Astrology is an ancient Indian science which explains planetary motions and positions with respect to time and their effect on humans and other entities on earth. Astrology can be traced thousands of years back. Early Astrology was only based on the movement of planets with respect to stars, but later on it started including zodiac signs as well.
According to Astrology there are 27 constellations made up of 12 zodiac signs, 9 planets and 12 houses with each house and planet representing some aspect of human life. Depending on when a person is born, the 12 signs are distributed among the 12 houses and 9 planets are placed in various houses.
This visual representation of the snapshot of the signs and planets is called a horoscope chart. Astrology is nothing but interpreting the meaning of these arrangements as it applies to humans and other entities.
Siddhars are saints in India, mostly of the Saivaite denomination in Tamil Nadu, who professed and practised an unorthodox type of Sadhana, or spiritual practice, to attain liberation.
Through their practices they are believed to have reached stages of insight which enabled them to tune into the powers hidden in various material substances and practices, useful for suffering and ignorant mankind. (Read more on wiki)