natal charts using inter-chart aspects and, when birth times are reliable, house overlays.
Do both people need exact birth times?
Exact times are strongly recommended for angles, houses, and overlays. Many planetary aspects can still be compared without them.
Can synastry predict whether a relationship will last?
No. It is a symbolic interpretive framework and cannot guarantee relationship outcomes.