Synastry is the part of astrology that compares two birth charts to describe a relationship. Most people first meet astrological compatibility as a Sun-sign match: Leo and Sagittarius good, Taurus and Aquarius hard. That kind of pairing is a headline, based on one planet out of ten in each chart. Synastry takes all of the planets in one person's chart and checks how they relate to all of the planets in the other's. The result is a far more detailed picture: where two people feel instantly at ease, where the attraction comes from, and where they are likely to rub each other the wrong way. This guide walks through how it works and how to read it.
Sign-level compatibility compares two Sun signs. It uses real logic: signs of the same element share a temperament (fire with fire, earth with earth), fire and air tend to feed each other, and earth and water tend to support each other. Signs that sit square to each other, such as Leo and Scorpio, share a mode but not an element, which is traditionally read as friction.
The trouble is that the Sun describes your core identity, not how you feel, how you love or how you argue. Two people with a "perfect" Sun match can have Moons that want completely different things, while two people with a "difficult" Sun match can share a Moon sign and feel deeply at home together. Sun-sign compatibility is a good first question. Synastry is the full answer.
The main tool in synastry is the inter-aspect: an aspect formed between a planet in one chart and a planet in the other. If your Moon is at 12 degrees Cancer and your partner's Venus is at 14 degrees Pisces, those two points are about 120 degrees apart, a trine. That trine connects your emotional needs with their way of showing affection, and tradition reads it as easy warmth.
The same five major aspects used in a single chart apply here:
Astrologers usually use tighter orbs in synastry than in a natal chart, often around 5 to 6 degrees for major aspects and a little more for the Sun and Moon. The closer the aspect, the more strongly it is felt.
Not all inter-aspects carry the same weight. For a romantic relationship, these tend to get the most attention:
Imagine a couple where one person is an Aries Sun and the other a Cancer Sun. On the Sun-sign level this is a square, fire against water, and a quick Aries and Cancer reading would flag tension: one charges ahead, the other wants security.
Now look at the full charts. Suppose the Aries person's Moon is in Taurus, trine the Cancer person's Venus in Virgo. The Aries person's Venus is conjunct the Cancer person's Mars. And the Cancer person's Moon sits right on the Aries person's Sun.
That changes the story. The Sun-Moon conjunction suggests the Aries person's basic nature is exactly what the Cancer person finds emotionally comforting. The Venus-Mars conjunction adds strong attraction. The Moon-Venus trine gives a steady, affectionate day-to-day feel. The Sun square is still there, and it may show up as different speeds when making decisions, but it sits inside a relationship with several strong connecting threads. This is why a "bad" sign match can work very well, and why a "good" one sometimes does not.
If both birth times are known, astrologers also look at house overlays: where one person's planets fall in the other person's houses. Your partner's Venus landing in your 7th house of partnership, for example, is often read as them feeling like "a partner" to you from early on. Their planets in your 4th house can make them feel like family or home. Their Mercury in your 3rd suggests easy everyday conversation.
A separate technique, the composite chart, averages the two charts into one chart for the relationship itself, using the midpoints between each pair of planets. Synastry describes how two people affect each other; the composite describes what the relationship becomes as its own thing. Both are useful, and they answer different questions.
In Vedic astrology, the traditional first step is different. Ashtakoota matching compares the Moon's nakshatra and Moon sign of each person across eight factors and scores them out of 36 points. Many families treat 18 points as the traditional minimum. Vedic astrologers then look further, at Mars placements, the 7th house, and planetary periods, much as a Western astrologer moves from Sun signs to full synastry. Both traditions agree on the principle: the Moon matters a great deal in how two people live together.
Every pairing has hard aspects. A chart comparison with no squares or oppositions at all is rare, and not necessarily ideal. Friction is often where growth and attraction come from.
Look for patterns, not single aspects. One Mars-Saturn square does not define a relationship. Several contacts pointing the same way, for example multiple Moon connections, say more than any one aspect.
Synastry describes, it does not decide. It can show where two people naturally meet and where they will need patience. How a relationship goes depends on the people in it. Treat a synastry reading as a way to reflect on the dynamic, not a verdict on whether to stay together.
On My Stella, you can compare two full charts planet by planet in the compatibility view, starting from your own chart: see your own birth chart.
Not for the planet-to-planet aspects, which work from birth dates. Birth times add the rising sign, house overlays and a more precise Moon position, which is useful since the Moon moves about 13 degrees a day.
There is no single one, but Sun-Moon, Moon-Moon, Moon-Venus and Venus-Mars contacts are traditionally given the most weight in romantic compatibility.
Yes. The same method applies to any two people. For friendships and family, Moon, Mercury and Jupiter contacts often matter more than Venus-Mars.
No. Synastry compares two separate charts to show how the people affect each other. A composite chart blends the two into one chart that describes the relationship itself.