Your sun sign depends only on your birth date. Your moon sign changes roughly every two and a half days. But your rising sign moves through all twelve zodiac signs in a single day, which means it shifts approximately every two hours. A birth time that is off by even an hour can produce an incorrect ascendant, and a birth time that is off by two hours or more will almost certainly give you the wrong rising sign entirely.
The Ascendant Moves Quickly
As Earth rotates, different parts of the sky appear to rise above the eastern horizon. The zodiac — a band of constellations — is always somewhere in that sky, and the ascendant simply marks which of the twelve signs is currently climbing above the horizon at a given location. Because Earth makes a full rotation every 24 hours, the ascendant completes a full cycle through all twelve signs in the same period — spending roughly two hours in each sign on average, though the actual duration varies by sign and latitude.
This speed is what makes the rising sign so personal. Two people born in the same city on the same day but two hours apart will have different rising signs and therefore entirely different birth chart structures, even if all their planet placements are nearly identical.
What Happens Near a Sign Boundary
The effect of a time error is magnified when the ascendant degree falls near the boundary between two signs — that is, near 0° or 29° of a sign. At these moments the ascendant is transitioning from one sign to the next, so a discrepancy of even 15 or 20 minutes can shift the result from one sign to the adjacent one.
If your ascendant degree comes out very close to 0° or 29°, this is a signal to verify your birth time carefully. A time that was estimated or rounded to the nearest half-hour has a meaningful chance of being on the wrong side of the sign boundary. In those cases, try entering a time 15 minutes earlier and 15 minutes later to see whether the sign changes — if it does, you have an ambiguous result and should work to find a more precise time.
How to Improve Accuracy
The most reliable source for birth time is an official document recorded at the time of birth. In order of reliability these are: an original long-form birth certificate (which often includes time), a hospital birth record or delivery room notes, and a baby book or diary entry made close to the birth. Notes made from memory years later are less reliable, and parental recollection without a written source is the least reliable of all.
If you live in the United States, you can request a copy of your original birth certificate from the vital records office of the state where you were born — most states include the birth time on the certificate. Other countries have similar official records, though availability varies.
If your birth time is genuinely unknown and no records exist, some professional astrologers use a technique called rectification — working backwards from significant life events to estimate the most likely ascendant. This is a complex and time-consuming process but can produce a working hypothesis when no other option is available.
Related: how to find your rising sign and what is a rising sign.