Synastry chart: how to read birth chart compatibility

What is synastry in astrology?

Synastry is the branch of astrology that compares two people’s birth charts. The word comes from Greek roots meaning ‘together’ and ‘star’. It is most often used for romantic compatibility, but the same method works for friends, family members, and business partners.

A Sun sign compatibility chart, such as ‘Aries and Libra’, uses only one placement for each person. Synastry uses the full chart: the Moon’s emotional needs, Venus’s way of loving, Mars’s drive and temper, Mercury’s communication style, and how each of these meets the other person’s planets.

What you need for a synastry chart

Many synastry chart calculators display the result as a bi-wheel: one chart in the inner circle and the other person’s planets in the outer ring, with lines showing the aspects between them. If only one birth time is known, you can still read that person’s houses with the other person’s planets placed inside them.

• Both birth dates. These are enough for the Sun and most planet signs.
• Both birth places, so the time zone and location can be calculated.
• Both birth times, if possible. Without them, the ascendant, houses, and sometimes the Moon’s sign are uncertain, so house overlays cannot be read.

How to read a synastry chart, step by step

1. Compare the Big Three. Look at both Sun, Moon, and rising signs and their elements. Fire and air signs often energize each other, while earth and water signs often stabilize each other.
2. Check the Moon. The Moon describes emotional needs and comfort. Aspects to the other person’s Moon show how easily you feel at home with each other.
3. Look at Venus and Mars. Venus shows how each person loves and what they value. Mars shows desire, drive, and how each person handles conflict.
4. Read Mercury. Mercury aspects show how easily you talk, plan, and resolve misunderstandings.
5. Find Saturn contacts. Saturn aspects to personal planets are traditionally read as commitment and responsibility, and sometimes as pressure or restriction.
6. Add house overlays. Note which of your houses the other person’s planets fall into, and vice versa.
7. Summarize the themes. Look for patterns that repeat, such as several links between Venus, Mars, and the Moon, rather than judging the relationship by one aspect.

Synastry aspects: what the angles mean

A synastry aspect is an angle between a planet in one chart and a planet in the other. Most astrologers use a tight orb, often about 5° or less, for the strongest contacts. The five major aspects are read in synastry as follows:

• Conjunction (0°): the two planets merge. The theme is intense and very noticeable to both people.
• Sextile (60°): friendly, cooperative energy that grows when both people make an effort.
• Square (90°): friction and attraction at the same time. It pushes both people to adapt.
• Trine (120°): natural ease and understanding. It feels comfortable, sometimes so comfortable that it is taken for granted.
• Opposition (180°): a strong pull between opposites. It can feel magnetic and requires balance.

Key synastry aspects for love and attraction

Aspects to the ascendant also matter. A partner’s Moon or Venus conjunct your ascendant is often read as instant familiarity or attraction, but it can only be found when your birth time is known.

• Venus conjunct Mars synastry: a classic sign of romantic and physical attraction. Venus is drawn to Mars’s energy, and Mars responds to Venus’s affection.
• Venus square Mars synastry: strong chemistry with some friction, because the two people want closeness and excitement in different ways.
• Moon conjunct Venus synastry: tenderness and emotional comfort. The Venus person often feels caring toward the Moon person’s needs.
• Sun conjunct Moon synastry: one of the most traditional compatibility indicators. The Sun person’s identity and the Moon person’s emotional needs fit together.
• Moon conjunct Mars synastry: passion and protectiveness, but also quick emotional reactions in arguments.
• Venus aspects to Pluto: intense attraction and depth. Squares and oppositions can bring jealousy or power struggles, so they are worth discussing openly.
• Saturn aspects to the Moon or Venus: a sense of seriousness and loyalty, sometimes with one partner feeling judged or restricted.

House overlays in synastry

House overlays show where a partner’s planets land in your chart. A planet in a house is read as that person activating the area of life the house represents. Some overlays that are often discussed in relationship astrology:

• Planets in your 1st house: the other person strongly affects how you see yourself and how you present yourself.
• Planets in your 5th house: fun, romance, creativity, and play.
• Planets in your 7th house: a natural sense of partnership and commitment.
• Mars in your 8th house in synastry: intense physical and emotional attraction, and sometimes control issues around shared resources.
• Planets in your 4th house: a feeling of home, family, and emotional safety.
• Planets in your 12th house: a hidden, dreamy, or spiritual connection that can be hard to put into words.

Synastry vs. composite chart

Synastry compares two separate charts and shows how each person affects the other. A composite chart combines the two charts into one, usually by taking the midpoint between each pair of planets, such as the halfway point between both Suns. It is read as the chart of the relationship itself.

Astrologers often use both. Synastry answers ‘how do we affect each other?’ while the composite chart answers ‘what is this relationship about as a whole?’ Both approaches depend on accurate birth details.

What does good synastry look like?

There is no single score for compatibility in synastry. Charts with many trines can feel easy but low in excitement, and charts with several squares can feel difficult but very motivating. Astrologers usually look for a mix: some harmonious contacts for ease, a few challenging ones for chemistry, and connections between the Moon, Venus, and Mars.

It also helps to remember the limits. Astrology has not been shown to predict relationship outcomes, and no aspect describes what a person actually thinks or feels. Use synastry to name differences, such as how each of you handles conflict or affection, and then talk about them with your partner.

Explore synastry with Astrology AI

In Astrology AI, add your own birth details, then open Chats and share your partner’s birth date, time, and place. You can ask: ‘Compare our charts, list the strongest synastry aspects between our Moon, Venus, and Mars, and explain one strength and one topic to talk about.’ Follow-up questions let you look at one aspect or house overlay at a time. For more prompts, see our guide to couple compatibility questions.