Synastry is the astrological practice of comparing two natal charts. Instead of asking whether two Sun signs are “compatible,” it looks at the relationship between both people’s planets, angles and houses. The result is a symbolic map of dynamics to explore — not a verdict on whether a relationship will succeed.
How synastry works
First, each person’s birth chart is calculated separately from their date, time and place of birth. The two charts are then placed together. An astrologer studies where one person’s planets fall in the other person’s chart and which angles or planets form aspects across the two charts.
Person A’s Venus in contact with Person B’s Mars
In an astrological reading, this might be explored as attraction, desire or a different way of expressing affection. It is a theme to reflect on, not proof that the two people must be together.
Which placements matter most?
Sun
Identity, vitality and the direction each person wants to grow toward.
Moon
Emotional habits, comfort and the ways each person responds under stress.
Venus
Affection, pleasure, values and what makes connection feel attractive.
Mars
Desire, initiative, conflict and how energy gets expressed.
Mercury is especially useful for communication. The Ascendant and the houses add context when both birth times are reliable. There is no single “best” placement: a useful reading considers several patterns rather than declaring a couple good or bad from one aspect.
Synastry and Sun-sign compatibility are not the same
A Sun-sign match is a very small slice of a chart. Two people with supposedly difficult Sun signs may communicate easily through Mercury, feel emotionally safe through their Moons, or share supportive Venus contacts. Conversely, two matching Sun signs can still have very different needs.
What if the birth time is unknown?
You can still compare many date-based placements, but the result should be labelled incomplete. The Ascendant and houses cannot be trusted without a reliable birth time, and the Moon may need checking if it changed signs that day. A responsible reading says what is certain and what is not instead of guessing.
What synastry can — and cannot — tell you
Synastry can offer language for discussing attraction, emotional needs, communication styles and recurring friction. It cannot establish that someone is your soulmate, prove what another person feels, predict a breakup, or justify staying in a harmful relationship. Real behaviour, consent, safety and communication matter more than any chart.
✦ Compare two birth charts free →Frequently asked questions
What is synastry?
Synastry compares two natal charts and interprets the symbolic relationships between their planets, angles and houses.
What information do I need?
Use both people’s birth dates and places, plus exact birth times when available. The time is especially important for the Ascendant and houses.
Does synastry predict the future?
No. It is a reflective astrological framework, not a scientific forecast or a guarantee about a relationship’s outcome.
Is synastry only for romantic couples?
No. The same method can be used to reflect on friendships, family, creative partnerships or work relationships.
Astrology is presented here as symbolic reflection and entertainment, not as a scientific prediction or relationship advice.