The conditions of man corresponds to the changes in the position of the nine grahas. A human being does not enjoy happiness all the time nor does he always suffer hardships-- that is he experiences a mixture of happiness and sorrow. While he may be pushed up to a high position today, he may be thrust down to the depths tomorrow. It is not man alone that is subject to changes of fortune. Establishments too have their ups and downs, so also nations.
The sages saw a relationship between the position and movements of the planets and the destiny of man, the sorrow and happiness experienced by him. There is a branch of astrology called "hora--skandha". If we knew the planetary position at the time of commencing a job or enterprise, with its help we should be able to find out how it would take shape, how we would fare in it. If our horoscope is cast on the basis of the configuration of the planets at the time of our birth, our fortunes over the entire period of our life can be predicted.
Different reasons are given for the ups and downs in a man's life for his joys and sorrows. It is similar to finding out the different causes of the ailment he suffers from. The physician will explain that the disease is due to an imbalance in the "dhatus". The mantravadin will say that it is due to the gods being displeased with the patient, while the astrologer will observe that it is all in his (the patient's) stars. The pandit versed in Dharmasastra will explain that the illness is the fruit of the man's past actions, his karma. And the psychologist will express the view that the bodily affliction is related to an emotional disturbance. What is the true cause?
All these different causes may be valid. All of them together go to create an experience. When it rains it becomes wet and the place is swarmed with winged white ants. Frogs croak. All these are indicators of the rain. Many outward signs manifest themselves as the fruits of our past karma. They are all related to one another. The course of the planets governing our life is in accordance with our karma. We come to know the consequences of our past actions in previous births in various ways. Astrological calculations help us to find out such consequences as indicated by the heavenly bodies.
For a general background, please see here