Synastry: How to Read a Synastry Chart (With Examples)

Quick Answer

Synastry is the astrological method of comparing two birth charts to see how two people affect each other. You place one person's planets around the other person's chart and look at two things: the aspects the planets form across the two charts, and the houses each person's planets fall into in the other's chart (called house overlays). Sun signs are only the starting point. The detail sits in contacts between the Moon, Venus, Mars and the Ascendant.

> Want to see your own? The free synastry calculator lists selected cross-chart planetary contacts with their orbs, measured in 3D. For the full side-by-side chart, use synastry chart compatibility.

Can You Read a Synastry Chart Without Birth Time?

Yes, but keep the missing information visible. Birth dates and places let you calculate provisional planetary positions; they do not establish the unknown person's Ascendant or house cusps. You can start with the free synastry preview without birth times: leave either unknown time blank and keep any recorded time you do have.

The preview uses local noon at the birthplace for each missing time. Noon is a calculation convention, not evidence of when someone was born. It gives one chart, not the full range of possible positions that day. A tight orb printed beside a noon estimate can look more certain than the input allows.

For the steps needed to check a possible Moon-sign change, read what you can find without a birth time. Start with the contacts you can support rather than interpreting an uncertain Moon or house placement as a fact about the relationship.

What a Synastry Chart Shows

A synastry chart, sometimes called a bi-wheel, is two natal charts drawn together. Your chart sits on the inner wheel with your houses. The other person's planets are placed around the outside, at their real zodiac degrees.

That layout answers three questions:

Synastry is always two-way. Your Venus trine their Mars is a different line in the reading from their Venus trine your Mars, and each person can experience the same contact differently.

How to Read a Synastry Chart in 5 Steps

1. Get accurate birth data for both people

You need date, time and place of birth for both. Without a birth time you can still read most planet-to-planet aspects, but the Moon's degree may be uncertain and the houses and Ascendant are unknown. If a time is missing, skip angles and house-grid claims that require that person’s time; the known-time chart may still provide its own houses, with appropriate uncertainty for the other person’s planets. Our guide to finding your signs without a birth time explains what still works.

2. Start with the personal planets

Read contacts between the Sun, Moon, Mercury, Venus, Mars and the Ascendant first. These describe day-to-day life together. Contacts from Jupiter and Saturn come next. The outer planets (Uranus, Neptune, Pluto) move slowly, so people born within a few years of each other share their positions; they matter mostly when they hit a personal planet or an angle.

3. List the major aspects and keep the orbs tight

The five major aspects carry most of the meaning:

Many astrologers use tighter orbs in synastry than in a single natal chart. Check the orb limit and angle convention stated by the result you are reading. A smaller orb means a contact is geometrically nearer its target angle; it does not measure how strongly either person feels it. A longitude orb and a true 3D orb need not agree.

4. Check the house overlays

Note where the other person's planets land in your houses. Their Sun in your 7th house tends to put the relationship itself in the foreground. Their Moon in your 4th house often feels like home. Their Saturn in your 10th can bring structure, or pressure, to your work and public life. The 12 astrology houses guide covers what each house rules.

5. Look for repeated themes

One aspect never defines a relationship. Look for themes that repeat: repeated Moon contacts invite an emotional-needs interpretation, Saturn contacts a discussion of responsibility, and Uranus contacts a question about change. These are symbolic themes, not verified facts about the pair. A relationship usually has both easy and hard contacts, and a challenging geometric contact does not by itself establish chemistry or conflict.

Read an Aspect Table Without Mixing the Charts

Your Venus contacting their Mars is a different comparison from their Venus contacting your Mars. Likewise, their planet in your house uses your house grid. A reliable time is needed for the owner of that grid; uncertainty in the visitor's planetary position can still affect a boundary placement.

For the actual aspect list, use the synastry preview. The full compatibility report adds a written reading and requires sign-in within the account allowance. Neither output is a measured probability of relationship success.

The Synastry Aspects That Matter Most

Sun and Moon. The classic marker of compatibility. One person's Sun on or opposite the other's Moon tends to create a sense of fitting together: one gives direction, the other emotional response.

Venus and Mars. The attraction axis. Conjunctions, trines and sextiles often feel like easy chemistry; squares and oppositions often feel like charged, restless attraction. For this pair alone, try the Venus Mars compatibility calculator.

Moon and Moon. Emotional rhythm. Harmonious Moon contacts make it easier to comfort each other; tense ones can mean you each need different things when stressed. The moon sign compatibility guide goes deeper.

Mercury contacts. How you talk, argue and make plans. Mercury square Mercury is a common source of "that is not what I said."

Saturn to personal planets. The long-term marker. Astrologers discuss Saturn contacts through responsibility, limits and time. Those symbols do not establish that a relationship will last or that either person is committed.

Contacts to the Ascendant. A planet on someone's Ascendant is felt immediately. It is often what people mean when they say they noticed someone right away.

A Worked Example: Longitude, Orb and House System

Take hypothetical positions: Anna's Moon is at 12° Cancer (102° longitude) and Ascendant at 3° Libra (183°). Ben's Sun is at 10° Capricorn (280°) and Venus at 1° Libra (181°). These are illustrative positions, not a calculated couple's complete charts.

• Sun–Moon longitude separation: 280° − 102° = 178°, giving a longitude opposition orb of 2°. A true 3D angle also needs the planets' ecliptic latitudes, so these longitudes alone cannot establish a 3D orb of 2°.
• Venus–Ascendant longitude separation: 183° − 181° = 2°. An astrologer may interpret this as a close contact under a chosen longitude orb. It does not establish mutual attraction.
• House convention: in whole-sign houses, a Libra Ascendant places all of Libra in the first house, including Venus at 1° Libra. In Placidus, the first cusp is the Ascendant at 3° Libra, so a position at 1° Libra is before that cusp; it must not automatically be labeled first-house. Check the actual house boundaries and result settings.

This example shows how to read the measurement before assigning a meaning. Compare the rest of both charts and the time confidence; two contacts cannot establish how a relationship will work.

Synastry vs Composite Chart

Synastry compares two separate charts to show how each person affects the other. A composite chart combines the two into one chart by taking the midpoints of each pair of planets, and describes the relationship as its own entity. Synastry answers "how do we affect each other?" The composite answers "what is this relationship for?" Make one with the composite chart calculator.

Frequently Asked Questions

What is synastry in astrology?

Synastry is the comparison of two birth charts. It looks at the aspects between one person's planets and the other's, and at which houses each person's planets fall into in the other chart, to describe how two people affect each other.

How do I read a synastry chart?

Start with accurate birth data for both people. Read the aspects between personal planets first (Sun, Moon, Mercury, Venus, Mars, Ascendant), keep orbs tight, then check where each person's planets land in the other's houses, and look for themes that repeat.

What are the best synastry aspects?

Astrologers often discuss Sun–Moon, Venus–Mars, Moon–Moon and Saturn contacts, but there is no measured list that guarantees a successful relationship. Interpret the actual aspects in context and compare the description with how the people communicate and behave.

Can synastry predict if a relationship will last?

No. Synastry describes dynamics: where things flow, where they rub, and what each person stirs up in the other. How a relationship goes depends on the people in it.

Do I need birth times for synastry?

Not for most aspects. Without birth times you can still read the planet-to-planet aspects, though the Moon's degree may be off. You need a reliable time for the chart whose houses or Ascendant you are using. Unknown times also make the moving Moon and any near-boundary placement uncertain, so check each contact separately.

What is the difference between synastry and a composite chart?

Synastry compares two charts side by side. A composite chart merges them into one chart built from midpoints. Synastry shows how two people interact; the composite describes the relationship itself.

Try It With Your Own Charts

• Synastry calculator: selected cross-chart contacts with measured orbs
• Synastry chart compatibility: the full two-chart comparison
• Composite chart: the chart of the relationship itself
• Moon sign compatibility: emotional fit
• Get your own birth chart: free, no account needed

Related Guides

• Aspects in astrology: what each aspect means in a single chart
• Why we measure aspects in 3D: why orbs can differ between calculators
• Planets in astrology: what each planet represents
• Zodiac compatibility guide: sign-by-sign pairings

Further reading

• Astrological compatibility on Wikipedia