What the contact does
It puts identity against instinct. The Sun person is doing what they are driving at; the Moon person responds to it without deciding to. That asymmetry is the whole contact and it is why the two people describe it differently.
The Moon person usually reports being read accurately, sometimes uncomfortably so. The Sun person usually reports feeling settled by the other's presence without being able to say why. Both descriptions recur often enough to be worth taking seriously.
It is the classic marriage contact in the older literature, and the modern reports are consistent with that. What it does not do is guarantee anything: plenty of brief relationships have it and plenty of long ones do not.
Why the orb decides how much of this you get
Every statement above assumes the contact is close. At half a degree both people usually name it without being told; at six degrees it is present in the arithmetic and largely absent from the experience.
Content about synastry routinely quotes contacts without saying how tight they are, which is how almost any two charts can be made to look significant. The form on this page reports the orb, and anything past three degrees should be read as background rather than as a feature of the relationship.
It is asymmetric
One person supplies the Sun and the other supplies the Moon, and the two experiences are not the same. The Sun person is generally the one doing something; the Moon person is generally the one responding to it.
That asymmetry is why the check on this page reports which direction the contact runs in. A comparison where both directions are present is uncommon and means the two bodies sit close to the same degrees in both charts.
What it cannot tell you
Whether the relationship works. A single contact describes one thread in a comparison that typically contains more than a hundred, and reading a relationship from one aspect is the commonest error in amateur synastry.
It also predicts nothing. Astrology describes texture rather than outcome, and a page that turns one geometric relationship into a forecast has stopped doing the thing it claims to be doing.
Where to look next
The tightest three contacts in a comparison, whatever they involve, describe more than any single named aspect. The synastry calculator lists every contact sorted by orb and both sets of house overlays.
House overlays are the half most tools omit and they decide the arena: the same contact reads as domestic in the fourth house and professional in the tenth. Both need birth times.
Questions people ask about this
How tight does this contact need to be?
Inside three degrees to be doing real work, and inside one to be unmistakable. Past six it is listed for completeness and should not carry a reading.
Does it matter which way round it is?
Yes. The contact is asymmetric and the two people experience it differently. The check on this page reports which direction is present.
Do we need birth times?
Not for this contact, which involves two bodies rather than an angle. Birth times add the house overlays, which decide where the contact plays out.
Is one aspect enough to read a relationship from?
No. A typical comparison contains well over a hundred contacts, and reading a relationship from one is the commonest error in amateur synastry.