It appears that Hellenistic astrologers worked more from tables of planetary positions than by direct astronomical observation, in much the same way that modern astrologers use an ephemeris. Before the time of Ptolemy, they used tables called "Perpetual Canons," which were not especially accurate. In fact, Valens [Bk VI, chapter 4] cites one Apollinarios that they were only accurate to a degree or two. This of course presents a serious problem for the astrologer using sign-based methods, particularly when the table had a planet in the very beginning or ending of a sign, because if the planet is placed in a different sign the entire rulership and attendant analysis changes. Valens even suggests that he often "rectified" the planetary positions based on the study of the chart.
Ptolemy's Almagest set an entirely new standard for the determination of planetary positions. His refined geometrical models of geocentric planetary motion allowed him to generate tables of planetary positions mathematically, so that the astronomer or astrologer did not have to rely on tables that were simply compiled from the somewhat imprecise astronomical observations that could be made at that time. Ptolemy also compiled convenient listings of planetary positions in a book called the "Handy Tables," which was the primary astronomical reference book until Arabian times, superceding everything else.
I believe that all Hellenistic astrologers would in fact have used the most accurate planetary tables they could have found. So much of their reasoning depended on having the planet in the correct sign, at least. However, it may seem odd that when dealing with certain other concepts, many astrologers continued to use more traditional numerical values (usually derived from Babylonian material) even when it was known that they may be observationally inaccurate. There are two especially important cases.
The first concerns the standard interval assigned for the heliacal rising or setting of a celestial body. This interval was set at fifteen ecliptic degrees distant from the Sun. However, the actual interval for the heliacal rising of a body - that is, the time that it becomes distant enough from the the Sun to be visible to the eye when it is ascending over the eastern horizon before the Sun in the morning - not only depends on visibility conditions, but also on geographical latitude and zodiacal position, as well as the intrinsic brightness of the body. It can actually be much more or much less than fifteen degrees.
The second relates to the ascensional times of the signs of the zodiac-- that is, the time it takes a sign to completely ascend over the horizon at a given geographical latitude. Ptolemy had shown in the Almagest that the traditional values used by Babylonian astronomers were often wildly incorrect, as far as observational astronomy is concerned. Yet many Hellenistic astrologers continued to use them.
What are we to make of this? Is this just conservatism on the part of the astrologers, or even worse, ignorance? On this subject we have to consider Plato's famous injunction to the astronomers in his circle: to save the phenomena using uniform circular motions. Now, this would seem to be a straightforward and reasonable demand to account for the observed motions of the heavens with geometrical hypotheses--and in fact this is how this injunction has been interpreted for most of the tradition. However, in the seventh book of the Republic Plato makes it very clear that he is far more interested in a kind of ideal astronomy that addresses the world of the forms or ideas. This ideal astronomy would deal with true motions governing ideal fastness and slowness as measured by true numbers in their true forms. The apparent motions of the heavens, however marvelous, are but an imperfect copy of these most real of motions. The expression "save the phenomena" has an interesting ambiguity because of the Greek word phainomenon. It could mean save or account for what is apparent in the observable motions; but it could equally mean account for that which appears, or that which is behind the observable motions. It is this latter sense that Plato is emphasizing in the Republic. Accordingly, one might think that the astrologers favored a standardized interval for heliacal phenomena and stuck to the simple arithmetical scheme of the Babylonians for ascensional times because they believed that these values were more consistent with the ideal astronomy.
However, I am not trying to exonerate the astrologers on the charge of conservatism or ignorance with this explanation. Despite the tremendous influence of the Platonic teachings in ancient times, I do not believe that the original Hellenistic astrologers were Platonists in this regard, although this issue requires much more study. I do not think that Hellenistic astrology was "metaphysical" in this manner. After all, it sought to account for everday events in the mundane world, not those in the ideal realm.
But there is another reason that they may have favored some of these standardized values. It is my hypothesis that all the astrological concepts are expressions of events occuring in the cosmic soul. And just as one's facial expressions can sometimes be misleading and not perfectly mirror the soul, so the observable celestial motions may sometimes exceed and sometimes fall short of exactly expressing a state of the cosmic soul. The standardized values, then, would be the true expression of the soul's inner state. In this way we need not invoke an ideal realm to find a justification for these astrological preferences.
This issue will actually be quite important as we begin to examine Hellenistic astrological practice in detail. For example, all Hellenistic astrologers, whether implicitly or explicitly, make the distinction between zodiacal methods (sometimes called "platic") that are based on the signs alone, and partile methods that are exact to the degree, although much more attention is given to the zodiacal methods. It is easy to account for this bias by making the assumption that the inaccuracy of the planetary tables made the partile methods unreliable, even though in principle they would give more exact information. It is much easier to locate a planet in a sign than in a degree. However, later in this book I will be explaining that the zodiacal and the partile methods actually express entirely different types of logic, or perhaps more accurately, the same logic operating at two different levels. So in this case the distinction has nothing to do with more or less perfect expression.
On the other hand, there is another distinction of astrological method commonly made by astrologers of the later period that does not so much concern accuracy as very possibility of giving a rationale for a given practice. As I mentioned earlier, both Ptolemy and Valens favor methods that they consider "natural" as opposed to those which they regard as "magical" or "mysterious" (as pertaining to the mysteries). Natural methods are ones that they can explain in terms of natural philosophy, numerological principles, and other such "reasonable" devices. As a matter of fact, the entirety of the first book of Ptolemy's Tetrabiblos has as its program the reconceptualization of astrological concepts in terms of Aristotelean natural philosophy, making astrological practice more "natural" and acceptable, and he purges out any concepts that he cannot account for in this manner. To me, the very fact that they would entertain such explanations is a sure sign that astrologers of that era had already lost touch with the logical substructure of Hellenstic astrology. Either that or they were silently repudiating the central Hellenistic hypothesis that celestial events are expressions of the inner workings of the cosmic soul.