This is a very common question and has been addressed numerous times in several fora. I really don't want to waste my time writing on this since google can reveal tons of information in a single click if searched with a proper search phrase. There are a number of western astrologers who have turned to Vedic given the limitations and inaccuracies of western astrology, no wonder astrology is much derided in the west. Here's a link from one such astrologer's website.
http://www.barbarapijan.com/bpa/Topics/Practice_Issues/Differences_western_Jyotisha.htm