What does Moon sextile Mercury mean in a birth chart?

That the part of the chart read for feeling and the part read for speech stand two signs apart, in elements that differ and get along. In practice it describes an available route between the two rather than any guarantee that a person uses it.

Does it make someone good at talking about feelings?

It makes them capable of it, which is not the same thing. The characteristic skill is accuracy rather than volume — a short true account rather than a long one — and it is perfectly possible to carry this contact and be a private person.

Why does the birth time matter so much for this one?

Because the Moon is the fastest thing in a chart. It crosses a sign in a little over two days, so its angle to Mercury changes measurably between breakfast and bedtime, and a contact counted over as little room as this one either holds at the moment of birth or does not.

Aspect

Moon sextile Mercury

Moon sextile Mercury sets the part of a chart that feels two signs from the part that says so. The Moon is read for emotional safety, what feels like home; Mercury for voice, communication, the way you frame what you do.

In easy contact the two make a route rather than a rule. It runs both ways — a feeling can be put into words, and words can be used to work out what the feeling was — and neither end is in charge of the other.

What it does not do is run itself. Somebody has to take the route, which is why this contact reads so differently in a person who writes things down and a person who does not.

Two homes, and the one that reaches the Moon

Mercury works differently out of each of its two home signs. Gemini collects: quick, plural, already interested in the next thing. Virgo corrects: slower, exact, interested in whether the thing is actually right. A Moon in easy contact with the second produces someone who audits their own mood. With the first, someone who narrates it.

The arrangement is not arbitrary. Cancer, where the Moon is at home, and Virgo, the sign that is both Mercury's home and where the tradition exalts it, stand exactly two signs apart — which is this angle. So a chart can hold this contact with both bodies working from their own ground, and that version is worth a second look: the two faculties are not merely cooperating, they are cooperating from strength.

Either way the recognisable trait is accuracy rather than fluency. Plenty of people can talk about their feelings at length. Rather fewer can give a short true account of one the following day, without dramatising it and without flattening it, and that is what this contact tends to supply.

Where it shows

In the impulse to write it down. Diaries, long messages composed and never sent, the note made at midnight that turns out to be exactly right in the morning — the traffic between these two has to go somewhere, and paper is where it goes when there is nobody to tell.

In other people's weather. Someone with this contact can often name what a friend is feeling before the friend gets there, and can do it without making a performance of having noticed. It is a social skill people register and almost never attribute to anything.

And in a failure mode specific enough to be worth naming: the explanation starts standing in for the experience. A feeling that has been described accurately can feel dealt with, and it is not. This is the contact most likely to produce someone who has understood a thing about themselves for years and never once felt it through.

What changes the reading

This is a birth-hour contact before it is anything else. The Moon crosses a sign in a little over two days while Mercury barely moves against it, so the figure forms and comes apart within hours. Two people born on the same date can be described by it and not described by it, and only the time separates them.

The margin allowed here is the tightest in the scheme, which matters more for this pair than for most. A generous reading is usually a misread: at that distance the two are merely in the same chart, which is true of everything in it.

The register comes from elsewhere. The same contact under a heavy Saturn is a person careful about what they say; under a loud Jupiter it is a person who says all of it. The angle supplies the route and not the traffic on it.

Questions

What does Moon sextile Mercury mean in a birth chart?: That the part of the chart read for feeling and the part read for speech stand two signs apart, in elements that differ and get along. In practice it describes an available route between the two rather than any guarantee that a person uses it.
Does it make someone good at talking about feelings?: It makes them capable of it, which is not the same thing. The characteristic skill is accuracy rather than volume — a short true account rather than a long one — and it is perfectly possible to carry this contact and be a private person.
Why does the birth time matter so much for this one?: Because the Moon is the fastest thing in a chart. It crosses a sign in a little over two days, so its angle to Mercury changes measurably between breakfast and bedtime, and a contact counted over as little room as this one either holds at the moment of birth or does not.