Your sun sign gets all the attention. Your moon sign is the one that explains why you get irritable when plans change, or why you go quiet instead of loud when something upsets you.
In traditional astrology, the Moon relates to emotional instinct. Not how you'd like to respond, but what you do automatically before you've had time to think about it. It also points to what you need in order to feel settled, which is information most people have never put into words.
Here's how it tends to show up, grouped by element.
Fire moons: Aries, Leo, Sagittarius
The instinct is to react immediately and move. Emotions arrive fast, get expressed, and often clear just as quickly, which can confuse people who process slowly.
What they usually need is space to say it, something to do with the energy, and not to be told to calm down. A fire moon who has to sit on a feeling for three days tends to be far worse by day three.
Earth moons: Taurus, Virgo, Capricorn
The instinct is to look for something solid. Emotion gets managed through practical action, which is why an earth moon might respond to a crisis by cleaning the kitchen.
What they usually need is stability, predictability and something tangible to do. Reassurance in the abstract lands weakly. A plan lands well.
Air moons: Gemini, Libra, Aquarius
The instinct is to understand the feeling before allowing it. Air moons often talk through emotion, or step slightly outside it to analyse it, which can be mistaken for coldness.
What they usually need is conversation, room to think out loud, and not to be pushed into a feeling before they've made sense of it.
Water moons: Cancer, Scorpio, Pisces
The instinct is to feel it fully and pick up what's happening in the room. Water moons frequently register other people's moods before those people have mentioned anything.
What they usually need is emotional safety, time to recover, and someone who doesn't rush them through it.
Why it matters in a relationship
Most recurring arguments between two people who genuinely care about each other are moon mismatches. One needs to talk it out now, the other needs an hour alone first. Neither is wrong, and neither is being difficult.
Naming that difference tends to defuse it faster than any technique.
Getting your own
You need your birth date, place and time, since the Moon changes sign roughly every two and a half days.
If you want to understand what yours means in your situation rather than in general, you can talk it through with an astrologer on My Astrology Advisor by live chat or voice call.