A synastry chart is two birth charts read together. Instead of asking what each person is like on their own, it asks a different question: what happens when these two people meet? It shows where one person's planets land in the other's chart and the angles they form, which can help explain why a connection feels warm, electric, steady or prickly.
If Sun sign matching has started to feel too simple, synastry is usually the next step. It works with the whole chart, so two people with supposedly clashing Sun signs can still share a lot of easy contact elsewhere.
What a synastry chart actually shows
Every birth chart is a snapshot of the sky at the moment someone was born. It maps the Sun, Moon, planets and the Rising sign across twelve houses. If you are new to that layout, How to read your birth chart: a first guide is a helpful place to start.
Synastry overlays one of those snapshots onto another. Most tools draw it as two rings: your chart on the inside, the other person's planets placed around the outside. Lines between the rings mark the aspects, or angles, between your planets and theirs.
The point is not to score the relationship. It is to see where your natures tend to meet easily and where they tend to rub, so you can understand the patterns you already feel.
How one person's planets land on another's
There are two main things to look at: aspects and house overlays.
Aspects: the angles between planets
An aspect is the distance in degrees between a planet in your chart and a planet in theirs. A few angles tend to matter most:
- Conjunction (0 degrees): the two planets sit together, so their themes blend and feel intense.
- Sextile (60 degrees): a light, friendly link that often feels easy to act on.
- Square (90 degrees): tension that can create spark, impatience or growth.
- Trine (120 degrees): a smooth flow, often felt as natural ease.
- Opposition (180 degrees): a pull between opposites that can attract and frustrate at once.
Astrologers usually give each aspect an orb, a small window of a few degrees either side. The tighter the orb, the more noticeable the contact tends to be. A Venus to Mars conjunction within one degree often feels much stronger than one six degrees apart.
House overlays: where their planets fall in your life
If both birth times are known, you can also see which of your houses their planets land in. Someone whose Moon falls in your fourth house, the house of home, may feel instantly familiar. Someone whose Saturn falls in your seventh house, the house of partnership, may bring a sense of seriousness or commitment to the bond.
House overlays are one way to see which area of life a person tends to activate for you, and it can be different in each direction. Your Venus may sit in their house of friendship while their Venus lands in your house of romance.
Contacts that tend to shape attraction, comfort and friction
Not every line in a synastry chart carries the same weight. These are the contacts many people look at first.
Attraction: Venus and Mars
Venus describes what you find lovely and how you like to receive affection. Mars describes desire, pursuit and how you go after what you want. When one person's Venus aspects the other's Mars, there is often a clear physical or romantic pull.
A trine or sextile can feel like easy chemistry. A square or opposition can feel more charged, the kind of attraction that comes with a bit of push and pull. For more on each planet alone, see Your Venus sign and the way you love.
Comfort: Moon and Sun
The Moon describes emotional needs and what helps you feel safe. The Sun describes identity and core purpose. When one person's Moon sits in harmony with the other's Sun, the relationship often feels supportive: one person tends to feel seen, the other tends to feel cared for.
Moon to Moon contacts matter too. Two Moons in a trine often understand each other's moods without much explaining. Two Moons in a square may need more words, because what soothes one can unsettle the other.
Friction: Saturn, Mars and hard aspects
Saturn brings structure, duty and limits. When it touches someone's personal planets, the other person can feel steadied or restricted, sometimes both. Saturn contacts often show up in long relationships because they add weight and staying power.
Mars in a hard aspect to the other person's Sun, Moon or Mars can point to arguments, competition or heat. That is not automatically a bad sign. Friction can keep a relationship lively, as long as both people have room to cool down.
A difficult aspect is not a verdict. It is a place where two people tend to need more patience, more words and more care.
How to read a synastry chart, step by step
It can look busy at first. A simple order helps.
- Get accurate birth data for both people. Date, place and, ideally, time. Without a time, skip houses and the Rising sign and focus on planet to planet aspects.
- Start with the personal planets. Look at contacts between the Sun, Moon, Mercury, Venus and Mars. These tend to describe daily life together.
- Note the tightest aspects. Contacts within two or three degrees are often the ones you actually feel.
- Look for repeated themes. If several contacts point to emotional ease, that theme likely matters more than one stray square.
- Check both directions. Your Venus to their Mars and their Venus to your Mars can describe slightly different experiences.
- Compare it with real life. The chart is a lens, so ask whether it helps describe what you already notice between you.
If you want the Sun, Moon and Rising basics in place before you compare charts, you can see your Sun, Moon and Rising and start from there.
What synastry cannot tell you
Synastry can describe tendencies, not outcomes. It cannot tell you whether a relationship will last, whether someone is right for you, or what either person will choose to do. People grow, communicate and make decisions that no chart predicts.
It also tends to be most useful alongside honest conversation. A chart might suggest that one person needs space while the other needs reassurance, but only talking about it turns that insight into something practical.
Used this way, a synastry chart becomes a shared vocabulary. It gives two people language for why certain moments feel easy and others feel hard, and that understanding is often where real compatibility begins.
Questions people ask
What is a synastry chart in astrology?
A synastry chart compares two birth charts by placing one person's planets around the other's. It shows the angles between their planets and which houses they fall into, which can help describe how the two people tend to interact.
Do I need a birth time for synastry?
You can read planet to planet aspects with just the birth dates. A birth time adds the Rising sign and house overlays, which tend to give a fuller picture of how the relationship plays out in daily life.
What is the difference between synastry and a composite chart?
Synastry compares two separate charts and shows how each person affects the other. A composite chart blends both charts into a single chart that describes the relationship itself as its own entity.
Which synastry aspects matter most?
Many astrologers start with contacts between personal planets, especially Venus to Mars for attraction and Moon to Sun or Moon to Moon for emotional comfort. Tight aspects within a few degrees often feel the strongest.
Can a relationship work with difficult synastry?
Yes, hard aspects like squares and oppositions often bring spark and growth along with tension. They usually point to areas that need more patience and communication, not to a relationship that cannot work.
