Your rising sign — the ascendant — is the sign that was climbing over the eastern horizon at the minute you were born. It is what people meet first, before they know anything else about you.
Three different questions get three different answers, and confusing them is why astrology often feels wrong about someone. The sun sign is what your birthday alone gives — the long arc of what you are becoming. The moon sign is the private one: how you feel and what settles you. The rising sign is the surface, the first ten minutes, the version of you that walks in. Someone can be a quiet Cancer sun with Leo rising and be described as outgoing by everyone who has met them once and by nobody who has known them for years.
Yes. The ascendant travels the full zodiac in twenty-four hours, so about two hours is one sign. Even twenty minutes matters if you were born near a boundary — and at high latitudes some signs rise in twenty minutes flat. A calculator that gives you an answer without the time has quietly assumed noon.
The birth certificate is the usual place; in many countries the hospital records it and the registry keeps it. Failing that, the people who were there often remember the part of the day, which at least narrows it to two or three candidates rather than twelve.
Two things. The city fixes the time zone, so your local clock becomes a real moment in universal time. And it fixes the latitude, which tilts the horizon — the same minute gives a different ascendant in Helsinki and in Lisbon.
Not more important — earlier. It is what shows first and what people describe you by before they know you. Astrologers read the whole chart together; the ascendant is simply the door you are met at.