Synastry Explained: How Astrologers Compare Two Birth Charts
Synastry is the astrological method of laying one person's birth chart over another's to see how their planets interact. Instead of asking whether two zodiac signs "match", it measures the angles between the actual planet positions in both charts, which is why it gives a far more specific picture than a sun-sign horoscope.
What is synastry?
Every birth chart is a snapshot of the sky at the exact time and place someone was born. It records where the Sun, Moon and planets stood, and in which zodiac sign and house.
Synastry (from the Greek for "coming together") puts two of those snapshots side by side. An astrologer then looks at three things: where each person's planets fall in the other's chart, which aspects (angles) form between the two sets of planets, and which areas of life one person's planets activate in the other's chart.
The result is not a verdict. It is a map of where two people tend to feel ease, spark, friction or pressure with each other.
What you need to start
Synastry needs a birth chart for each person, and a chart needs three facts: date, time and place of birth.
| Detail | What it unlocks | If it is missing |
|---|---|---|
| Date of birth | Sun, Mercury, Venus, Mars and the outer planets | Nothing works without it |
| Time of birth | Rising sign, house positions, an exact Moon | Rising sign and houses become guesses |
| Place of birth | The time zone and local sky, which fix the rising sign | Houses and rising sign are unreliable |
The rising sign changes roughly every two hours, so a birth time that is off by an hour or two can move it. If you do not know a birth time, you can still compare the planets, but treat anything involving the rising sign or houses as unconfirmed.
The three layers of a synastry reading
A full reading works in layers. Each one answers a different question.
- Planets in signs. What does each person bring? A Venus in Taurus loves differently than a Venus in Gemini, before the other chart is even involved.
- Aspects between the charts. How do the two people's planets relate? This is the heart of synastry and carries most of the weight.
- House overlays. Where do one person's planets land in the other's chart? Their Venus in your seventh house colours partnership; their Mars in your fourth stirs up home life.
If you only have time for one layer, make it the second. The aspects between the charts explain most of what people describe as chemistry or friction.
The aspects, in plain English
An aspect is the angle between two planets, measured in degrees around the zodiac wheel. Five do most of the work.
| Aspect | Angle | What it usually feels like |
|---|---|---|
| Conjunction | 0° | Fusion. The two planets act as one; intense and hard to ignore |
| Sextile | 60° | Easy cooperation; a friendly opening you can choose to use |
| Square | 90° | Tension and friction that pushes both people to act |
| Trine | 120° | Natural flow; things work without effort, sometimes too comfortably |
| Opposition | 180° | Magnetic pull and push, each person mirroring what the other lacks |
Trines and sextiles are often called "good" and squares and oppositions "bad", but that is too simple. A relationship made only of trines can be pleasant and flat. Squares are frequently where the spark and the growth come from, as long as both people can work with the friction.
Orbs: how close is close enough?
Planets are rarely at the exact angle, so astrologers allow a margin called an orb. A common working range is about 6 to 8 degrees for the major aspects between personal planets, with the tightest aspects (within 2 or 3 degrees) felt most strongly. Astrologers disagree on exact orbs, so treat any single number as a convention rather than a law.
Which planet pairs matter most
Not all aspects carry equal weight. The personal planets, which move fast and describe how someone feels and acts day to day, matter most between two charts.
| Pair | The question it answers |
|---|---|
| Sun and Moon | Does one person's identity feel at home with the other's emotional needs? |
| Moon and Moon | Do you soothe and comfort each other in similar ways? |
| Venus and Mars | Is there attraction, and do desire and affection line up? |
| Mercury and Mercury | Do you understand each other when you talk? |
| Sun and Sun | Do your core drives pull in compatible directions? |
| Saturn to a personal planet | Is there commitment and stability, or a feeling of being judged or held back? |
For the attraction pairs in particular, see our guide to Venus and Mars in synastry. For the Sun and Moon themselves, start with Sun, Moon and Rising.
A worked example
Take two invented people, Ava and Liam. These placements are made up to show the method.
- Ava's Venus is at 12° Taurus. Liam's Mars is at 14° Virgo. Taurus and Virgo are four signs apart, so the angle is about 122°: a trine with a 2° orb. Affection on her side and desire on his flow into each other easily.
- Ava's Sun is at 20° Libra. Liam's Sun is at 18° Aquarius. Libra and Aquarius are also four signs apart, giving about 118°: another trine, with a 2° orb. Their core identities are naturally compatible.
- Ava's Moon is at 3° Pisces. Liam's Moon is at 5° Gemini. That is three signs apart, about 92°: a square, with a 2° orb. This is the friction point. She needs emotional immersion and he needs emotional space, and the clash is tight enough to matter.
The honest summary of this pair: attraction and shared direction come easily, and their main work is learning each other's emotional rhythm. That is far more useful than being told "Libra and Aquarius are a good match."
Synastry vs composite vs sun-sign matching
These are often confused.
- Sun-sign compatibility compares only two zodiac signs. It ignores everything else in both charts, so it can only give broad, generic answers.
- Synastry keeps both charts separate and studies how they interact. It describes the dynamic between two individuals.
- A composite chart blends the two charts into a single new chart for the relationship itself, using midpoints. It describes the relationship as a third entity.
Most astrologers use synastry and composite together. Synastry shows how each person experiences the other; the composite shows what the pairing is like as a unit.
What synastry cannot tell you
Synastry is a framework for reflection. It is not scientifically validated as a predictor of relationship outcomes, and no chart can say whether two people will stay together. Strong aspects do not guarantee a good relationship, and difficult aspects do not doom one. How two people communicate, what they value and how they treat each other matter more than any angle.
Used well, synastry gives you language for patterns you already sense, and questions worth asking each other.
Frequently asked questions
Do I need a birth time for synastry?
You can compare planet positions without one, but the rising sign and house overlays depend on it, and the Moon's sign can be uncertain on days when it changes sign. A birth certificate or hospital record is the most reliable source.
What is a good synastry score?
There is no official scale. Tools that give a percentage are summarising how many harmonious and challenging aspects appear and how tight they are. Read the score as a starting point and look at which planets are involved, because a single tight Moon square can matter more than several loose trines.
Can two signs that "don't match" still work?
Yes. Sun signs are one input among many. Two people with a "difficult" sun-sign pairing can have strong Moon, Venus and Mercury connections that make the relationship feel easy.
Is synastry only for romantic relationships?
No. Astrologers use synastry for friendships, family, business partners and co-workers. The same planets and aspects apply, but you read them for the kind of bond in question.
Orbit provides astrology for entertainment and self-reflection. Readings are not predictions and are not medical, psychological, legal or financial advice.