Synastry Basics: How to Compare Two Birth Charts Fairly
Every astrology fan has checked whether their sign is “compatible” with someone else’s. Sun-sign compatibility is a playful starting point, but astrologers who study relationships use a much fuller method called synastry: comparing two complete birth charts. This guide explains how it works, what to look for and — just as importantly — what it cannot tell you.
Beyond Sun signs
Simple compatibility tables match Sun signs by element. Fire and air are said to energise each other; earth and water to nourish each other; signs of the same element to understand each other; opposite signs to attract. The compatibility checker on our homepage uses exactly these classic rules and explains each result.
These rules are a useful sketch. But a Sun sign is only one of many placements, and two people with “incompatible” Suns can have deeply harmonious Moons, Venuses and ascendants.
The placements that matter most
- Sun and Moon. When one person’s Sun connects harmoniously with the other’s Moon, astrologers traditionally see mutual understanding and support.
- Venus and Mars. These describe attraction and desire. Links between one person’s Venus and the other’s Mars are classic indicators of chemistry.
- Mercury. Compatible Mercury signs suggest easy conversation; clashing ones suggest different thinking styles that require patience.
- The ascendant and descendant. One person’s planets falling near the other’s descendant (seventh-house cusp) are traditionally read as a sense of partnership.
- Saturn. Connections to Saturn can bring commitment and stability — or a feeling of restriction.
Aspects: the angles between charts
Synastry looks at the angles between one person’s planets and the other’s. Conjunctions (same position) blend energies strongly. Trines (120 degrees) and sextiles (60 degrees) are considered flowing. Squares (90 degrees) and oppositions (180 degrees) are considered challenging but often stimulating — much attraction comes from tension.
House overlays
If you know both birth times, you can see which of your houses the other person’s planets fall into. Their Venus landing in your fifth house may feel romantic and playful; their Saturn in your tenth may feel like career support or pressure.
What synastry cannot do
This is the most important section. No chart comparison can:
- Guarantee that a relationship will succeed or fail.
- Tell you whether someone is trustworthy or kind.
- Replace communication, shared values and mutual respect.
Real relationships depend on choices, effort and circumstances. Many happy couples have “difficult” synastry, and many “perfect” matches part ways. Astrology at its best offers a vocabulary for understanding differences — not a verdict on whether to stay or leave.
Using synastry well
- Build both charts in the Natal Blueprint.
- Compare each person’s elemental balance using the Element Scale. Does one supply what the other lacks?
- Compare Mercury signs to understand communication differences.
- Compare Venus and Mars to understand how each of you shows and seeks affection.
- Use what you notice as a conversation starter, not a judgement.
Friendship and family
Synastry is not only for romance. Comparing charts with friends, siblings, parents or colleagues can explain recurring misunderstandings and help you appreciate different temperaments.
Learn more about the elements behind compatibility in the elements section of the Star Codex.