[QUOTE=Derleth]
OK, tell me this:
What is the underlying theory of astrology? A ‘theory’ is similar to the atomic theory underlying chemistry.
[/QUOTE]
Oh, so that’s what a theory is! Who knew? There is no theory in the scientific sense because, as you must know, astrology is not a science.
Astrology was developed when human understanding of the cosmos and our place in it was inherently religious. The first observations of the heavens were a mixture of charting the location and movement of heavenly bodies along with an attempt to relate that to natural events and the human condition. Like the rest of what was beyond understanding, much less control, the movement of stars, planets and luminaries was thought to be directly controlled by certain gods. A correct interpretation of celestial activity was believed to give an indication of the activity of the associated deities. Correlating cosmic events with terrestrial events, the observers gradually built up centuries of associations and interpretations.
Plus, if you needed some confirmation, you could always check out an entrail or two.
It was the 17th century before astronomy began the slow process of disentangling itself from its disreputable conjoined twin. Nonetheless, astrology is a sticky sort of thing. It popularly persists because we want explanations, we want meaning, we want everything to fit together and make sense, to know we have our rightful place in the cosmos. Because of its fluid, interpretive nature, astrology can shoehorn just about anything into place, which is a very satisfying thing. 
Current understandings vary widely, from those who insist that there is some scientific basis [“The Wave Structure of Matter allows the planets to affect us.” “Planetary motion orchestrates solar activity and geomagnetism, which imprints the neural network.”] to a generalized kind of all-is-one philosophy (religious connotations optional) [“There exists an ordering principle which influences events and situations and affects the inner constitution.” “The patterns found in the vastness of the universe are a larger exposition of the same patterns within the individual.” “It is the window of divinity available to mankind.”] to my personal favorite: “The movements of the planets have metaphorical value as symbols of the mind.” aka, it’s all made up.
I think any value lies in the fact that the system was made by human minds for human minds. It is one way among many to a better understanding of oneself and one’s relation to the world. I’m an artist and I naturally think in (and enjoy) pictures, symbols and metaphors. Mileage varies.