Synastry: how astrologers compare two birth charts

What is synastry in astrology?

Synastry (from the Greek syn, together, and astron, star) is the technique of comparing two natal charts to understand a relationship. Instead of asking what each person is like, it asks what happens between them: which of your planets touch which of theirs, and where their planets fall in the map of your life. Astrologers have used it for centuries for marriages, business partnerships and family; today it is the engine behind any compatibility reading worth the name, including AstraLink's.

A synastry reading has two halves. Inter-chart aspects are the angles between one person's planets and the other's: your Venus conjunct their Mars, your Moon square their Saturn. House overlays are where one person's planets land in the other's houses: their Sun in your 7th house, your Moon in their 4th. Aspects describe the feeling; overlays describe the arena. Both matter, and a good synastry chart shows both at once as a bi-wheel: one chart in the inner ring, the other in the outer.

If you want a number before the theory, the birth chart compatibility calculator gives a first read from two birth dates. The rest of this page explains what the app computes when you add times and places, and how to read a synastry chart yourself.

The key aspects to check in synastry

Start with these seven contacts, in roughly this order of weight. Each works in both directions (your planet to theirs, theirs to yours), and the closer the orb, the stronger the effect.

Sun–Moon

The classic. One person's Sun conjunct, trine or sextile the other's Moon means the Sun person's identity feels nourishing to the Moon person, and the Moon person's care feels natural to the Sun person. Traditional astrologers called it the marriage aspect. Sun square or opposite Moon is still a strong pull, with a recurring feeling that one person's way of being does not quite meet the other's needs.

Moon–Moon

How you each feel safe, and whether those match. Moons in the same element or in trine decompress the same way. Moons in square want different things after a bad day (talk versus silence, closeness versus space) and can feel abandoned by each other without anyone being at fault. This aspect decides how a relationship feels on Tuesday night, not on a date.

Venus–Mars

The attraction aspect, and the one behind every search for "Venus and Mars compatibility." Venus is what you find beautiful and how you love; Mars is what you want and how you pursue. Venus conjunct or trine the other person's Mars is effortless chemistry. Venus square or opposite Mars is chemistry with friction: strong pull, frequent arguing, a pattern of make-up and break-up. No Venus–Mars contact at all is not a deal-breaker, but it is common in relationships that stay friendships. Read Venus in astrology for how each Venus sign loves.

Venus–Venus and Sun–Venus

Venus–Venus shows whether your love languages overlap. Sun–Venus shows whether the Venus person finds the Sun person's core self attractive, and whether the Sun person feels appreciated. Harmonious contacts here are why some couples simply like each other, which turns out to matter more than passion over ten years.

Saturn contacts

Saturn is the glue and the judge. Saturn conjunct or trine a partner's Sun, Moon or Venus makes the relationship feel serious, real and durable; couples with strong Saturn synastry tend to stay. Saturn square or opposite those same points feels like being graded, restricted or made responsible for the other's fears. Hard Saturn contacts do not end relationships by themselves; they make the relationship a place where one person is always slightly on trial, which needs to be named.

Pluto contacts

Pluto conjunct, square or opposite a partner's Sun, Moon, Venus or Ascendant is obsession, transformation and power. It is the aspect behind the relationships people describe as changing their life, and also behind the ones they describe as consuming it. Pluto synastry is strongest when both people are honest about control; without that honesty, one becomes the investigator and the other the suspect. The Leo–Scorpio pairing is the Sun-sign version of this dynamic.

Chiron and the lunar nodes

Chiron contacts show where you can heal each other and where you can reopen a wound; Chiron conjunct Venus, for instance, is love that touches an old injury. The North Node is direction: a partner's Sun, Moon or Venus on your North Node feels like the relationship is pulling you toward who you are becoming. Planets on the South Node feel instantly familiar, often too familiar, as if you have done this before and are at risk of doing it again.

House overlays: where their planets land in your chart

House overlays need exact birth times for both people, because houses are set by the time and place of birth. Once you have them, they answer a question aspects cannot: what area of your life does this person light up?

7th house. Their planets in your 7th house feel like a partner. Their Sun or Venus here is the classic "I could marry this person" overlay; their Saturn here is a partner who feels like a responsibility. See the 7th house in astrology for how this house describes what you attract.

5th house. Romance, play, creativity, dating. Their Venus or Mars in your 5th is fun, flirtation and the feeling of being on holiday together. Great for the beginning; not by itself enough for the long term.

8th house. Intimacy, merging, shared money, transformation. Their Moon, Venus or Pluto in your 8th is deep, sexual and slightly frightening. These relationships change people. They need trust more than most.

12th house. The unconscious, the hidden, the self-undoing. Their planets in your 12th house feel mysterious, spiritual or hard to see clearly. Some of the most tender synastry lives here; so does the pattern of not quite knowing what the other person is doing to you until later.

1st and 4th houses. Their planets in your 1st house affect how you see yourself; you feel more visible around them. In your 4th house, they feel like home and family.

A house overlay calculator does this for you: it takes both charts and lists where each planet lands. The AstraLink app does it automatically once birth times are entered.

Orbs: how exact does an aspect need to be?

An orb is the margin around an exact angle within which an aspect still counts. Synastry uses tighter orbs than natal work because the contacts are between two separate charts. Reasonable defaults:

• Conjunction, opposition, trine, square: up to 6–8° for Sun and Moon, 5–6° for Venus, Mars and Mercury, 3–4° for Jupiter, Saturn and the outer planets.
• Sextile: up to 3°.
• Minor aspects (quincunx, semi-square): 2° or less, and only when the planets involved are personal.

An aspect within 1° is strong enough to define a relationship; one at 7° is background. When you read a synastry chart, list the tightest five contacts first. They are the relationship. Applying aspects (the faster planet moving toward exactness) are traditionally read as slightly stronger than separating ones, though in synastry, where both charts are fixed, most astrologers treat the orb alone as the measure of strength.

Four synastry patterns you will see again and again

Once you have read a few dozen synastry charts, the same shapes keep appearing. Recognising them saves time and prevents over-reading a single aspect.

The friendship chart. Strong Mercury, Sun and Moon contacts, Jupiter to personal planets, and little or no Venus–Mars or 5th/8th house activity. These two like each other enormously, talk for hours, and one of them may be waiting for a romance that the chart does not describe. It can become romantic if other contacts exist; if not, it is a great friendship, which is not a consolation prize.

The obsession chart. Pluto conjunct, square or opposite the other person's Sun, Moon, Venus or Ascendant, often with 8th house overlays and Venus–Mars squares. Intense, transformative, hard to leave, prone to power struggles. Worth it when both people are honest about control; exhausting when one becomes the investigator.

The marriage chart. Sun–Moon harmony, Moon–Moon compatibility, Venus–Venus or Venus–Mars ease, and at least one stabilising Saturn contact. Often less dramatic at the start than the obsession chart, and far more likely to be standing in ten years. Astrologers look for several of these within tight orbs, not one perfect aspect.

The karmic chart. Planets on the lunar nodes, Saturn conjunct personal planets, 12th house overlays and Chiron contacts. The feeling of having known each other before, of unfinished business, of a relationship that is teaching something. Rewarding and demanding in equal measure; the North Node contacts point to the lesson.

Most real charts are mixtures. The point of naming the patterns is to see which one dominates, because that is the relationship you actually have, whatever the Sun signs promised.

Synastry beyond romance

Synastry is not only for couples. The same method reads any two charts: business partners, co-founders, parent and child, close friends, a manager and a report. The weights change. For a business partnership you care less about Venus–Mars and more about Mercury contacts (can you communicate), Saturn contacts (can you rely on each other), Jupiter (does the partnership grow) and the 10th and 2nd house overlays (career and money). For a parent–child chart, Moon contacts and Saturn contacts explain most of the emotional weather. The AstraLink compatibility feature focuses on romance because that is what most people ask, but the communication and friction scores are readable for any relationship, and the compatibility hub explains each dimension.

Composite vs synastry

Synastry compares two charts. A composite chart makes a third chart from the midpoints of the two, describing the relationship as an entity with its own Sun, Moon and Ascendant. (The Davison chart is a related method that uses the midpoint in time and space.) In practice: synastry tells you how you affect each other; the composite tells you what the relationship is for and how it behaves in the world. Read synastry first. If the synastry is good and the composite Sun is in the 12th house, you have a private, perhaps hidden, relationship; if the composite Sun is in the 10th, the relationship is public and possibly a partnership in work as well as love.

How AstraLink computes synastry

The app uses the tropical zodiac, Placidus houses and ephemeris-grade positions. It calculates every aspect between the two charts within the orbs above, weights each by aspect type, orb and the planets involved (a Sun–Moon conjunction at 1° counts far more than a Mercury–Jupiter sextile at 3°), places each person's planets in the other's houses, and folds the results into four scores: love (Venus, Mars, 5th and 7th house overlays), communication (Mercury and Moon contacts, 3rd house), friction (hard aspects from Mars, Saturn, Uranus, Pluto and Lilith, 12th house overlays) and growth (Sun–Moon, stabilising Saturn, Jupiter, nodes). An AI reading then explains the top contacts in plain English, with the tension named rather than smoothed over. The same engine powers Cosmic Dating, which suggests matches by synastry and shows you the reasons before you message anyone. Unlimited synastry readings are part of Premium; early-access members get founding-member pricing.

The free calculator on this site is a subset: Sun, Moon, Venus and Mars from birth dates, no houses. It is a good first filter; the app is the reading.