Synastry Explained: How Birth Chart Compatibility Works

Synastry is the astrological practice of laying one person's birth chart over another's to see how the two fit together. Instead of asking whether a Leo and a Scorpio are "compatible", synastry asks something far more specific: where does your Venus fall in their chart, how does your Moon speak to their Mars, and which areas of life does each of you light up in the other? It is a more detailed picture than a Sun-sign match, and it is also more honest, because it rarely produces a simple yes or no. Here is how birth chart compatibility works, what astrologers look at and how to read it sensibly.

What is synastry in astrology?

The word comes from the Greek for "coming together of stars". In practice it means comparing two natal charts side by side. Each chart is a snapshot of the sky at one person's birth, so synastry compares two snapshots and looks for the points where they touch.

There are three main layers to a synastry reading:

• Sign placements. Which sign each person's planets sit in, and how those signs get on (the same element, complementary elements, or friction between them).
• Aspects between the charts. The angles that form between one person's planets and the other's, such as a conjunction (the same degree) or a square (90 degrees apart).
• House overlays. Where one person's planets land in the other's houses, which shows the area of life each person tends to affect in the other.

Synastry looks at the relationship from both directions. Your Moon touching their Venus is not the same experience for each of you, and a good reading describes both sides.

Why is Sun-sign compatibility only one layer?

A Sun sign tells you about a person's core identity and what they are working towards. That is real information, but it is one planet out of ten or so, and one sign out of a chart of many. Two people can have Sun signs that textbooks call "a perfect match" and still struggle because their Moons want different things from daily life. Two others, with Suns that look awkward on paper, can get on beautifully because their Moons and Venuses are in harmony.

Sun-sign pairings are a useful starting point, and our compatibility pages cover every combination, for example Aries and Leo. But think of them as the first chapter. Synastry reads the rest of the book.

Which planets matter most in a compatibility reading?

Astrologers tend to start with the personal planets, because they describe how a person feels, loves and acts.

Sun and Moon

The Sun is identity and the Moon is emotional need. When one person's Sun makes a friendly aspect to the other's Moon, it is traditionally seen as a sign of ease: one person's way of being feels natural to the other's emotional world. Moon-to-Moon contacts say a lot about whether you feel at home with each other's moods, routines and ways of coping. You can look up yours with the free Moon sign calculator.

Venus and Mars

Venus is how we give and receive affection, what we find beautiful and how we like to be treated. Mars is drive, desire and how we go after what we want. Contacts between one person's Venus and the other's Mars are classically linked with attraction and chemistry, though a tense aspect between them can bring friction alongside the spark. Venus-to-Venus shows whether you share a language of affection, which is explored further in our guide to Venus sign meaning.

Mercury

Mercury covers conversation and thinking style. Good contacts here are often described as "we talk easily"; difficult ones as "we keep misreading each other". It matters more in long relationships than the headline planets sometimes suggest, because talking is what most of daily life is made of.

The Rising sign and Ascendant

Your Rising sign, or Ascendant, describes your approach to the world and first impressions. When someone's planets land close to your Ascendant, you tend to feel strongly "seen" by them. The Rising sign needs an accurate birth time, and our rising sign calculator will find yours. For a refresher, see what a rising sign is.

Saturn, Jupiter and the outer planets

Saturn contacts are about commitment, responsibility and, sometimes, restriction. They are often described as the glue of long-term partnerships, and also as a source of heaviness. Jupiter contacts feel generous and encouraging. Outer planets such as Uranus, Neptune and Pluto matter most when they touch a personal planet precisely, and they tend to colour the relationship with unusual, dreamy or intense qualities.

What do aspects between two charts mean?

An aspect is a measured angle between two planets. In synastry, you compare planets from different charts. The main ones are:

Two things matter beyond the type of aspect. The first is the orb, how close the angle is to exact. The tighter the contact, the stronger it is felt. The second is which planets are involved. A square between two Mercuries is a very different matter from a square between a Moon and a Saturn.

Be wary of treating "hard" aspects as bad and "soft" ones as good: squares and oppositions often supply the energy that keeps a relationship alive.

How do house overlays work in synastry?

House overlays show where in your life someone else's planets land. If you place a partner's planets into your own chart's houses, you see which areas they activate for you. For example:

• Their Sun or Venus in your 7th house points to partnership and one-to-one commitment.
• Their Moon in your 4th house can feel like home and family.
• Their Mars in your 8th house is associated with intensity and strong physical or emotional pull.
• Their Saturn in your 1st house can make you feel measured or held to account.

Then you repeat the exercise the other way round. Houses depend on an accurate birth time, because they are calculated from the Ascendant. Our reports use Placidus houses, one of the most widely used systems. Without a birth time you can still compare planets by sign and by aspect (the Moon is the one to be careful with, since it moves quickly), but the overlay layer is lost. If houses are new to you, how to read your birth chart is a good place to start, with astrology houses explained going deeper.

Can synastry tell you if a relationship will last?

No, and be wary of anyone who says it can. Synastry describes tendencies: where two people click, where they are likely to rub and what each one brings out in the other. It does not predict whether a relationship will work, because that depends on communication, values, timing and choices that no chart contains.

The most useful way to treat a synastry reading is as a conversation starter. If it says your Mercuries are in tension, you can decide to be more patient about misunderstandings. If it shows a Moon-Venus trine, you can enjoy knowing there is natural warmth to build on. We offer astrology for reflection and entertainment, not as a verdict on anyone's love life.

How do you read your own synastry step by step?

If you want to try it by hand, here is a sensible order:

1. Get both charts right. You need date, time and place of birth for each person. Without a time, skip houses and the Ascendant.
2. Start with the Sun, Moon and Venus signs. Note the elements and how they relate.
3. Look for tight aspects between the personal planets. Sun, Moon, Mercury, Venus and Mars, within about 5 degrees at most, are the headline contacts.
4. Add Saturn and Jupiter to see where commitment and generosity show up.
5. Check the house overlays, both ways.
6. Look for patterns, not single items. Several contacts pointing the same way tell you more than one dramatic aspect.

Where can you get a full synastry reading?

Our Two Skies compatibility report does the comparison for you. It runs to around 20 pages and compares both full charts, covering the planet-to-planet contacts, the aspects and the house overlays described above, written out in plain English for the two of you. It uses the tropical zodiac and astronomically accurate positions, and you need both birth dates, with times and places if you want the house layer included. For a quick first look, start with the free compatibility pages for any pair of signs.