Zodiac compatibility: synastry in PDF

What synastry compares

Two natal charts are calculated separately, from two birth moments and two birthplaces, and then laid over each other. The comparison happens on two levels, and they use different machinery.

The first level is aspects. Every planet in one chart sits at a degree, and every planet in the other chart sits at a degree. The angular distance between any two of them, one taken from each chart, is measured and compared against the recognised angles: conjunction, sextile, square, trine, opposition. Each contact gets an orb, a permitted margin around the exact angle, and the size of that orb is a convention. A wide orb describes a looser contact than a tight one.

The second level is houses. The house system divides one chart into twelve areas by birth time and latitude, and those divisions have no counterpart in the other chart. Placing one person's planets into the other person's houses asks a separate question: which area of life do those planets land in. This level only works when the birth time of the person whose houses are being used is accurate, because the cusps move with the clock. The Ascendant travels through the zodiac once a day, roughly one sign every two hours, and a small error in the recorded time can shift a cusp enough to move a planet into the neighbouring house.

Both levels need date, time to the minute, and place for both people. Without a time, the planets can still be placed to a degree in most cases, but the houses fall away and the comparison loses its second level. The natal chart page covers what a single chart contains before any comparison is made.

Why sign distance is geometry

Each sign occupies thirty degrees of the zodiac. That single fact produces most of what people call sign compatibility, and it does so by arithmetic rather than taste.

Signs of the same element sit one hundred and twenty degrees apart. Aries, Leo and Sagittarius are all fire, and the step from one to the next is four signs of thirty degrees. One hundred and twenty degrees is a trine, the angle traditionally read as easy and mutually supportive.

Signs that share a modality but not an element sit ninety degrees apart. Aries is cardinal fire and Cancer is cardinal water. The distance between them is three signs, ninety degrees, which is a square. The square is read as friction that requires work, not as failure.

Opposite signs sit one hundred and eighty degrees apart, six signs away, and form an opposition: same modality, opposing elements, a relationship of tension and balance.

Adjacent signs sit thirty degrees apart. That is the smallest step in the system, and it is also the step that shares nothing. Take Aries and Taurus: fire and earth, cardinal and fixed. Neighbours differ in element and in modality at the same time, so they hold no structural feature in common. This is why the sign immediately before or after your own is the weakest pairing in the system. It is arithmetic, not opinion. The same logic explains the sextile: two signs sixty degrees apart share a compatible element pairing and differ in modality, which is why the angle reads as cooperative.

Element	Signs	Modalities in order
Fire	Aries, Leo, Sagittarius	cardinal, fixed, mutable
Earth	Taurus, Virgo, Capricorn	fixed, mutable, cardinal
Air	Gemini, Libra, Aquarius	mutable, cardinal, fixed
Water	Cancer, Scorpio, Pisces	cardinal, fixed, mutable

Read across a row and every pair inside it is a trine, since same element always means one hundred and twenty degrees. Read down a column and the modalities line up across elements, which is what produces the squares. The glossary lists the angles and their orbs.

What it cannot tell you

• It does not grade a couple. There is no score, no ranking and no verdict. A comparison with several squares is not a worse relationship than one with several trines, because hard angles describe friction and friction is not the same thing as incompatibility.
• It does not predict fidelity or a breakup. No arrangement of degrees monitors behaviour or forecasts an event. The chart describes contacts between positions, not choices made afterwards.
• It does not work without birth times for the house comparison. With one time missing, the aspects remain and the house level collapses into one sided guesses, which is a thinner reading than it appears.
• It does not measure effort, history or circumstance. Two pairs with identical contacts can have entirely different relationships, because the chart is not the relationship.

Common questions

What if I do not know my birth time?

The aspects can still be calculated, since planetary degrees are mostly stable across a day. The Moon is the exception, moving roughly half a degree per hour, so an uncertain time leaves its position uncertain too. The house comparison needs a time, and one side missing it means the reading stays at the aspect level only. This note on what to check first goes through the order of operations.

Is a square worse than a trine?

It is a different angle, not a lower grade. A trine tends to describe ease, and ease can mean two people never have to negotiate anything. A square describes friction, and friction is often what makes a pair define terms. Neither angle decides how a relationship turns out.

Are Sun sign compatibility columns useless?

They are coarse rather than useless. A Sun sign column sorts everybody into twelve buckets of thirty degrees, while synastry measures the actual distance between two Suns, which might be anywhere from zero to twenty nine degrees. The Sun is also one body out of ten, and a pair can share an easy Sun contact while squaring each other on the Moon or Mars.

Can more than two charts be compared at once?

A comparison is a pair. Three people produce three separate pairs, and a family or household reading is a set of pair comparisons rather than one combined chart.