Synastry Chart: How to Read Relationship Compatibility in Astrology

A synastry chart is not a compatibility score. It is a map of how two people’s planetary energies interact - where they flow easily, where they create friction, and where they push each other to grow.

Astrologers have used synastry (from the Greek “syn,” meaning together, and “astron,” meaning star) for centuries to examine why some relationships feel like coming home and others feel like running into a wall. The roots of synastry analysis trace to the Hellenistic tradition - second-century astrologers like Ptolemy were already documenting how planetary angles between two charts revealed relationship dynamics. The technique works by overlaying one person’s birth chart directly on top of another’s, then examining every point where their planets form geometric angles called aspects.

This guide covers how synastry works, which aspects matter most, how to read house overlays, and what astrologers look for when analyzing a relationship chart.

How a Synastry Chart Works

Your birth chart is a snapshot of the sky at the exact moment you were born - the positions of the Sun, Moon, and planets relative to Earth, laid out in a 360-degree wheel divided into 12 houses. (If you haven’t calculated yours yet, the birth chart calculator guide covers the three inputs you need: date, time, and location.)

In a synastry reading, your chart and a partner’s chart are placed together. Each planet in Person A’s chart is compared to every planet and point in Person B’s chart. The analysis focuses on the geometric angles - aspects - formed between these planetary pairs.

An aspect is exact when two planets fall at a specific angular relationship. In practice, aspects are read within a range called an orb - the acceptable degree of deviation from the exact angle. Professional astrologers typically use orbs of 6-8 degrees for major aspects between personal planets, narrowing to 1-3 degrees for points like the Ascendant or Midheaven. The tighter the orb, the stronger and more felt the interaction.

The core question synastry asks: when your planetary energies meet, do they cooperate, clash, or transform each other?

For a deeper look at how aspects work in a single chart before applying them to synastry, see Astrology Aspects: Conjunction, Square, and Trine Explained.

The Five Synastry Aspects

Conjunction (0deg) - Planets in the same degree of the zodiac. The energies merge and intensify. This can be magnetic and absorbing, or overwhelming, depending on which planets are involved. As astrologer Liz Greene wrote in Relating: An Astrological Guide to Living with Others (1977): “Two people placed side by side in the same life situation will experience it utterly differently, because they are utterly different - and it is this difference which makes the alchemical interaction between them possible.” (Wikipedia)

Sextile (60deg) - A harmonious angle of opportunity. The planets cooperate naturally and can develop easily with effort. Less charged than a trine, but productive and supportive.

Square (90deg) - The friction aspect. Planets in square create tension, challenge, and conflict. Squares often appear in long-term relationships because they generate enough activation to keep two people engaged. A relationship with no squares may eventually feel flat.

Trine (120deg) - Natural ease and flow. The planets work together with minimal effort. Trines describe compatibility that feels effortless - shared values, similar rhythms, automatic understanding.

Opposition (180deg) - Planets directly across from each other create push-pull dynamics. The opposition is the “mirror” aspect: you see something in the other person that reflects, completes, or directly challenges your own chart. Oppositions in synastry often explain magnetic attraction between people who seem very different.

The Most Important Synastry Aspects

Not all planetary combinations carry equal weight. These are what astrologers examine first.

Sun-Moon Aspects

When one person’s Sun makes an aspect to the other’s Moon, it describes the core emotional dynamic of the relationship. A Sun-Moon conjunction or trine indicates a natural sense of support - the Moon person feels nurtured by the Sun person, who in turn feels appreciated and recognized. A square or opposition creates friction between core identity and emotional needs.

The Sun-Moon aspect is most consistent in long-term relationships. Stephen Forrest wrote in The Inner Sky (1988): “The birth chart is not a prison. It is a map of potential.” (Wikipedia) Applied to synastry: the Sun-Moon connection is the map’s most important road.

Venus-Mars Aspects

Venus-Mars contacts are the primary indicators of physical attraction and romantic chemistry. When Person A’s Mars makes an aspect to Person B’s Venus, or vice versa, there is usually strong attraction. The conjunction and trine generate obvious heat. The square generates tension-filled attraction - the kind that burns bright and can be difficult to sustain long-term.

For context on how each partner’s Venus sign shapes their love language, see the Zodiac Signs Compatibility Chart and the Sun, Moon, and Rising Sign Meaning article.

Moon-Moon Aspects

Two people’s Moons in aspect show how emotionally compatible they are day to day. A Moon-Moon trine means emotional life flows in sync - similar rhythms, similar needs, natural comfort in domestic life. A Moon-Moon square creates friction around feelings and habitual responses.

To understand how Moon sign affects emotional baseline before reading synastry, see the Moon in Scorpio and Moon in Taurus articles on the Sidera blog.

Saturn Aspects to Personal Planets

Saturn contacts are the commitment aspects. When one person’s Saturn falls on another person’s Sun, Moon, Venus, or Mars, it creates a lasting bond - but also pressure, responsibility, and sometimes restriction. Saturn-Sun or Saturn-Moon in synastry appears consistently in long-term partnerships and marriages. The Saturn person often feels like a stabilizing force; the personal planet person may feel constrained over time.

Venus-Venus Aspects

Two Venuses in harmonious aspect describe shared values, aesthetic compatibility, and the same basic idea of what love should look like. A Venus-Venus square or opposition can mean very different love languages and financial priorities - one person wants security and routine, the other wants novelty and spontaneity.

House Overlays: Where Your Energy Lands

House overlays are one of the most overlooked elements of synastry. When a planet from Person A’s chart falls into a specific house of Person B’s chart, it activates that area of their life.

Understanding the 12 houses first makes overlays far more legible. See 12 Astrology Houses Explained for a full breakdown.

Partner’s Sun in your 1st house - They illuminate your sense of self and identity. You feel seen, energized, and more yourself around them. There is a natural sense of vitality when this person is present.

Partner’s Venus in your 7th house - Their Venus naturally orients toward partnership with you. This is one of the clearest indicators of romantic interest and long-term compatibility potential in the overlays.

Partner’s Moon in your 4th house - Emotional resonance around home, family, and domestic life. This person feels like family. Strong indicator of wanting to build a home life together.

Partner’s Mars in your 8th house - Charged, transformative energy. Mars here activates depth, sexual connection, and sometimes power dynamics. This overlay rarely produces indifferent relationships.

Partner’s Saturn in your 12th house - Saturn falls on the house of hidden fears, solitude, and the unconscious. The Saturn person may activate the other person’s anxieties or draw attention to unresolved patterns. Can feel heavy or therapeutic, depending on the rest of the chart.

Partner’s Jupiter in your 9th house - Expansion, travel, learning, and philosophical alignment. Relationships with this overlay often feel growth-oriented - like this person makes you want to explore and discover.

Bonus Points: Juno and the North Node

Two points beyond the traditional planets appear in many synastry readings.

Juno - The asteroid associated with committed partnership and marriage patterns. When one person’s Juno conjuncts or trines the other’s Sun, Moon, or Venus, it often indicates a sense of “this is my person” - even before conscious awareness of why.

North Node connections - When one person’s North Node (the point of soul direction in the chart) contacts another’s personal planets, the relationship tends to feel purposeful or karmic. The planet person may be helping the Node person move in their intended direction. North Node contacts are common in relationships that feel like they carry long-term significance.

Green Flags and Red Flags in Synastry

No synastry chart is all harmonious aspects or all difficult ones. But certain patterns signal compatibility - or significant challenge.

Strong compatibility indicators:

• Sun-Moon conjunction or trine (core emotional compatibility)
• Venus-Mars aspect of any kind (attraction and desire)
• Moon-Moon trine or sextile (emotional sync)
• Jupiter contacting personal planets (generosity, growth, optimism toward each other)
• Ascendant-Descendant axis connections (natural orientation toward partnership)

Patterns that require awareness:

• Saturn squaring or opposing the Moon (emotional restriction or criticism dynamic)
• Mars squaring or opposing Mars (constant friction and power competition)
• Pluto conjunct Moon or Venus (deep transformation, but also control dynamics)
• Multiple 12th house overlays without counter-balancing positives (hidden relationship, secrecy, or unresolved patterns)

Synastry vs. Composite Chart

Synastry and composite charts are two tools for the same question.

Synastry: Shows how two individuals’ charts interact. It is about the experience each person has of the other.

Composite chart: Creates a single chart from the midpoints between two charts. It represents the relationship itself as a third entity - not who you are individually, but who you are together.

Most astrologers use synastry first to understand the raw chemistry and friction, then the composite chart to understand the relationship’s nature and long-term trajectory.

How to Read a Synastry Chart Step by Step

Step 1: Start with the Luminaries. Look at Sun-Moon and Moon-Moon contacts first. These describe the emotional foundation of the relationship.

Step 2: Check Venus and Mars. Look for Venus-Mars aspects between the two charts. These describe attraction, desire, and what drives each person romantically.

Step 3: Look at Saturn. Saturn contacts explain why people stay together even when things get difficult. They add gravity and longevity - sometimes weight.

Step 4: Examine House Overlays. Note which houses each partner’s planets activate. This reveals the practical shape of the relationship - where you influence each other most.

Step 5: Check bonus points. Juno and the North Node add context about partnership patterns and karmic connections.

Step 6: Read the whole chart. One difficult square does not make a relationship impossible. Synastry is cumulative. Multiple patterns across planets tell the complete story.

What Sidera Shows in Synastry

Most compatibility tools compare Sun signs. Sidera’s synastry feature does the actual astrological work: it calculates precise planetary positions for both people, identifies every aspect formed between the two charts, and shows the orb for each aspect.

Rather than “Aries and Sagittarius are compatible,” Sidera shows you that your Venus at 14deg Taurus forms a 3deg trine to your partner’s Mars at 17deg Capricorn - a tight, specific aspect with real astrological meaning.

This is the difference between a generic compatibility reading and actual synastry analysis.

For more on how to interpret planetary positions in the full chart context, see How to Read a Birth Chart for Beginners, Sun, Moon, and Rising Sign Meaning, and the Planets in Astrology Cheat Sheet.

FAQ

What is a synastry chart? A synastry chart overlays two people’s birth charts to show how their planetary energies interact. Astrologers examine the aspects (geometric angles) formed between Person A’s planets and Person B’s planets to understand compatibility, attraction, friction, and growth dynamics in the relationship.

How do I create a synastry chart? You need both people’s birth data: date of birth, time of birth, and place of birth. Enter both sets of data into a synastry chart calculator. Sidera calculates synastry using precise planetary positions and shows all inter-chart aspects with exact orb measurements.

What is the most important aspect in synastry? Sun-Moon aspects are generally considered the most significant for long-term compatibility. They describe the core emotional dynamic between two people. Venus-Mars aspects are the primary indicators of physical attraction and romantic chemistry.

What does Saturn in synastry mean? Saturn contacts - especially when one person’s Saturn makes an aspect to the other person’s Sun, Moon, or Venus - create lasting bonds with a sense of commitment and structure. The Saturn person may feel like a stabilizing influence; the personal planet person may feel pressure or limitation over time. Saturn contacts are common in long-term partnerships and marriages.

What is the difference between synastry and a composite chart? Synastry shows how two individual charts interact - the experience each person has of the other. A composite chart creates a single new chart using the midpoints between two charts, representing the relationship itself as a third entity. Synastry reveals chemistry and friction; the composite chart reveals the relationship’s purpose and direction.

What does it mean when someone’s planet falls in your 7th house? The 7th house is the house of partnership and committed relationships. When a partner’s planet - especially their Venus, Sun, or Moon - falls in your 7th house, it creates a strong pull toward partnership. This overlay indicates the other person naturally activates your relationship instincts and may feel like an obvious fit for a committed role.

Can synastry predict if a relationship will last? Synastry shows the dynamics at work between two people - not a guaranteed outcome. Strong Sun-Moon, Saturn, and Juno contacts are associated with long-term bonds. Difficult Mars-Mars or Saturn-Moon contacts describe friction that requires conscious work. The chart shows the terrain; the people decide how to navigate it.

What is the significance of the North Node in synastry? The North Node is the point of soul direction in the birth chart. When one person’s North Node makes a strong contact with another person’s personal planet (especially the Sun, Moon, or Venus), the relationship often carries a sense of direction or long-term purpose. The planet person tends to support the Node person’s development.

Explore Compatibility by Sign

Synastry aspects carry different textures depending on the signs involved. Explore sign-specific compatibility patterns:

• Zodiac Signs Compatibility Chart
• Moon in Scorpio in Relationships
• Moon in Taurus: Stability in Love
• Sun in Libra: The Partnership Sign
• Sun, Moon, and Rising: The Three Core Placements
• How to Read a Birth Chart for Beginners

Or check your daily horoscope to see how current transits are affecting your relationships: Aries | Taurus | Gemini | Cancer | Leo | Virgo | Libra | Scorpio | Sagittarius | Capricorn | Aquarius | Pisces