What is synastry? A beginner’s guide to relationship astrology

Updated: 26 October 2026

Synastry is the branch of astrology that compares two people’s birth charts to describe their relationship. Instead of asking “are our signs compatible?”, it looks at both charts in full.

How does synastry work?

A birth chart maps where the planets were at the exact moment and place you were born. In synastry, two charts are laid on top of each other and astrologers look at the angles, called aspects, that one person’s planets form with the other’s. If one person’s Venus sits on the same degree as the other’s Mars, for example, astrology reads that as strong romantic and physical attraction.

Synastry works in both directions. Your Moon might touch their Sun while their Venus touches your Saturn. Each contact tells a different story about who feels what in the relationship.

What information do you need?

• Date of birth: sets the sign and degree of each planet.
• Time of birth: sets the Ascendant, the houses and the Moon’s exact position. The more accurate the time, the more detailed the reading.
• Place of birth: sets the time zone and the latitude and longitude used for the Ascendant.

If you don’t know a birth time, you can still read synastry; the Ascendant and house overlays are simply left out.

What do synastry aspects mean?

The aspect between two planets shows how their energies interact. It doesn’t have to be exact: the allowed margin is called the orb, and the tighter the orb, the more strongly the contact is felt.

For a closer look at each aspect, see the synastry aspects guide.

The most important contacts in synastry

• Sun–Moon: the classic “soulmate” contact, linking one person’s identity with the other’s emotional world.
• Venus–Mars: the best-known sign of romantic and physical chemistry.
• Moon–Venus: tenderness, warmth and nurturing each other with love.
• Moon–Moon: similar emotional needs and a similar idea of “home”.
• Saturn contacts (Moon–Saturn, Venus–Saturn): commitment, responsibility and staying power. Demanding, but common in long relationships.
• Venus–Pluto: intense, magnetic and transformative attraction.
• Ascendant contacts: first impressions and how you feel around each other.

House overlays: which part of your life does your partner touch?

With a birth time, you can also see which house of one chart the other person’s planets fall into. If your partner’s Venus falls in your 7th house, astrology reads them as someone who affects you in the area of partnership and marriage. The 5th house describes romance and fun, the 4th a sense of home, and the 8th deep intimacy.

How to read a compatibility score

The compatibility score on Astro Sinastri weighs every contact between the two charts by the importance of the planets involved and how tight the aspect is. Looking at the separate areas (romance, passion, emotional bond, communication and long term) tells you far more than a single number. A low area doesn’t mean “impossible”; it means “this takes more effort”.

The limits of synastry

Astrology is not a science, and no chart decides a relationship’s future. Synastry offers a language for understanding each other and talking about your relationship. It is at its best when it gives you a new angle on the question “why do we feel this way?”