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Synastry is the astrology of relationships. It compares two birth charts to see how two people affect each other. An astrologer lays one chart over the other and studies the angles between their planets and the houses each person’s planets land in. That gives a far richer picture than matching Sun signs.
The word comes from Greek roots meaning “stars together.” It works for any pair, whether partners, friends, a parent and child, or two coworkers who share a desk. The method stays the same. Only the questions change.
Why Sun-sign compatibility is only a start
Most people meet relationship astrology through Sun-sign matches, such as whether a Taurus suits a Virgo. That is a fair first step. Our compatibility pages start there, because the Sun shows the core of how each person wants to live.
Still, the Sun is one body among ten. Two people with “difficult” Sun signs may share a warm Moon contact and an easy Venus link. Two “ideal” Sun signs might have Mars squaring Saturn between them. Synastry looks at all of it. To see why the Moon and rising sign matter so much, read the guide to the big three first.
Aspects between two charts
Synastry rests on the cross-chart aspect, sometimes called an inter-aspect. You take a planet in one chart and measure its angle to every planet in the other chart. The angles are the same as in a single chart. They are the conjunction at 0°, sextile at 60°, square at 90°, trine at 120° and opposition at 180°. Most astrologers keep orbs a little tighter between charts, around 5 or 6 degrees for the major aspects.
Some contacts carry more weight than others. Astrologers check these first:
- Sun and Moon: one person’s identity meets the other’s emotional needs. A conjunction or trine feels natural and familiar.
- Moon and Moon: shows how easily two people share moods, routines and comfort.
- Venus and Mars: the classic contact for attraction, desire and romantic chemistry.
- Mercury and Mercury: how well two people talk, joke and solve problems together.
- Saturn to personal planets: commitment, duty or weight. It can steady a bond or make it feel tight.
- Planets to the rising sign: strong first impressions and a feeling of recognition.
Easy aspects (trines and sextiles) describe flow and comfort. Hard aspects (squares and oppositions) describe friction, and friction is also spark. Many lasting bonds rely on a few hard contacts that keep things alive, plus enough easy ones to rest in.
Outer planets in synastry
Contacts from Uranus, Neptune and Pluto to personal planets deserve a look, with care. Uranus on someone’s Venus feels exciting and unpredictable. Neptune on the Moon feels dreamy and kind, sometimes with blurred boundaries. Pluto on Mars or Venus brings intensity. The outer planets move slowly, so people born a year or two apart share the same outer-planet positions. These contacts mean more when they touch a personal planet closely.
Reading a contact from both sides
An aspect between two charts is never quite symmetrical. If your Moon trines their Sun, you may feel seen and supported by them. They, in turn, may feel that you understand what they are trying to be. Each person feels the contact through their own planet. Ask “how does this feel from my side, and how from theirs?” and a list of aspects starts to look like daily life.
House overlays
The second layer is the house overlay. Here you place one person’s planets into the other person’s houses. It shows which area of your life the other person stirs up.
If your partner’s Venus falls in your 7th house, they may feel like a natural partner in the committed sense. Their Sun in your 5th house brings play and romance. Their Saturn in your 4th house might make Sunday dinners feel more serious when they are around. Their Moon in your 10th can make them invested in your public goals.
Overlays run in both directions, and the two directions rarely match. One person may light up the other’s 7th house while the other lands in their 11th, the house of friendship. That gap explains a lot when two people describe the same relationship in different words. Overlays depend on house cusps, so both birth times need to be fairly accurate. If one time is missing, read the unknown birth time guide before you lean on houses.
The composite chart, briefly
A composite chart is a separate technique. It blends two charts into one. Each planet in the composite sits at the midpoint between the two people’s positions of that planet. The composite Sun, for example, sits halfway between both Suns.
Astrologers read it as the chart of the relationship itself, almost like a third person in the room. Its Sun shows the shared purpose of the bond. Its Moon shows the emotional weather. Its Venus describes how affection flows between the two, and its Saturn shows where the bond asks for patience. A composite can look very different from either person’s own chart, which makes it fun to study. Many astrologers use synastry to see how two people meet, and the composite to see what they build together.
How planet strength changes the picture
A contact is only as reliable as the planets involved. A Venus-Mars trine between charts reads one way when both planets are well placed. It reads another way when one of them is under strain in its own chart. So our method rates every planet as Strong, Harmonious, Weak or Afflicted before reading any comparison, and reads the whole chart before any single link. The method page explains how those levels are set.
How to start reading synastry yourself
Begin with both birth charts. Our birth chart calculator gives exact degrees for each planet, which you need to spot aspects. Read the Sun, Moon, Venus and Mars contacts first. Then move to rising-sign contacts and Saturn. Save house overlays for last, and only if you trust both birth times.
Write down the three or four tightest contacts and ask what each one looks like on a normal Tuesday. Look for themes that repeat, because a chart says important things more than once. And keep the limits in view. Synastry describes how two people meet each other. It cannot decide whether a relationship should start or end, and it never replaces an honest talk.
Questions
What is synastry in astrology?
Synastry is the comparison of two birth charts to see how two people affect each other. The astrologer looks at the aspects between one person's planets and the other's, and at which houses each person's planets fall into in the other chart.
Do we need birth times for synastry?
Aspects between planets only need birth dates, though the Moon is more precise with a time. House overlays and anything involving the rising sign or Midheaven need accurate birth times for both people.
What are the most important aspects in synastry?
Contacts between the Sun, Moon, Venus, Mars and the rising sign carry the most personal weight. Sun-Moon, Venus-Mars and Moon-Moon links are often central, and Saturn contacts show where a bond feels committed or heavy.
What is the difference between synastry and a composite chart?
Synastry keeps the two charts separate and compares them. A composite chart blends them into one new chart built from the midpoints between each pair of planets, and it is read as the chart of the relationship itself.
Can synastry tell if a relationship will last?
No. Synastry describes how two people tend to meet each other, where things flow and where they rub. It cannot predict outcomes, and how two people choose to treat each other matters far more.