Synastry chart: two birth charts on one wheel

Two charts, not a third one

A natal chart is the sky at one person's birth, seen from where they were born. Synastry keeps two of them whole and reads one against the other — one person's Venus against the other's Mars, one person's Moon against the other's Sun — with nothing averaged and no new chart cast for the pair.

Which contacts count

Only the five major aspects are read: the conjunction (0°), sextile (60°), square (90°), trine (120°) and opposition (180°), each within an orb of 4°. Two charts make well over a hundred pairs of points; the wider orbs used inside one chart would return most of them, and the tight one keeps the contacts two people would actually feel.

And at least one end of a contact must be a point the tradition reads between people: the Sun, the Moon, Mercury, Venus, Mars, Saturn, the North Node, the Ascendant or the Midheaven. Uranus in one chart against Neptune in the other is a fact about two birth decades, shared by nearly everyone born in those years, not about two people. One such end is enough: their Moon on your Jupiter still counts.

One contact, read from both sides

A contact reads differently from each end. Take their Saturn conjunct your Moon. From your side — the Moon's — a need to feel safe meets something steady and serious. It can come across as shelter, someone who stays when things are hard, or as a cool hand on your moods, as if a feeling had to justify itself first.

Turn the line round — the Saturn yours, the Moon theirs — and you may find yourself the one who holds things together, meeting another person's moods with patience, or with a measuring look you did not intend. One line, two experiences, and neither is the truth about either person. That is why the reading always speaks from the side of the one who asks.

The contacts read first

Some pairs are read first, and have been for a long time: Ptolemy, in the second century, already set two people's Suns and Moons side by side, writing of marriage and of friendship alike. Sun–Moon is what makes one person shine meeting what makes the other feel at home — an everyday comfort when the aspect is easy, one person's aims arguing with the other's need for rest when it is hard. Moon–Moon is two ways of needing safety: one who talks a mood out, one who needs quiet.

Venus–Mars is what one values meeting how the other wants and acts — desire between partners, and between friends or colleagues the push that turns a shared taste into something done. Mercury–Mercury is how two minds talk: a trine finishes each other's sentences, a square argues until something is understood, and it is what friends and colleagues are read for first. Saturn's contacts bring weight and reliability, steadying or heavy, often both. Their planet on your North Node touches the direction you are growing in.

None of these marks a soulmate or a doomed bond: a square between two Moons is a difference in how two people recover, not a reason to part, and a trine between Venus and Mars is an ease, not a promise.

House overlays: where one person lands in the other's life

A chart's twelve houses are the areas of a life. Synastry also places each person's planets in the other's houses: their Venus may fall in your seventh house, the house of partnership; your Saturn in their tenth, the house of work and reputation. An overlay is read from the side of the person whose house it is, and it runs both ways.

An overlay needs only the receiving chart's houses, and houses need that person's exact birth time and place: the Ascendant, where the first house begins, crosses a sign in about two hours. So with your own time and place, their planets land in your houses even if all you know of them is a date; a planet that could have stood in either of two houses that day is left out. Your planets land in their houses only when their time and place are known too.

What two dates fix, and what they leave out

With two dates and nothing more, the calculator treats each birth as any moment of that day, anywhere on Earth. What stays the same across that span is shown; what changes is marked “not fixed by the date alone”. Most planets keep their sign throughout, and so does the Sun, except around the day it changes sign. The Moon moves about 13° a day: its sign often changes within the span, and even when it does not, no contact to it can be held to 4°. The Ascendant, the Midheaven and the houses need the time and place.

A contact is kept only if it holds whatever the hour was. Leaving a point out is deliberate: a Moon placed at noon could stand more than six degrees from where it really was, and a contact drawn from that guess could be one that never existed, or hide one that did. “Not fixed” is more use to you than a guess that looks like a fact.

Composite and Davison charts

Two other methods build a third chart for the pair. The composite puts each planet at the midpoint of the two people's positions and reads the result as the chart of the relationship itself — a sky that never existed, whose Ascendant astrologers derive in more than one way. The Davison chart is a real sky, cast for the moment and the place halfway between the two births. This page and the app use synastry, where two charts stay two people.

Why there is no score

Many compatibility sites end in a percentage. This one does not, on purpose. A score adds up what does not add — a square two people work through for years, a trine they never notice — and the same contact means one thing between partners and another between a mother and her son. A number is also the one thing a reader could screenshot and act on — stay, leave, stop trying — and it is exactly what two charts cannot support.

Nor does any chart hold what a score pretends to settle — whether someone will come back, how long a bond lasts, what the other person really feels. What two charts can help with is about you: what you need to understand about this bond, and what in it is yours to do.

The reading in the app

In the app, the compatibility door reads both charts in full. The other person's date of birth is enough; a time and a place, if you have them, add the houses and fix the Moon to its degree. The door asks who they are to you — a partner, an ex, a friend, a parent, a colleague — where things stand between you, and how the reading should refer to them: she, he or this person. A name you type stays on the page and is never sent.

Their details are used only inside that one reading and deleted with it, never added to your profile or to anyone else's. A reading for two costs one reading, like any other; without an account you can try it once, as your free astrology try.