My knowledge about Astrology is limited to the one that is followed in South India. My discussions will be restricted to that. Astrology is extensively used by the Hindus in India. The underlying philosophy of Astrology or for that matter any Hindu knowledgebase is the "Karma Siddhanta" or "The Theory of Karma". Simply put, as you sow so shall you reap. That is the dictum of the karma theory and its so exacting that even gods when they descend to the Earth, have to abide by it.
According to the Hindu belief there are three types of Karma: Sanchitha, Prarabdha and Agami.
Sanchitha karma is the sum total of all mental impressions that are left in and after any deed is done.
Prarabdha karma is the set of deeds that you are born to resolve in this birth (a sub set of Sanchitha).
Agami karma is what is yet to unfold.
When a child is born, the place and time of birth is recorded and the horoscope is cast based on the Place of birth, date and time of birth and the sunrise on the day of birth.
The Astrological chart when studied carefully by a qualified astrologer, will help reveal the Prarabdha Karma of the person whom the chart represents.
But, why choose the position of the planets for casting a chart?
We live in a 4 dimensional world - 3 dimensions of space (length, breadth and height) and 1 dimension of time. By making a chart of the position of planets at a given place and time, the conversion to any other place can be done easily and validated for any other calendar system or place. Secondly, the planets movement around the Zodiac is very consistent across a time scale...like Moon take 2.5 days to transit from one house in the zodiac to the other.
Hence, calculating the relative time is also easy given the starting position of the planets. It is my opinion that the planets by themselves DO NOT produce results and they are used to keep the timing precise.
Having said this, in the next article in this series, we will analyze what constitutes Karma, the three types of Karma and which of these Karma can be modified, etc.,