We can use tables in an Ephemeris (or use a computer program) to determine the exact Sign and degree of that point on the horizon, which is called the Ascendant or "Ascendant Point".

As the Earth turns, the Rising Sign moves up from the horizon about one degree every four minutes, so the exact degree of the Rising Sign which is sitting on the horizon is constantly changing over the course of a few minutes. When the 30th degree has risen after about two hours (30 x 4 minutes = 120 minutes), a new Sign appears on the horizon and becomes the new Rising Sign at that time and place.

The Ascendant Point (located somewhere from 0 to 29 degrees of the Ascending or Rising Sign) is always the first degree of the First House in all systems used to calculate Houses. As the Ascendant moves minute-by-minute, so does the Cusp of the First House and ALL of the other 11 house cusps. So the Sun or Moon or a Planet which might have been in one House at a given time might be in the next House any time from a few minutes to 2 hours later!

This is why your exact birth time is needed to know exactly what House they were in at your time of birth. The Sun and Planets move around the zodiac only one degree or less per 24 hours, so they don’t change Sign that quickly (unless they were already very close to the cusp at the time); but the House each is in will change at least every two hours - and for some, the House might change within a few minutes.

You can tell the time of birth (approximately) just by looking for the position of the Sun in your birth horoscope - which is also called a "birth chart", or natal horoscope (and you are called the native). When the Sun is located near the Ascendant, you must have been born close to Sunrise (because it indicates the Sun was rising then). When the Sun is at the very top of the chart near the Midheaven, you were born close to Noon. When the Sun is on the horizon line at the right of the chart (near the 7th House cusp) you were born close to Sunset.

If you happened to be born at exactly the moment the Sun was rising, the Sign and degree of the Ascendant and the Sign and degree of the Sun would be the same. If you were born exactly when the Sun was setting, the Sign & degree of the Sun would be the same as the Sign & degree of another Point called "the Descendant", which is always exactly 180 degrees opposite the position of the Ascendant (and thus in the opposite Sign), since they are both on the same Horizon Line.