At the time of the sunrise, the rising sign is always the same as the sun sign. The rising sign changes every two hours. So to calculate your rising sign, you need to know the time of sunrise at that date and place, then add one sign for every two hours until you reach your time of birth. Make sure to consider daylight saving time. Alternatively, get an automatic calculation on websites like AlwaysAstrology.com or CafeAstrology.com.
According to this formula, for example, if you were born in Virgo, at 11.00 p.m., and on that day the sunrise was at 7.00 a.m., your rising sign, also called ascendant or East Point, is Scorpio. If you were born at 5.00 a.m., your rising sign is Leo.
This method is most accurate if you were born close to the equator. Geographical location and the time of year can influence the results. Due to the tilting in the Earth's axis, it can take more or less than two hours for different signs to cross the horizon. For the best precision, enter your date, time and place of birth on an astrology website. You can also download an astrology software, like Astrolog or AstroCalc 6.2.
Astronomically, the ascendant is the ecliptical geocentric longitude, or Celestial Longitude, of the rising point of the ecliptic.