How synastry works: reading two birth charts together

What synastry actually compares

Every birth chart is a snapshot of where the Sun, Moon and planets were at one moment from one place on Earth. Synastry takes two of those snapshots and places them on the same 360-degree wheel. Because both charts use the same zodiac, any planet in one chart sits at some measurable angle from every planet in the other. Those angles are the raw material of the whole method.

This is a different question from sun-sign compatibility. A sun-sign pairing like Taurus and Scorpio describes millions of people at once, so it can only speak in broad strokes. Synastry uses the exact degrees of ten planets and the angles for both people, which produces dozens of specific contacts. Two Taurus and Scorpio couples can end up with very different synastry, and that difference is the point.

Astrologers usually work with two layers. The first is inter-aspects: the geometric angles between one person's planets and the other's. The second is house overlays: which areas of one person's chart are lit up by the other person's planets. A third, related technique called the composite chart blends the two charts into one, but strictly speaking that is a separate method built on the same data.

Inter-aspects: the core of the reading

An aspect is a meaningful angle between two points. The main ones used in synastry are the conjunction (0 degrees, the two planets fused together), the sextile (60 degrees, easy cooperation), the square (90 degrees, friction that pushes for change), the trine (120 degrees, natural flow) and the opposition (180 degrees, attraction and tension across a gap). Each aspect gets an orb, a margin of a few degrees within which it still counts.

Tighter orbs carry more weight. A partner's Moon sitting one degree from your Sun is read as a strong, central contact. The same aspect at seven degrees is usually treated as background. Many astrologers use orbs of about six to eight degrees for the Sun and Moon and tighter ones, around three to five, for the other planets, though conventions vary by school.

The planets involved shape the meaning. Sun-Moon contacts are traditionally read as a sense of basic fit and familiarity. Venus-Mars contacts describe attraction and chemistry. Mercury contacts describe how easily you talk and argue. Saturn contacts describe commitment, duty and sometimes heaviness. A useful habit is to note which planet belongs to whom: your Saturn square their Moon reads differently from their Saturn square your Moon, because the person whose Saturn it is tends to play the role of the one setting limits.

Squares and oppositions are not bad news by default. Many long relationships show plenty of them, and they often describe exactly where a pair grows. Charts with only trines and sextiles can read as comfortable but low on spark. Most readers look for a mix rather than a clean sheet.

House overlays: where you land in each other's lives

Houses divide a chart into twelve areas of life — self, money, communication, home, romance, daily routine, partnership and so on. House overlays ask where one person's planets fall when placed in the other person's houses. If your Venus lands in their seventh house, the traditional reading is that you naturally show up in the partnership area of their life.

Overlays need accurate birth times, because house cusps shift roughly one degree every four minutes. A birth time that is off by an hour can move several overlays into neighboring houses. If one person does not know their time of birth, it is safer to lean on planet-to-planet aspects and treat house overlays as provisional.

Overlays run in both directions and rarely match. Your planets might cluster in their fourth house of home and family, while their planets fall into your tenth house of career and public life. That asymmetry is common and often matches lived experience: one person feels the relationship as domestic and grounding, the other as something that changes their direction or ambitions.

Reading a synastry chart step by step

Start with both full charts calculated from accurate birth data. Note each person's Sun, Moon, rising sign, Venus and Mars before comparing anything, so you understand what each person brings on their own. Synastry interprets how two existing patterns meet; it cannot replace knowing the charts individually.

Next, list the tight inter-aspects, beginning with the personal planets and angles: Sun, Moon, Mercury, Venus, Mars, the Ascendant and the Midheaven. Write down the aspect, the orb and whose planet is whose. Then add contacts from Jupiter and Saturn, which describe growth and commitment. Outer-planet contacts from Uranus, Neptune and Pluto are worth noting but move slowly, so many people born in the same few years share them.

Then look for repetition. One Venus-Mars trine is a detail. Venus-Mars trine, Moon conjunct Venus and Sun trine Mars together suggest a consistent theme of warmth and attraction. Astrologers often say a theme only counts once it appears at least two or three times in different forms, which protects against over-reading a single contact.

Finally, read the house overlays and summarize in plain language: what feels easy, what needs negotiation, what each person tends to activate in the other. Framed this way, a synastry reading becomes a set of conversation starters for a couple rather than a forecast.

Contacts astrologers pay attention to

Sun-Moon conjunctions and oppositions are classic markers of mutual recognition, and they appear often in traditional discussions of marriage charts. Moon-Moon aspects describe whether two people process feelings in similar ways, which matters a lot in daily life. Venus-Mars aspects are read as physical and romantic chemistry, and Mercury-Mercury aspects as how naturally conversation flows.

Saturn contacts get special attention because they are associated with longevity and seriousness. A partner's Saturn on your Venus can feel steady and committed or restrictive, depending on how both people handle it. Many astrologers note that relationships with some Saturn ties tend to feel more durable than those with none, while also warning that heavy Saturn contacts can bring a sense of obligation.

Contacts to the Ascendant and Descendant matter too. A planet conjunct someone's Descendant, the cusp of the seventh house, is a traditional signature of being drawn to that person as a partner. None of these contacts is a requirement or a guarantee; they are simply the places readers look first.

What synastry cannot tell you

Synastry describes tendencies and themes. It cannot tell you whether a relationship will work, whether someone is trustworthy or whether you should stay or leave. Real relationships depend on choices, communication, circumstances and timing that no chart captures. Treat any reading as a prompt for reflection, not as advice about a decision that belongs to you.

It also cannot be fairly reduced to a single percentage. Compatibility scores are a convenient summary, but they compress dozens of contacts with different weights into one number, and two very different charts can land on the same score. If you see a score, look at which aspects drove it.

Finally, the method assumes accurate data. A wrong birth time or birthplace changes the angles and houses. If something in a reading feels completely off, check the inputs before drawing conclusions.

Try it with your own chart on Nuvastro

The first step in any synastry reading is knowing your own chart well. Nuvastro calculates your full birth chart from your date, time and place of birth, and the chart reveal is free, so you can see your Sun, Moon, rising sign and planet placements before comparing them with anyone else's.

The 7-day trial unlocks the full natal chart with houses, degrees and aspects, which is exactly what you need to work through the steps above with a partner's chart beside yours. No card is needed to start. It is designed for reflection and entertainment — a structured way to think about a relationship, not a verdict on it.

For a quicker first look, Nuvastro's free compatibility guides cover every sun-sign pairing, and the free moon calendar follows the daily sky. Start with your own chart, compare a partner's when you are ready, and read the results as questions to explore together.

Common questions

What is synastry in astrology?

Synastry is the comparison of two birth charts. It reads the angles between one person's planets and the other's, plus where each person's planets fall in the other's houses, to describe how two people tend to interact.

Do you need a birth time for synastry?

Planet-to-planet aspects can be read with birth dates alone, though the Moon may be uncertain. House overlays and contacts to the rising sign need accurate birth times for both people.

What is the difference between synastry and a composite chart?

Synastry keeps the two charts separate and compares them. A composite chart averages the two sets of positions into a single chart that astrologers read as the relationship itself.

Which synastry aspects are most important?

Readers usually start with tight aspects involving the Sun, Moon, Venus, Mars and the Ascendant, then look at Saturn for commitment themes. Repeated themes matter more than any single aspect.

Can synastry predict if a relationship will last?

No. It describes tendencies and areas of ease or friction. Whether a relationship lasts depends on the people in it, their choices and their circumstances.