Hellenistic astrology is a kind of astrology that is widely regarded as the original astrology and it appeared sometime after Alexander the Great conquered Egypt in 332 B.C.E. and sometime before the beginning of the Christian era. It is impossible to pinpoint more exactly the period in which the original astrology appeared, but it is widely believed that the writings that constituted the original Hellenistic astrological material were composed somewhere in the second century before the Christian era. The authorship of those original writings is also unknown and mysterious as well, as they feature a cast of semi-divine figures with names such as Hermes, Asclepius, Anubis, Orphias and many others.
The Hellenistic astrology marks a completely new chapter in the history of astrology. It is almost certainly the case that Hellenistic astrologers got the idea for doing astrology from the Babylonians or possibly from the pre-Hellenistic Egyptians, or perhaps from both. Either way, both were practicing astrology in some form prior to the Hellenistic period. During the Hellenistic era, a large number of new concepts, techniques, theories and practices were introduced which cannot be traced back to either the Babylonians or pre-Hellenisitc Egyptian antecedents. There are no Greek astrology signs like in Western astrology and it is only one of the many differences between the two. All those newly-found astrological practices contain new and original astrological concepts such as houses, zodiac signs, sign characteristics, quadruplecities, triplicities, masculine and feminine signs and others.
Hellenistic astrology played a key role in the history of astrology, as it is the primary source of all the Western astrology that appeared later. It is also related to the Hindu astrology. And yet, many details surrounding the tradition, theory and practices of Hellenistic astrology are still unknown. In point of fact, we do not have much material from those early astrologers, who we can refer to as the founding astrologers of Hellenistic tradition. However, there is a corpus of Hellenistic astrological writings that survived, consisted of a vast of texts, most of which written in Greek and some in Latin.
These Hellenistic astrological writings that survived are so few that we can actually count them up: 4 Substantial General Treatises in their original languages (3 in Greek and 1 in Latin). “Tetrabiblos” is by far the most popular book and it was written in the 2nd century A.D. by Claudius Ptolemy. This text on astrology is one of the most important ones ever written. Vettius Valens' “Anthology” is another significant writing on Hellenistic astrology which translation to English is now available online for free. The “Apotelesmatics” of Hephaistion of Thebes is the third writing in Greek and forth, written in Latin, is the “Mathesis” of Firmicus Maternus.
In addition to these four writings, there are also some remained fragments of what seems to be a fifth Greek writing by Dorotheus of Sidon. Unfortunately, being fragmentary at times and having been corrupted by interpolations by Persian translators has made this piece of evidence unreliable. Nevertheless, Dorotheus' text is still considered as one of the best sources for the original astrology and it influenced Persian, Arab and Christian astrologers to dig deeper.
All of the material that is so recognizable in the modern sense was introduced during the Hellenistic period. That's why we could easily say that Hellenistic astrology is the origin of all later Western astrology. The Hellenistic tradition had a major influence on the development of the Western tradition. In the 8th century, much Hellenistic material was translated into Arabic either directly from Greek sources or through Persian interpolated translations. For at least the few first centuries of the Arabian flowering of astrological practice, astrology was still for all intents and purposes Hellenistic.
It is important to mention, however, that the Arabian astrologers evidently did not get all of the Hellenistic astrological practices and theories. As primary sources when they were formulating their own astrology, they relied upon Ptolemy and Dorotheus of Sidon. The only problem is, Ptolemy is not exactly a representative of the Hellenistic astrology. While he indeed included a lot of traditional Hellenistic astrological material in his “Tetrabiblos”, Ptolemy had his own agenda and discarded a lot of traditional Hellenistic material, which is why Ptolemy is not considered as a typical representative.
The other highly influential author for the Arabian astrology was Dorotheus of Sidon. He, actually, has a plenty of material that is traditional, or put in other words, material that is representing the original Hellenistic form of astrology. But the problem with him is that his work was not only fragmentary, but also written in didactic verse. It is actually a long astrological poem that unfortunately didn't survive in its original form.
Shortly after the second century B.C.E., there was a very large corpus of astrological material made by some mysterious persons, named Nechepso and Petosiris. Namely, these persons were real, with data showing that Nechepso was an Egyptian pharaoh and Petosiris was a high priest. However, it became evident that these two lived in different eras, so they may have been fictional names for the actual authors of the texts.
Nechepso and Petosiris were believed to be the explicators and expositors of the original material going back to Hermes Trismegistus. However, these vital pieces of astrological material did not survive in tact, so we don't have all the volumes of all the different books that these two important astrological figures introduced. What we know of their writing is primarily interpolated translations presented by later authors, occasional paraphrases and simple references that in many cases do not have a lot of content.
In any case, the material that was formulated in this lost work by Nechepso and Petosiris became the source of all the later Hellenistic astrological writings. The Hellenistic tradition could be understood to abstand from the founding period all the way up til somewhere around the 7th century of C.E. Even though this covers the Roman period and the early Byzantine period, it can still be referred to as Hellenistic astrology for the only simple reason that it has all of the earmarks of this original astrology and there are no other traditions that have influenced it.
Upon first read, you may get an impression like this so-called ancient Greek astrology is a collection of precepts and doctrines, gathered by a small number of individuals who used to star gaze and think alike. However, the original astrology has been in the making over centuries, with plenty of individuals (such as the ones we've mentioned in the article) leaving their own contribution to the astrological practice of their period. Thanks to their efforts, although not everything, we still know a lot about the origins of Hellenistic astrology.