To appreciate the importance of Ptolemy’s Tetrabiblos we must witness its transit in history, i.e. what led to its becoming and what has become of the system he formulated. First what was astrology before Ptolemy? Let's see what Ptolemy says.
“We shall decline to present the ancient method of prediction, which brings into combination all or most of the stars, because it is manifold and well-nigh infinite, if one wished to recount it with accuracy.… Besides, it depends much more upon the particular attempts of those who make their inquires directly from nature than of those who can theorize on the basis of the tradition; and furthermore we shall omit it on account of the difficulty in using it and following it.” (Ptolemy, p 227)
He seems to consider his system a simplification of older systems. He justifies this simplification on the grounds that because it is easier to follow it can be based upon more of a tradition. It seems then that one of the strengths of Ptolemy’s astrological synthesis is regularity and consistency. This is confirmed when Ptolemy makes the further point that the older systems are based upon natural observation and the diviner’s talent. His system of astrology avoids this problem because it is based upon recorded planetary positions combined with a plausible theory. Maybe these dual aspects of simplification and consistency are the reasons why Ptolemy’s system has survived while the other systems have become extinct. Note that his system requires less natural observation than the older systems; we shall come back to this point when we talk about modern astrology which uses no naturalistic observation.
The ancients all had their methods of divination but it is thought that the Chaldeans of Babylon with their two systems, i.e. liver divination and star-gazing, laid the true foundation of Ptolemy’s horoscope based astrology. (Lindsay, p.9) The practice of liver divination was motivated by specific questions, whose answer was revealed by the nature of the liver at the moment of sacrifice.
“The Babylonian diviner somehow felt that the blow of the sacrificial knife linked the victim with the whole universe in a sort of lightening stroke.” (Lindsay, p.26)
This sense of the importance of the moment is carried over into astrology where the moment of birth has ultimate significance. Ptolemy, besides devoting a section of his introduction to the justification of this notion of a starting point, katarche, also stressed the importance of the exact moment of birth, necessary to “the fraction of the hour of the birth.” (Ptolemy, p.228) So the significance of the sacrificial moment becomes linked to the moment of birth.
While Chaldean liver divination had a personal characteristic that was determined by the individual destiny of those performing the sacrifice or posing the questions, their second system of divination, through the stars, had a more general nature. It dealt with the heavenly bodies in relation to the national destiny; this concerned itself primarily with the undertakings of the ruler. The phenomena of primary interest were those most readily observable, i.e. eclipses, the relation of the sun and moon, and the appearance of the sky in general. Because the eclipse was the most dramatic of the celestial phenomena it came to have special significance. Its appearance portended danger. (Lindsay, Page 3)
Ptolemy’s system reflects the Chaldean emphases. For instance eclipses still play an important role. And they are still considered to be dangerous. “The positions most dangerous and hardest to avoid are those in which either of the luminaries is possession of the place of the eclipse, or the degree opposite.” (Ptolemy, p.191) Additionally the eclipse comes to have significance as the starting point or horoscopal birth point for a nation or world just as the time of birth would for an individual. The eclipse becomes the sacrificial moment for the earth. Ptolemy, like the Chaldeans, also stresses the primary importance of the national destiny in considering the total effect of the stars.
“Prognostication by astronomical means is divided into two great and principal parts. … that which relates to whole races, countries, and cities, which is called general, and the second and more specific is that which relates to individual men. … And since weaker natures always yield to the stronger, and the particular always falls under the general, it would by all means be necessary for those who purpose an inquiry about a single individual long before to have comprehended the more general considerations.” (Ptolemy, p. 119)
Thus in Ptolemy’s astrology, as for the Chaldeans, the stars come to stand first for general matters, such as the nation, and only second for individual matters – and this in relation to the first. This is similar to Planetary Astrology where an individual's planetary transits are subservient to the general planetary configuration. (See General vs Individual.)