Astrological compatibility: how synastry actually works

By Ines Vasquez

Synastry compares two whole birth charts by laying one over the other and reading the contacts between them — where one person's planets fall in the other's chart, and what angles they make. It does not rank pairs or score them. What it describes is the texture of a particular relationship: where it is easy, where it catches, and what each person tends to activate in the other.

How does astrological compatibility work?

Two birth charts, each a map of the sky at a moment of birth, are set against each other. The astrologer looks at three things: which of person A's planets land in which of person B's houses, what aspects (angular relationships measured in degrees along the zodiac) form between the two sets of planets, and how the two charts' angles — the Ascendant, Descendant, Midheaven and Imum Coeli — are touched.

Nothing in that procedure produces a verdict. It produces a description. A Venus–Saturn contact between two charts is read as a particular flavour of attachment: slow, weighty, inclined to duty and to endurance, sometimes to withholding. Whether that is good depends entirely on what the two people want and what they are each prepared to do. The technique tells you what is there. It does not tell you whether to stay.

This is the single most common misunderstanding, and the entire sun-sign compatibility industry rests on it.

Why are sun-sign compatibility charts unreliable?

Because they compare one factor out of roughly forty, and the one they choose is not the factor most relevant to relating.

A sun-sign grid — Aries goes with Leo, Taurus clashes with Aquarius — uses the element and aspect pattern between two signs as a proxy for two people. Trines (120 degrees) between signs of the same element are called compatible; squares (90 degrees) are called difficult. The geometry is real, and Ptolemy set out the aspect doctrine in the Tetrabiblos in the second century. The application is a shortcut so severe it stops carrying information.

Consider what it omits. Two people born under the same sun sign three weeks apart can have Moons in opposite signs, Venus in different elements, Mars most of a sign apart, and Ascendants that share nothing. In synastry the Moon and Venus contacts usually describe the daily experience of the relationship more accurately than the Sun contacts do. A sun-sign grid cannot see any of it.

There is also a mechanical problem. The Sun spends about a month in each sign, so sun-sign compatibility sorts the whole human population into twelve boxes and then into 144 pairings. Any system with 144 outputs applied to billions of relationships is describing categories, not couples.

Sun-sign compatibility is not astrology done badly. It is a different thing wearing astrology's clothes — closer to a magazine quiz, and fine as that, so long as nobody mistakes it for a reading.

What does synastry actually look at?

Six things, in roughly this order of weight, though astrologers differ on the ordering.

What is examined	What it is read as describing
Moon contacts between the charts	Emotional recognition; whether the two people's default responses make sense to each other
Venus and Mars contacts	Attraction, style of affection, how desire is expressed and received
Sun contacts	Whether each person's core direction is visible to the other
Contacts to the Ascendant and Descendant	Instinctive physical response; whether one person matches the other's picture of a partner
Saturn contacts	Where the relationship becomes serious, binding, restrictive, or all three
House overlays — whose planets fall in whose houses	Which areas of life each person activates in the other

The house overlay is the piece most often left out of popular accounts and it does a great deal of work. If your Venus falls in someone's second house, the house traditionally governing resources and worth, you are read as touching their sense of value. If it falls in their tenth, the house of vocation and public standing, you touch their ambition and how they are seen. Same planet, entirely different function in the relationship.

Modern synastry also reads the nodal axis — the two points where the Moon's orbit crosses the ecliptic, always exactly opposite each other. Contacts from one person's planets to the other's North Node are read as pulling that person toward unfamiliar ground. This is a twentieth-century reading. The nodes are ancient and were used for eclipse prediction and, in Hellenistic and Indian astrology, as malefic or benefic points; the idea of the North Node as a direction of growth comes from Dane Rudhyar and the evolutionary astrologers writing from the 1930s onward.

What is the difference between synastry and composite charts?

Synastry keeps two charts and compares them. A composite chart makes a third chart out of them by taking the midpoint of each pair of planets — your Sun at 4° Cancer and theirs at 4° Capricorn produce a composite Sun at 4° Aries or 4° Libra, depending on which midpoint is taken.

The composite is read as the chart of the relationship itself, as though the partnership were a separate entity with its own nature. It is entirely a modern technique. Robert Hand's Planets in Composite (1975) is the book that established it in English-language practice, and it remains the standard reference.

Astrologers disagree about whether composites mean anything. The objection is that a midpoint between two real bodies is not a body, has no physical existence, and the resulting chart describes no moment that ever occurred. The defence is that the technique produces recognisable descriptions in practice, and that plenty of accepted astrological technique — house cusps, Arabic parts, the Ascendant itself — refers to calculated points rather than objects. There is also the Davison relationship chart, which takes the midpoint in time and space between two births and casts a real chart for that moment and place, which sidesteps the objection at the cost of being harder to compute.

Most working astrologers use synastry as the primary tool and treat the composite as a second opinion.

Can a chart comparison say whether a relationship will last?

No, and the honest astrologers have said so for a long time.

What a comparison is held to describe is the nature of the contact — what each person tends to bring out in the other, where friction is structural rather than incidental, what the relationship demands. Duration is a different question, and it depends on decisions neither chart contains.

There is a further difficulty that popular compatibility writing tends to hide. Charts described as easy — many trines and sextiles, elements in agreement — are frequently described by the people in them as pleasant and inert. Charts with hard aspects, squares and oppositions between significant planets, are often the ones people stay in for thirty years, because the friction generates attention. Astrologers from Liz Greene onward have made this point repeatedly: difficulty in a synastry chart is not a warning, and ease is not a promise.

Anyone who tells you a chart comparison predicts an ending is overreaching, and the same caution that applies to any predictive claim applies here.

What do the hard aspects between two charts mean?

Squares (90 degrees) and oppositions (180 degrees) between two people's planets are read as points of friction, and in synastry specifically as points where each person keeps triggering something in the other that neither chose.

The classical distinction is old. Ptolemy in the second century treated trine and sextile as harmonious and square and opposition as inharmonious, on the grounds that trine and sextile connect signs sharing gender and often element, while square and opposition connect signs that do not. That is the geometric logic, and it has been in continuous use since.

The modern reading is different in emphasis rather than in substance. Twentieth-century psychological astrology, which came largely through Rudhyar, Greene and Stephen Arroyo, recast hard aspects as productive tension rather than misfortune — the place where something has to be worked out rather than the place where something goes wrong. Traditional astrologers tend to find this softening excessive and point out that Valens, writing around 150 AD, described difficult configurations as difficult and meant it.

You will meet both, often without being told which you are reading. Our page on ruling planets sets out which to read first and why. A synastry reading in the modern style will usually tell you that a Mars square Venus contact between two charts describes attraction with a disagreement about pace built into it. A traditional reading will tell you it describes discord. The two are not as far apart as they sound.

Where does the technique come from?

Chart comparison for relationships is old in principle and recent in its current form.

Hellenistic and medieval astrologers compared nativities, but the questions asked were usually different — whether a marriage should be made, whether children would come, and these were often answered by horary or electional methods rather than comparison. Abu Ma'shar in the ninth century and the medieval Arabic tradition after him used chart comparison, though the emphasis fell on rulership relationships between the charts rather than on aspects between planets.

Synastry as now practised — the Moon and Venus weighted heavily, house overlays read as areas of activation, hard aspects read as psychological material — is a twentieth-century construction. It comes out of the same movement that turned natal astrology from a prediction of events into a description of character, and it carries the same assumptions. It should be assessed as what it is: a modern interpretive system, roughly a hundred years old, built on a much older geometric frame.