Synastry chart: how to read compatibility between two birth charts

A synastry chart compares two birth charts. You lay one chart over the other and check how one person's planets and angles connect with the other person's planets, and which of the other person's houses they fall into. Astrologers read it to understand a couple's dynamic — what draws two people together, what rubs, and where they complement each other — not to score whether the relationship will last.

TL;DR: Synastry looks at two things: the aspects between two people's planets, and the houses one person's planets land in on the other person's chart. Tradition reads the Sun–Moon, Moon–Moon and Venus–Mars contacts first, then contacts with the angles and with Saturn. Houses and angles need birth times, and synastry doesn't give you a compatibility percentage.

What does a synastry chart show?

A birth chart maps the planets at the moment and place of one person's birth. In synastry, two of these charts are placed side by side, each calculated separately for its own date, time and place. Nothing is averaged: each person keeps their own planets, signs and houses. The comparison covers three things:

• Aspects between the charts. These are the same angles you find within one chart — conjunction, sextile, square, trine, opposition — measured between a planet of one person and a planet of the other. The calculator uses the same orbs (the allowed distance from the exact angle) in synastry as in a birth chart; our guide to aspects in astrology lists them.
• Planets in the other person's houses. Each of your partner's planets falls into one of the twelve houses of your chart. Tradition reads this as that planet's themes touching the area of life the house describes: the 5th house for fun and romance, the 10th for career and goals.
• Contacts with the angles. One person's planet close to the other person's Ascendant, Descendant or Midheaven (MC, labeled Medium Coeli in the calculator) is among the most visible contacts in tradition. When a partner's planet sits on your Descendant, the start of your 7th house of partnership, tradition ties its theme directly to the relationship.

Synastry isn't the same as sun sign compatibility. Comparing “a Cancer and a Capricorn” looks at one point of each chart, the Sun's sign. Synastry compares whole charts down to the degree, so two Cancer–Capricorn couples can have completely different Moon, Venus and Mars contacts. The same method works for friends, business partners or a parent and child, though most people ask about it for couples.

Which contacts matter most in synastry?

Two charts usually produce dozens of aspects between them — 68 in the example below. Tradition sorts them by the planets involved and starts with the ones that describe everyday closeness, attraction and staying power:

• Sun and Moon. The Sun shows what someone looks for in a relationship for themselves and what they bring to it. The Moon shows what a person needs to feel safe with someone. Tradition sees a contact between one person's Sun and the other's Moon as the classic sign of a good fit in everyday life: who one person is trying to be matches what the other needs.
• Moon and Moon. Tradition reads an easy link between two Moons as everyday life that is easier to share: a similar rhythm of rest, similar ways of showing care. Tension between the Moons is read as different needs for rest and closeness, which are worth naming instead of guessing.
• Venus and Mars. In astrology Venus is the main planet of attraction and affection, and Mars describes how someone pursues another person and how they argue. One person's Venus in contact with the other's Mars is the classic sign of attraction. Tradition reads Mars in tension with Mars as rivalry that works better when it's turned into a shared goal.
• The angles and the planets. The Ascendant is the first impression and the way someone approaches the world. Tradition reads a partner's planet close to your Ascendant as a contact noticed right away, its theme showing in how you act around that person. A planet on the Descendant is linked with the partnership itself, and one on the Midheaven with goals and career. These contacts need the exact birth time of the person whose angles you're looking at.
• Saturn. Tradition links a contact between Saturn and the other person's planet with durability and a sense of commitment, and sees Saturn as the stabilizing factor. It has another side too: coolness or a feeling of being judged, especially in squares and oppositions. It works best when the couple talks about expectations. Tradition describes Venus–Saturn contacts as lasting, but with some coolness to work through.
• Mercury. Tradition reads Mercury contacts as how easily two people understand each other: a similar style of talking as fewer misunderstandings, a different one as a call for patience with important decisions.

Uranus, Neptune and Pluto move so slowly that their aspects to each other are shared by people born years apart. Pluto conjunct Pluto between two people of a similar age says more about their generation than about the couple. These planets become personal only when they touch the other person's Sun, Moon, Venus, Mars or angles. Tradition then links Neptune with a sense of special closeness and the risk of idealizing, and Uranus with excitement and freshness, but also restlessness.

The type of aspect also reads a little differently in synastry than in a single chart, because it describes two people meeting:

How to read a synastry chart, step by step: an example

Here is a fictional couple calculated with the Numato engine, the same one the calculator uses. Person 1: New York City, July 4, 1995, 9:20 a.m. (EDT, UTC−4), the same fictional person as in our 7th house example. Person 2: Manchester, January 17, 1992, 4:50 p.m. (GMT, UTC+0). Without names, the calculator labels them “Person 1” and “Person 2.”

1. Start with both people's Big Three. The calculator shows it for each person separately. Person 1 has the Sun in Cancer, the Moon in Virgo and a Leo Ascendant. Person 2 has the Sun in Capricorn, the Moon in Gemini and a Leo Ascendant too.
2. Pick the classic pairs from the list. The engine found 68 aspects between these two charts. The calculator shows the 25 strong ones, major aspects and quincunxes with an orb of up to 3°, right away, and the other 43 under “Show weaker aspects (43).” The list runs from the tightest aspect, not the most important one, so the very top is Person 1's Ascendant quincunx Person 2's Sun (0°04′). Look for the Sun, Moon, Venus, Mars, Saturn and the angles first.
3. Read each aspect from both sides. Tradition reads the same aspect differently for the person whose planet it is and for the person whose point it touches.
4. Check the houses on the couple's wheel. They show which area of life the relationship touches for each person.
5. Leave the generational aspects for last.

Person 1's Moon trine Person 2's Sun (0°24′). Both sit in earth signs, and this is the classic Sun–Moon pair. A Capricorn Sun describes a person whose way of being themselves runs through goals, responsibility and perseverance. A Virgo Moon describes feeling safe when things are in order and help is practical. Tradition reads the trine this way: who Person 2 is trying to be fits what Person 1 needs to feel at ease.

Moon square Moon (0°30′). Someone with the Moon in Virgo settles down by sorting things out and fixing what's broken; someone with the Moon in Gemini, by talking, moving around and hearing new ideas. Tradition reads a square between the Moons as different needs for rest and closeness, which are worth naming instead of guessing. Day to day, it can look like one person quietly tidying up after an argument while the other wants to talk it through right away.

Person 1's Mars square Person 2's Venus (0°22′). This is the tightest of the classic pairs. Venus in Sagittarius describes affection shown through freedom, humor and plans for the future, while Mars in Virgo describes someone who pursues the other person carefully and precisely. In tradition a Venus–Mars contact is the classic sign of attraction, and the square adds friction: different styles that irritate but also keep things lively, so both people need to put in some work.

Person 1's Saturn: square the Moon (1°09′) and sextile the Sun (2°04′). One planet makes two contacts here, and they show both sides of Saturn in synastry. The sextile to Person 2's Sun is read as the stabilizing side: durability, commitment and support for Person 2's goals. The square to Person 2's Moon is read as the other side: coolness or a feeling of being judged in emotional matters. Tradition says this works best when the couple talks openly about expectations instead of assuming them.

Person 1's Ascendant sextile Person 2's Moon (0°59′). The Ascendant is the first impression and the way someone approaches the world. Tradition reads this as Person 2's moods and need to talk finding an easy response in Person 1's warm, expressive Leo way of meeting people. A sextile is an opening, not a given: it shows up once someone makes the first move. This contact would vanish without Person 1's birth time, because there would be no Ascendant.

Generational planets on a personal point. Person 1 was born in 1995 with Neptune and Uranus late in Capricorn, close to Person 2's Sun at Capricorn 26.8°: Neptune conjunct the Sun at 2°19′ and Uranus conjunct the Sun at 2°20′. People born around then have these planets in roughly the same place, so on their own they describe a generation. Here they touch Person 2's Sun, which makes them personal: tradition links Neptune with a sense of special closeness and the risk of idealizing, and Uranus with excitement, but also restlessness.

Houses: where one person's planets land in the other's chart

Person 1 has a birth time, so on the couple's wheel Person 1 sits in the middle with houses and angles, and Person 2's planets form the outer ring. The wheel shows Person 2's Sun, Mercury and Mars, all in Capricorn, landing in Person 1's 5th house, which the calculator describes as “Creativity, joy, romance and children.” Tradition reads this as Person 2 bringing fun, play and romance into Person 1's life. Person 2's Venus lands in Person 1's 4th house (home, family and origins), and Person 2's Moon in the 11th (friends, groups and plans for the future).

The calculator doesn't list the other person's houses in a table; you read them off the wheel. To see the other side, enter the two people in reverse order. Then Person 2 sits in the middle, and you can see Person 1's Moon and Mars landing in Person 2's 3rd house, the house of conversation and learning, and Person 1's Jupiter landing in Person 2's 5th.

Put together, tradition would describe this couple as an easy match between one person's goals and the other's needs (Moon–Sun), attraction with some friction (Mars–Venus), and two areas to work on: different needs for rest and closeness (Moon–Moon) and expectations around feelings (Saturn–Moon). That describes a dynamic. It doesn't say whether these two belong together.

How to compare two charts in the calculator

1. Open the synastry calculator and enter both people's birth date, time and place (names are optional). For the time, choose how much you know: “I know the exact time,” “I know the approximate time,” “I only know the time of day” or “I don't know the time.”
2. In the result, each person gets a “Moment of birth” card and their own “Big Three”: Sun, Moon and Ascendant.
3. “The couple’s wheel” shows Person 1's chart in the middle, with houses and angles, and Person 2's planets on the outside. The lines are aspects between the charts. If only Person 2 has a birth time, the calculator puts Person 2 in the middle.
4. “Aspects between the charts” lists the aspects from the tightest. Each row shows one person's name and point, the aspect, the other person's name and point, and underneath the orb and phase, such as “orb 0°24′ · applying.” Major aspects and the quincunx come with a short explanation.

In the calculation settings you can change the house system and the zodiac. The houses on the couple's wheel belong to the person in the middle.

Synastry without birth times: what you lose

Planets move slowly, so their positions depend mostly on the date. The birth time and place set the Ascendant, the Midheaven and the houses. That's why a synastry chart from birth dates alone is possible, but incomplete:

• The houses and angles of the person without a time drop out. You can't see which of their houses the partner's planets fall into, and there are no aspects to their Ascendant or Midheaven. If Person 2's birth time were unknown in the example, the six aspects to Person 2's Ascendant and Midheaven would disappear, and Person 2's Big Three would say: “The Ascendant depends on the exact time of birth — we don't calculate it without one.”
• The Moon becomes a range. The Moon moves more than ten degrees a day: on January 17, 1992, it went from Gemini 15.6° to Cancer 0.4°. Person 2's Moon card would then read: “The Moon changes sign within this time range: from Gemini to Cancer. A more precise time will settle it.” Eighteen aspects, 13 of them with Person 2's Moon, including the Moon–Moon square, would carry the note “depends on the exact time — it may not have occurred within the time range.”
• The wheel loses its houses only when neither person has a time. If one person knows it, the calculator puts that person in the middle with their houses. Without both times, the note under the wheel says: “Without a time of birth the wheel has no axes or houses — it shows the planets in their signs.”

With an approximate time, the calculator shows a person's angles and houses only if neither the Ascendant nor the Midheaven changes sign anywhere in the time range you entered. If you're looking for a recorded time, our guide on how to find your birth time explains which official birth records include it in the US and the UK.

Synastry, the composite chart and numerology

A composite chart doesn't set two charts side by side; it builds a single chart in which each planet sits at the midpoint between the same planet in both birth charts. So it describes the relationship as a unit, while synastry shows how two particular people meet — see composite vs. synastry in our composite chart guide.

Numerology describes a couple with a different system that needs no birth time or place. At Numato that's the Relationship analysis, based on birth dates and names, and the “Destiny Matrix compatibility,” based on birth dates alone. Both come in handy when one of you doesn't know their birth time.

What synastry can't tell you

We calculate the planets in both charts astronomically, from NASA JPL ephemerides. The meanings of aspects and houses come from astrological tradition, explained here in plain language. That tradition has limits:

• It doesn't score compatibility. Synastry gives no percentage and no points. A trine isn't worth more than a square: ease can go unused, and tension can keep a relationship alive.
• It doesn't answer “are we meant to be?” It describes a dynamic: what attracts, what irritates and where the work is. Whether a relationship works out depends on the choices of two people.
• It doesn't tell you what will happen. A synastry chart doesn't give a wedding date or a breakup.
• It isn't about gender and doesn't judge. One person's Venus in contact with the other's Mars means the same whoever is a woman and whoever is a man.

If a relationship is going through a crisis, talking to a professional, such as a couples therapist, will help more than any chart.

Frequently asked questions

What does a synastry chart show?

A synastry chart shows how two birth charts connect: the aspects between both people's planets, the contacts between one person's planets and the other's angles, and which houses one person's planets land in on the other's chart. Tradition reads from this what draws the two together, what causes friction and where they complement each other. It describes the dynamic of a relationship, not whether it will succeed.

Can you do synastry without a birth time?

Yes, partly. The date gives the positions of the Sun and planets, so most aspects between the charts can still be calculated. Without a birth time, though, that person has no Ascendant, Midheaven or houses, and some Moon aspects are uncertain because the Moon moves more than ten degrees a day. The calculator flags those aspects with a note saying they depend on the exact time.

What is the difference between synastry and a composite chart?

Synastry keeps two separate charts and compares them: it shows how each person's planets meet the other's. A composite chart blends two charts into one, placing each planet at the midpoint between the two people's positions of that planet, and reads the relationship itself as a separate whole. Synastry answers how the two of you interact; the composite chart describes what the relationship is like as a whole.

Which aspects matter most in synastry?

Tradition treats the conjunction as the most important contact between two charts, because it stands for a strong sense of togetherness in the matters of both planets. The orb counts too: an aspect within 1° weighs more than one at 8°. Contacts involving the Sun, Moon, Venus, Mars, Saturn and the angles say the most. Aspects between Uranus, Neptune and Pluto of people born close together describe their generation, so they come last.

Is a difficult synastry a deal-breaker?

No. Squares and oppositions between two charts describe friction, not a verdict. Tradition reads a square as different styles that irritate but also keep the relationship lively, and an opposition as opposites that attract and complement each other. A couple with tense contacts can do well by talking about their differences, while easy trines can go unused. Synastry shows where the work is, not whether to do it.

This content is for educational purposes and supports self-reflection; it does not replace professional advice.