Birth chart compatibility — synastry — is the comparison of two complete charts rather than two star signs. It looks at how each person's planets fall against the other's: which placements support each other, which ones grind, and where the friction in a particular pair is likely to keep showing up. It does not produce a score, and it cannot tell you whether to stay.
The word most people arrive with is compatibility, and it carries a promise the method cannot keep: that two people can be graded, and the grade will tell them what to do.
What a chart comparison actually produces is different and more useful — a description of where the specific friction lives in this pair, and what each person is likely to be doing that the other reads as something else entirely.
The tables that rank Aries with Leo and warn Taurus off Aquarius compare one element from each chart and ignore the other thirty-nine.
Consider what that means arithmetically. There are 144 possible star sign pairings, and every couple on earth falls into one of them. The table therefore treats several million people as interchangeable with your partner. It is not that the tables are subtly imprecise; they are working with about two per cent of the available information.
There is a second problem, less often noticed. The tables describe compatibility between signs, but a relationship is not conducted between signs. It is conducted between two people whose Moons need different things, whose Mercuries argue in different registers, and whose Mars placements escalate at different speeds. None of that is visible from two Sun signs.
Synastry lays two charts over each other and looks at the angles between the planets of one and the planets of the other. The question it asks is not *are these two compatible* but what happens where these two charts touch.
A few of those contacts are worth naming, because they account for most of what couples actually argue about.
A chart comparison is a description of mechanism, not a verdict. It is worth being exact about the boundary, because this is the point at which most compatibility material starts overpromising.
It can tell you where a specific pair's friction is structural rather than situational — the difference between *we keep arguing because of a hard year* and *we keep arguing because one of us settles by talking it out and the other settles by going quiet, and both experience the other's method as abandonment*.
It cannot tell you whether a relationship will last, whether someone is faithful, whether a particular person will come back, or whether you should leave. None of that is in a chart. What is in the chart is the shape of the difficulty; what to do about it is a decision, and it stays a decision.
Synastry uses houses on both sides — where your partner's planets fall in your chart, and where yours fall in theirs. Houses require an exact birth time from both people.
Without it, the planet-to-planet contacts still work: those depend on positions in the zodiac, not on the horizon. What disappears is the part that says *which area of your life this person occupies* — and that is usually the half people most wanted.
If one of the two times is missing, it is worth trying to recover it before running the comparison. What can be narrowed and how is set out in birth chart without a birth time.
Calculate both charts first, separately, and read them as two people rather than as a pair. Most of what looks like a relationship problem turns out to be one person's Moon doing what it has always done, in a situation where somebody is now watching.
Then look at the four contacts above, in that order. Moon first, because it accounts for the arguments that repeat; Mercury last among the everyday ones, because it accounts for the arguments that seem to be about nothing.
You can calculate both charts here for free — every planet by sign and house for each person, and the chart wheel. No account is needed, and the calculation runs in your own browser.
A synastry chart is two birth charts drawn against each other so the angles between one person's planets and the other's can be read. It is the technical name for what is usually called birth chart compatibility. Unlike a star sign table it uses both complete charts, which means it needs the date, time and place of birth for both people rather than just two birthdays.
Yes, and this is one of the clearest failures of the table approach. Sign-to-sign compatibility compares one element out of roughly forty from each chart. Two people whose Sun signs are listed as a poor match routinely have easy Moon and Venus contacts, which is what daily life actually runs on. The reverse happens too: a pairing the tables approve of can be exhausting in practice.
For the full reading, yes. Contacts between the two sets of planets can be calculated from dates alone, but houses cannot, and the houses are what say which area of life each person occupies for the other. Without both times you get an accurate description of how the two of you operate and no information about where it plays out.
No, and any reading that claims to is describing something a chart does not contain. What a comparison describes is the mechanism of the friction and the support in a specific pair — which differences are structural and which are circumstantial. Whether two people stay together is decided by what they do about that, and no configuration of planets forecloses it.