What the Moon describes
If the Sun is who you are when you are being deliberate, the Moon is who you are when you are tired, upset or comfortable. It describes emotional need: what settles you, what unsettles you, and what you reach for without deciding to.
It is also the placement people recognise most immediately when they read it, precisely because it describes the unedited version.
Why you need a birth time
The Moon moves through a sign in roughly two and a half days — about twelve degrees per day. That means being born in the morning rather than the evening can genuinely put your Moon in a different sign, and there is no way to guess it from your birthday.
If you do not have a birth time, calculate the chart anyway: you will still get an accurate answer for everything else, and often a probable Moon sign with a caveat.
Why it matters in relationships
In synastry the Moon does a disproportionate amount of the work. Sun-sign compatibility describes whether two people make sense on paper. Moon contacts describe whether they can live together — whether one person’s way of being upset is legible to the other.
It is very common for a pairing that looks difficult on sun signs to be extremely comfortable, and the Moon is usually why.