Astrology is a part of astronomy and mathematics, put together. It is a vast subject in itself, having indefinite boundaries in the world of predictions.
The ancient Indian astrologers, Varahamihira and Bhaskaracharya were examples of astrologers with great predicting abilities. These astrologers took Indian astrology to great heights. Indian astrology is also known as Vedic astrology. When it comes to accuracy in predictions, vedic astrology has always shown its supremacy.
According to Indian astrology, stars (or nakshathras) and grahas(planets) play a big role, in the predictions. Everyday, every hour, every month and the whole year is ruled by planets and the stars below them.
Place of birth and time is very much needed to create a person's horoscope. As Varahamihira has rightly said, Time and place can change anyone. He was excellent and superb in predicting things. He was the most respected astrologer during the reign of Vikramaditya, the king of Ujjain.
http://hubpages.com/hub/Time_and_Place_can_Change_Anyone gives details of Varahamihira very clearly.
And these books are helpful guides to know the interesting topics on Vedic atrology.