Most of us have checked astrology compatibility at some point in a relationship — typed both names into a search bar, found out that Scorpio and Aquarius are "a challenging match," and felt vaguely unsettled. The thing is, that kind of check is barely scratching the surface. Sun-sign compatibility is real, but it tells you about roughly one-tenth of what is actually going on between two people.
The fuller picture has a name: synastry.
Why Sun-sign compatibility is not enough
Your Sun sign is determined by the month you were born. That means everyone born in roughly the same four-week window shares your Sun sign — hundreds of millions of people. Saying a Taurus and a Virgo are compatible tells you something about how two particular energies tend to relate in the abstract. It says almost nothing about two specific humans.
Your chart has ten planets, each in a different sign, each in a different house. So does theirs. The interaction between those twenty points — and the geometric angles they form — is what actually shapes how two people experience each other. Two Scorpio-Taurus couples can feel completely different depending on what the rest of their charts are doing.
Sun-sign comparison is like judging a film by the genre label on the poster. Useful as a rough orientation, misleading as a full review.
What synastry actually is
Synastry is the comparison of two natal charts placed side by side — or more precisely, overlaid on top of each other. Astrologers call this "chart overlay." You take the wheel of one person's birth chart and draw the other person's planets into it. Then you look at what happens.
Some of their planets will fall right on top of yours. Some will form harmonious angles (trines at 120°, sextiles at 60°). Others will create tension (squares at 90°, oppositions at 180°). Each of those conversations between planets carries meaning. And the combination of all those conversations is what tells the real story.
What you get is not a score. It is a map of the dynamic — where things flow easily, where there is friction and why, what draws you together, what might create repeated patterns of conflict.
The placements that matter most
Not every planet-to-planet connection carries equal weight. Here is where synastry gets interesting.
Sun and Moon contacts. When one person's Sun closely touches the other's Moon — by conjunction (same degree) or trine — there is often a deep sense of ease, of being seen. One person's core identity resonates with the other's emotional life. It is one of the most relationship-sustaining contacts in synastry.
Venus contacts. Venus shows what we find beautiful, how we express affection, what we need to feel loved. When your Venus connects strongly with someone's Sun, Mars, or Ascendant, attraction tends to be real and felt. Venus-Venus harmony (especially trine or sextile) often shows up in long-term partnerships where people genuinely enjoy each other.
Mars contacts. Mars is desire, drive, how you take action. Mars contacts in synastry tend to generate chemistry — but the nature of the contact matters a lot. A Mars-Venus trine reads differently from a Mars-Mars square. The first flows. The second creates energy that can feel exciting or exhausting depending on how both people handle friction.
Ascendant connections. The Ascendant is how you show up in the world — the first impression you project. When someone's planet (especially Sun, Venus, or Moon) lands close to your Ascendant, you may feel seen or drawn in ways that are hard to explain immediately. It tends to create strong first impressions that linger.
Moon-to-Moon compatibility. Two people's Moons in compatible elements (both fire signs, or both water signs, for example) often feel instinctively safe with each other. Their emotional registers are in the same key. This matters more for long-term compatibility than it might seem — you can love someone passionately and still feel emotionally homeless around them if your Moons are out of sync.
What synastry shows and what it does not
Synastry shows the shape of the dynamic — not who you should be with, and definitely not a verdict on whether a relationship will work.
A chart full of harmonious contacts does not mean the relationship will be easy. Sometimes too much ease creates stagnation. A chart with multiple squares can indicate a relationship with real friction — and also real growth, real electricity, real presence. Plenty of lasting, meaningful partnerships have challenging synastry. The friction is part of what keeps them alive.
What synastry cannot do:
- Tell you whether someone is a good person
- Predict specific events ("you will break up in March")
- Account for the choices both people make every day
- Replace actually knowing each other
Think of it the way Eira thinks about all astrology: weather, not fate. A trine from their Venus to your Moon means conditions are favourable for feeling loved and accepted by this person. Whether you build something real together is still entirely up to you.
How Eira can help you explore this
The first step in synastry is having an accurate natal chart — yours. Without that foundation, any compatibility reading is working in the dark.
In Eira, you can build your natal chart in about two minutes (you will need your date, time, and place of birth). The app maps your chart across eleven life areas, including relationships. You can see where Venus sits in your chart, what your Moon sign is, what your seventh house (the house of partnership) looks like — all in plain language, without needing to decode symbols.
Once you have your chart, Eira's AI astrologer is there for the questions synastry tends to raise. Ask about the planets in your relationship sphere, ask what a Moon-Venus connection tends to feel like, ask what your Ascendant says about how you draw people in. You will get an answer that is grounded in your actual chart, not generic advice.
The deeper exploration — what a specific person's chart does against yours — is something you can begin here, with your own map first.
Open Eira and build your natal chart
What synastry actually looks at: five points
Sun and Moon
The most durable link. One person's Sun beside the other's Moon produces the feeling of being at ease together, with no explanation available.
Moon and Moon
Governs daily life and whether your ways of processing feelings match. Two Moons in conflict: people love each other and still get irritated over small things.
Venus and Mars
Attraction. Works fast and strongly, but holds nothing on its own — this is the spark, not the shared life.
The Ascendants
How easily you read each other in the first seconds. Unexplained liking, or its absence, often hides here.
Saturn
The most underrated point. One person's Saturn on the other's personal planets brings weight and duration — and with them a sense of obligation. Relationships that last almost always have this contact.
What synastry does not do
It does not deliver a verdict. There are no couples "incompatible by chart." There are couples for whom certain themes will be harder — and that can be known in advance.
It does not explain everything. A chart cannot see upbringing, experience, or whether someone is willing to work on a relationship.
It does not replace the conversation. What synastry does well is show which conversation to have.
Frequently asked questions
What is synastry? A comparison of two natal charts: how one person's planets land on the other's chart.
Do I need both birth times? For the full picture, yes. Without them a comparison is still possible, but without houses or Ascendants.
Can synastry tell me whether we will marry? No. It shows dynamics, not outcomes.
So is Sun-sign compatibility simply false? Not false — just a very rough approximation: one point out of ten.