Quick answer
A synastry chart compares two people's birth charts to show how their planets interact. Astrologers look at aspects, meaning the angles between one person's planets and the other's, and house overlays, meaning where one person's planets land in the other's houses. Contacts between the Sun, Moon, Venus and Mars matter most. Synastry describes tendencies in a relationship, not its outcome.
Key takeaways
- Synastry compares two full birth charts to show how two people tend to affect each other.
- Aspects between the charts describe ease, tension or blending between specific parts of each person.
- Contacts between the Sun, Moon, Venus and Mars usually matter most in a love comparison.
- House overlays show which areas of life each person activates for the other, and they need birth times.
- A mix of easy and hard aspects is normal, and no single contact decides a relationship.
If you have ever wondered why one person calms you down and another sets you on edge, synastry is astrology's way of looking at that question. It takes two birth charts, lays one over the other, and looks at how the planets meet.
A synastry chart does not tell you whether a relationship will work. It describes where two people flow easily, where they rub, and where they blend. This guide explains how it is built, what the aspects mean, which contacts matter most, and how to read one without panicking over a single square.
What is a synastry chart?
A synastry chart is a comparison of two birth charts. It shows how one person's planets interact with the other person's, and it is the main method astrologers use to read compatibility.
A quick reminder of the basics. Your birth chart is a map of where the Sun, Moon and planets were at the moment you were born, seen from your birthplace. Each planet describes a part of you: the Moon your emotional needs, Venus your style in love, Mars your desire and drive.
Synastry compares two of these maps. It is usually drawn as a bi-wheel: one chart in the middle and the other around the outside. Astrologers then look at two things:
- Aspects: the angles between your planets and theirs.
- House overlays: which of your houses their planets fall into, and the other way around.
How does synastry work, step by step?
Synastry works by measuring the distance between each of your planets and each of theirs, then noting which distances form meaningful angles. Here is the process in order:
- Cast both birth charts. Each needs a birth date, time and place. The positions come from an ephemeris, a table of where the planets were at any moment.
- Compare every planet pair. Your Moon to their Sun, your Venus to their Mars, and so on.
- Find the aspects. When two planets are close to a key angle, such as 90 or 120 degrees, that is an aspect.
- Check the orbs. An orb is the margin allowed around an exact angle. Tighter orbs mean stronger contacts.
- Look at house overlays. Place each person's planets in the other person's houses.
- Read the pattern. Weigh the strongest contacts, especially between personal planets, and look at the overall balance of ease and tension.
What do the aspects mean in synastry?
Each aspect describes a different kind of interaction. Easy aspects bring flow, tense aspects bring friction and energy, and conjunctions blend two planets together.
| Aspect | Angle | How it tends to feel between two people | Orb Venara uses |
|---|---|---|---|
| Conjunction | 0 degrees | Blended, intense, hard to ignore | 7 degrees (8 with Sun or Moon) |
| Sextile | 60 degrees | Friendly, easy, supportive | 4 degrees (5 with Sun or Moon) |
| Square | 90 degrees | Friction, drive, a push to grow | 6 degrees (7 with Sun or Moon) |
| Trine | 120 degrees | Natural ease and understanding | 6 degrees (7 with Sun or Moon) |
| Opposition | 180 degrees | Attraction and tension, like a seesaw | 7 degrees (8 with Sun or Moon) |
A conjunction depends heavily on which planets are involved. Your Venus on their Moon is usually tender. Your Mars on their Saturn can feel like pressing the gas while someone else holds the brake.
Hard aspects are not bad. Squares and oppositions often create the pull that keeps two people interested. Soft aspects are not automatically good either. A chart full of trines can feel comfortable and a little sleepy. Our guide to aspects in astrology covers each one in more depth.
Which planet contacts matter most?
In a love comparison, contacts between the Sun, Moon, Venus and Mars matter most. These are the personal planets, the ones that describe daily feelings, attraction and identity.
Here is what the key pairs usually describe:
- Moon and Moon: whether your emotional rhythms match. Easy contacts mean you tend to want the same thing at the same time.
- Moon and Venus: comfort and affection. A classic sign of tenderness between two people.
- Venus and Mars: chemistry. Affection meets desire. Our article on Venus and Mars synastry covers every combination.
- Sun and Moon: a traditional partnership signature, where one person's identity supports the other's needs.
- Sun and Venus: genuine liking and admiration.
- Mars and Mars: how you handle conflict and shared goals.
- Saturn to a personal planet: commitment, structure and sometimes heaviness.
Contacts from the slow outer planets, Uranus, Neptune and Pluto, are read more carefully. These planets stay in one sign for years, so everyone born around the same time shares them. They matter most when they touch a personal planet or angle very closely.
What are house overlays?
A house overlay is where one person's planets land in the other person's houses. It shows which area of life each person tends to activate for the other.
A few examples:
- Their Venus in your 7th house can make them feel like a natural partner to you.
- Their Moon in your 4th house can make them feel like family or home.
- Their Mars in your 5th house can bring romance, play and physical chemistry.
- Their Saturn in your 1st house can feel like they take you seriously, or like they are judging you.
House overlays need accurate birth times for both people. The rising sign, which sets where the houses begin, changes roughly every 2 hours. If either time is unknown, skip the overlays and focus on planet-to-planet aspects.
Does synastry go both ways?
Yes, and the two directions can feel different. Your Venus trine their Mars is not the same contact as their Venus square your Mars. Both appear in the same comparison.
This is why synastry often explains why two people experience a relationship differently. One person may feel deep comfort because of a Moon contact, while the other feels most drawn by a Mars contact. Neither is wrong. They are just responding to different links between the charts.
The astrologer Liz Greene wrote about how partners often activate parts of us we do not fully own. Synastry is a map of which parts each person activates. That makes it a useful starting point for conversations about what each of you needs.
What can synastry not tell you?
Synastry cannot tell you whether a relationship will last, whether someone is right for you, or what will happen next. It describes tendencies and dynamics.
It also has technical limits. Without birth times, the Moon's exact degree is uncertain, since the Moon moves about 12 degrees a day. Houses and rising signs are unknown. And like all astrology, synastry is a symbolic tradition, not a science. The planetary positions are real astronomy. The meanings are interpretations that many people find useful for reflection.
Most importantly, synastry does not see behavior. How two people handle stress, money, family and conflict shapes a relationship far more than any aspect.
How can you try synastry yourself?
Three steps to start:
- Find your own Moon, Venus and Mars. You can get your free reveal in about 90 seconds to see yours.
- Collect your partner's birth data. Date, time and city. A time from a birth certificate is best.
- Compare Moons first, then Venus and Mars. If you want a written reading, Venara's Compatibility report compares the personal planets between your charts, reads both Moon needs and adds a composite chart. When you read any report, start with the contacts between the Moons.
Frequently asked questions
What is a synastry chart?
A synastry chart is a comparison of two birth charts, usually drawn as one chart inside the other. It shows the angles, called aspects, between one person's planets and the other person's, and which of your houses their planets fall into. Astrologers use it to describe how two people tend to affect each other in a relationship.
What are the most important aspects in synastry?
Contacts between the personal planets matter most. Moon to Moon and Moon to Venus describe emotional comfort. Venus to Mars describes attraction. Sun to Moon is a traditional partnership signature. Saturn touching a personal planet often describes commitment and weight. Contacts to the rising sign also matter when both birth times are known.
Do you need birth times for synastry?
You can compare most planets without birth times, because the Sun, Venus, Mars and slower planets move little in a day. The Moon moves about 12 degrees a day, so its exact position is uncertain. Rising signs and house overlays need accurate birth times for both people. Without them, focus on planet-to-planet aspects.
What is the difference between synastry and a composite chart?
Synastry compares two separate charts to show how each person affects the other. A composite chart combines the two into one chart, using the midpoints between each pair of planets, to describe the relationship as its own unit. Synastry answers how we affect each other. The composite answers what this relationship is like.
Can bad synastry be overcome?
Challenging synastry means there are squares or oppositions between important planets. That points to areas of friction, not a verdict. Many lasting couples have hard aspects between their charts, and those contacts often add chemistry and growth. Talking openly about the differences they describe is usually more useful than worrying about them.
Sources
- Liz Greene, Relating: An Astrological Guide to Living with Others (1977)
- Marion D. March and Joan McEvers, The Only Way to Learn Astrology, Vol. 1
- Astronomy Engine (open-source ephemeris library by Don Cross)
About this article
Written and checked by the Venara editorial team against the sources listed and Venara's chart engine. Astrology here is a tool for reflection, not a prediction or professional advice. How we write and check articles.
For entertainment and self-reflection. Not a substitute for professional advice.