Maybe this site will help you out.
Anybody can help me out about astrology and ascendants? I need to write a paper for school and don't really know where to start!
This is where astrology and astronomy (almost) meet. The ascendant is the point where the Eastern Horizon and the ecliptic intersect (the ecliptic is the apparent path the sun takes through our sky). Through the year, this means that the ascendant passes through many constellations as the Earth makes one orbit of the Sun. For astrological purposes these are divided into 12 (to "sortof" coincide (badly) with the months of the year). Thus there are 12 signs of the Zodiac, each corresponding to the constellation at the ascendant at the time.
(I think that's right, but I'm an (amateur) astronomer, not an astrologer).