Synastry Chart: How to Read Two Birth Charts Together

Learn how to read a synastry chart step by step: each natal chart first, then inter-aspects, house overlays, the 7th house, and an unknown birth time.

Mystic Birth Chart Editorial StudioUpdated October 20, 2026

A synastry chart is what you get when you place two birth charts side by side and ask how they meet. One person's planets are compared with the other person's planets, angles and houses. The word comes from Greek roots meaning "stars together," and the practice is old. Ptolemy, writing in the second century, already noted that partners tend to get along more easily when the Sun and Moon of one chart relate harmoniously to the Sun and Moon of the other.

Synastry is not a verdict on a relationship. It describes the climate two people create together: where they feel easy, stirred up or tested. This guide walks through a practical order of reading, from the two separate charts to the contacts between them.

What Synastry Actually Compares

A birth chart (also called a natal chart) is a map of the sky for the exact time and place someone was born. It shows the Sun, Moon and planets in zodiac signs, and, when the birth time is known, the Ascendant (the sign rising on the eastern horizon) and the twelve houses, which divide life into areas such as home, work and partnership.

Synastry compares two of these maps in three main ways:

Inter-aspects. These are angles between a planet in one chart and a planet in the other. If your Venus sits at 12 degrees of Libra and the other person's Mars sits at 13 degrees of Aries, the two planets are opposite each other. That is an inter-aspect.

House overlays. These show which of your houses the other person's planets fall into, and the reverse. If their Moon lands in your 4th house, their emotional life touches your sense of home.

Each person's relationship significators. These include the 7th house, its ruler, Venus, Mars and the Moon in each individual chart. They describe what each person brings to any partnership before the two charts even meet.

Synastry is different from a composite chart, a modern technique that blends two charts into one. Synastry keeps two distinct people and two distinct charts.

Step 1: Read Each Chart on Its Own First

The most common mistake in synastry is skipping straight to the contacts. A Venus-Mars trine between two charts means little until you know what Venus and Mars are like in each person's own chart.

Before comparing anything, ask a few questions of each chart separately:

What does this person's Moon need? The Moon describes emotional rhythm and what makes someone feel safe. Our guide to Moon sign needs by element is a good starting point.

How do Venus and Mars work here? Venus shows affection, taste and what someone values. Mars shows desire, pursuit, anger and how someone acts. Their sign, house and condition matter as much as their contacts. See Venus sign vs Venus house for how placement changes meaning.

What does the 7th house describe? We will come back to this below.

Where is Saturn? Saturn often shows where someone is cautious, serious or slow to trust, and also where they can commit with great steadiness.

If you do not yet know your own placements, the free birth chart calculates them without asking for an email. For a full natal reading of one chart before you compare it with anyone else's, the Essential Birth Chart Reading is a whole-chart study written from your calculated chart.

Step 2: Look at the Inter-Aspects

Once you know both charts, list the aspects between them. The main aspects are:

Conjunction (0 degrees): the two planets blend and intensify each other.

Sextile (60 degrees): an easy opening that rewards a little effort.

Square (90 degrees): friction, pressure and motivation.

Trine (120 degrees): natural flow and mutual ease.

Opposition (180 degrees): strong awareness of the other, attraction and tension at once.

Many astrologers use orbs of around 5 or 6 degrees for contacts between the Sun, Moon and personal planets in synastry, and tighter orbs for slower planets. The closer the aspect, the more noticeable it tends to be. If the terms are new to you, aspects in astrology explains each one.

Start with the contacts that carry the most weight.

Sun and Moon Contacts

These are the oldest contacts in relationship astrology. When one person's Sun or Moon aspects the other's, the pair often feels a basic recognition. A trine or sextile can feel like ease of temperament. A conjunction can feel like being understood without long explanations. A square or opposition can still work well, but daily rhythms may need more negotiation: one wants to talk now, the other wants quiet first.

Moon Contacts in General

The Moon describes comfort. When someone's planet touches your Moon, they reach your emotional life directly. Their Venus on your Moon can feel tender and soothing. Their Mars on your Moon can feel protective, exciting or irritating, depending on condition and aspect. Their Saturn on your Moon can feel steadying or cooling, sometimes both.

Venus and Mars Contacts

Venus and Mars describe affection and desire. Contacts between one person's Venus and the other's Mars are classic signatures of attraction, while Venus-Venus contacts often show shared taste and similar ways of giving care. We cover these in detail in Venus and Mars in synastry.

Saturn Contacts

Saturn contacts are often feared and often misunderstood. When one person's Saturn touches the other person's Sun, Moon or Venus, the relationship may feel serious, responsible and sometimes heavy. It can also give a relationship structure and staying power. The Saturn person may feel like a source of stability, or like a critic. Which one shows up depends on both charts and on how both people handle responsibility. Our article on why Saturn is not punishment applies here too.

Step 3: Check the House Overlays

House overlays require a known birth time for the person whose houses you are using. Take each of the other person's planets and see which of your houses it falls into.

A few patterns to watch:

Read this in your own chart

If this sounds like your love life, your Venus sign is only the first clue. A full reading asks where Venus is placed, what she rules, which aspects reach her, and how love, money, desire, and self-worth repeat through the chart.

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Planets in your 7th house tend to feel like partnership material. That person may seem to occupy the role of "the other" in your life very naturally.

Planets in your 1st house affect how you feel about yourself around them. Their Venus in your 1st can feel flattering. Their Saturn in your 1st can feel like being evaluated.

Planets in your 4th house touch home, family and private life.

Planets in your 5th house touch play, pleasure, romance and creativity.

Planets in your 10th house touch career and public life.

Read the house and the aspect together: a planet in your 5th house that also squares your Moon describes fun with a sharp edge. For a refresher on what each area covers, see houses in astrology.

Step 4: Study the 7th House and Its Ruler in Each Chart

The 7th house describes partnership and one-to-one encounter. In each chart, note the sign on the 7th house and the planet that rules it. Traditional rulers are:

Aries and Scorpio: Mars

Taurus and Libra: Venus

Gemini and Virgo: Mercury

Cancer: the Moon

Leo: the Sun

Sagittarius and Pisces: Jupiter

Capricorn and Aquarius: Saturn

Then ask: where is that ruler, and does the other person's chart touch it? If your 7th house ruler is Jupiter and the other person's Moon sits right on your Jupiter, that is a meaningful link, because their emotional nature lands on the planet that describes partnership in your chart. This kind of contact is often more telling than a generic Sun-sign match, and it only appears when you read each chart properly first.

Step 5: Look for Repeated Themes

A single contact rarely tells the story. If three different contacts all involve Saturn, commitment and responsibility are a real theme. If several contacts link one person's Venus or Moon with the other person's Jupiter, generosity and encouragement may run through the relationship. If Mars appears in many hard aspects, the pair may need clear ways to disagree.

Weigh both directions. Your Sun on their Moon is not the same experience as their Sun on your Moon.

What to Do When a Birth Time Is Unknown

Many people do not know their birth time, and synastry can still be useful. You simply have to be honest about what changes.

Without a birth time for one person:

That person has no reliable Ascendant, Midheaven or houses. You cannot read their 7th house, its ruler, or house overlays that use their chart. Their planets can still fall into the other person's houses, as long as the other person's birth time is known.

The Moon may be uncertain. The Moon moves roughly 12 to 15 degrees a day, so on any given birth date it may sit anywhere in a wide range. If it was near the end or beginning of a sign that day, it may even have changed signs. In that case, read both possible Moon signs as possibilities, not facts.

The other planets stay much steadier. Mercury and Venus rarely move more than a degree or two in a day, so their positions are usually clear. The Moon is the one to treat with care, especially in tight aspects.

A noon chart is fine for a rough picture, as long as nobody treats its houses as real. Our article on reading a chart with an unknown birth time explains more.

A Worked Example: Robin and Sam

Here is a short example with two fictional people. Robin knows their birth time. Sam does not.

Robin has Virgo rising, so in whole-sign houses the 7th house is Pisces and its ruler is Jupiter, which sits at 5 degrees of Scorpio in Robin's 3rd house. Robin's Sun is at 10 degrees Taurus, Moon at 14 degrees Cancer, Venus at 22 degrees Aries, Mars at 8 degrees Leo and Saturn at 16 degrees Capricorn.

Sam has the Sun at 18 degrees Capricorn, Venus at 10 degrees Aquarius, Mars at 12 degrees Cancer, Jupiter at 20 degrees Sagittarius and Saturn at 11 degrees Taurus. Because Sam's birth time is unknown, the Moon could be anywhere from late Libra to around 9 degrees Scorpio.

Now the contacts:

Sam's Sun opposes Robin's Moon (4 degrees). A classic luminary contact. There is strong mutual awareness. Sam may feel like a clear, steady presence to Robin, while Robin's emotional responses may be very visible to Sam. Their daily rhythms may need conscious adjustment.

Sam's Sun conjoins Robin's Saturn (2 degrees). Robin may feel responsible for Sam, or may sometimes become the cautious voice when Sam wants to move forward. This contact often gives seriousness and loyalty.

Sam's Mars conjoins Robin's Moon (2 degrees). Energy meets feeling. This can feel protective and lively, or it can leave Robin feeling rushed by Sam's pace.

Sam's Venus opposes Robin's Mars (2 degrees). A strong attraction signature. The pull is real, with some push and pull built in.

Sam's Saturn conjoins Robin's Sun (1 degree). Another Saturn link. Robin may experience Sam as reliable, or as someone whose approval matters a lot.

Sam's Jupiter trines Robin's Venus (2 degrees). A generous, warm contact. Sam may encourage Robin's affection and enjoyment.

House overlays. Only Robin's houses are available. Sam's Sun falls in Robin's 5th house, which links Sam with Robin's sense of romance and play. Sam's Jupiter falls in Robin's 4th house, warming home life. Sam's Venus lands in Robin's 6th house, which ties affection to daily routines and practical care.

The 7th house link. Robin's 7th house ruler, Jupiter, sits at 5 degrees Scorpio. If Sam's Moon is in Scorpio, it may sit close to Robin's Jupiter, which would be a meaningful partnership link. If Sam's Moon is in late Libra, that link disappears and the Moon instead falls in Robin's 2nd house. Without a birth time, this stays a possibility to hold lightly.

The repeated theme. Saturn appears twice and both luminaries are involved. That suggests a relationship where commitment, responsibility and timing matter a great deal. With Venus-Mars and Jupiter-Venus also present, warmth and attraction are real too. This is not a "good" or "bad" chart. It describes a pair that may feel serious and drawn together at the same time, and who would benefit from talking openly about pace and expectations.

Common Myths About Synastry

"There is a perfect synastry chart"

Every pairing has ease and friction. Trines can feel pleasant but low in motivation, while squares can carry energy and growth. What people do with their contacts matters more than the contacts themselves.

"Hard aspects doom a relationship"

Squares and oppositions describe tension, not failure. Plenty of lasting partnerships show them.

"Sun signs tell you everything"

Sun-sign matching uses one point out of many. "Incompatible" Sun signs can sit beside a beautiful Moon trine. For a fuller approach, see our guide to birth chart compatibility.

"Synastry can predict whether you will stay together"

It cannot. Choices, communication and care belong to the people involved.

Going Further

Synastry rewards patience: each chart, then the contacts, the houses, the 7th house rulers and what repeats. If you want a written study of two charts together, the Synastry and Compatibility Reading covers both natal charts, the main contacts between them, house overlays where birth times allow, and the themes that repeat, with clear notes about anything an unknown birth time leaves open.

You can also read about love astrology beyond compatibility for the pattern each person brings into any relationship.

Sources and further study

Ptolemy, Tetrabiblos, Book IV, the chapter on marriage. Translated by F. E. Robbins (Loeb Classical Library, Harvard University Press, 1940).

Dorotheus of Sidon, Carmen Astrologicum, Book II, on marriage. Translated by David Pingree (Teubner, 1976); also translated by Benjamin Dykes (Cazimi Press, 2017).

William Lilly, Christian Astrology (1647), sections on marriage questions and the 7th house.

Ronald C. Davison, Synastry: Understanding Human Relations Through Astrology (1977).

Liz Greene, Relating: An Astrological Guide to Living with Others on a Small Planet (1977).

Stephen Arroyo, Relationships and Life Cycles (1979).

Robert Hand, Planets in Composite (1975), for the composite method mentioned above.

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