The sky turns once a day
The Earth turns once a day, so the whole zodiac passes over the eastern horizon in about 24 hours. That averages out to a sign every two hours, which is why the Ascendant needs a birth time. It is not a steady average: signs rise at different speeds, and the speed depends on the latitude of the birth place.
Use the slider to move the birth time. The ribbon shows which sign was rising when, and the wheel recomputes for the moment you pick.
Marta's birth minute. Color is the rising sign.
06:42 on the clock · 05:42 UTC
24°48′ Aquarius
The same rising sign as at 06:42.
The Ascendant moves about 22.3° an hour here.
06:42 on the clock: 24°48′ Aquarius. Same rising sign as at birth.
The wheel shows a chart computed for this moment.
What the clock said
A birth time is written in local time, and local time is UTC plus an offset that countries have changed many times. The engine converts using the offset the place really had on that date, including any daylight saving.
A tool that applies today's offset to an old birth is silently wrong by the difference. That is one way a rising sign comes out wrong.
When the time is not known
With no birth time the chart is computed for local noon. The Sun's sign is almost always right, and the Moon's may not be. The Ascendant, the Midheaven and the houses are not reliable, so Genastro hides them instead of showing a guess (see unknown birth time).
Place matters too. The same moment at a different latitude and longitude gives a different Ascendant.
Try it on your chart
Check your birth time
Enter your birth time and place. The chart reports the offset it used and the window your rising sign held.