Astrology is a limb of Vedas that is why it is called Vedanga(Veda+Anga). Vedas are repository of knowledge. The knowledge comes from Lord Brahma, the creator who gave it to Narada Muni ,who in turn gave it to Rishis and Maharishi Parashara admits to get it from Rishis. Maharishi Parashara was the son born from mental power of Lord Brahma. We get this knowledge from Brihat Parashara Hora Shastra which is written in the form of dialogues between Sage Maitreya who asks questions which are answered by his learned Guru Sage Parashara.