Of all the ancient astrologies, Indian and Hellenistic are closer to one another in fundamental conception and practice than either of them is to Chinese or Meso-American astrology, for instance. They are also closer to one another than either is to modern astrology. Indian and Hellenistic astrology may both be called "horoscopic" in the sense that much of their chart analysis depends heavily on the determination of the rising degree at the time of birth, and not merely the zodiacal position of the planets on the year, month, or day of birth, as is the case in many other ancient astrologies. They are also very similar in their use of sign-based aspects and in their fundamental house assignments. The Hellenistic time-lord procedures have a role analogous to that of the celebrated Indian dasas, although the procedures for determining the sequence and duration of the planetary rulers are different in the two traditions. And they are alike in innumerable points of detail.
Nevertheless, there are significant differences. Indian astrology makes extensive use of the naksatras, which are a division of the zodiac into 27 or 28 portions corresponding to the daily motion of the Moon. Although the "lunar mansions" are known in Medieval Western astrology, they are not found in the original Hellenistic material and seem to have been imported later from India. Then again, Indian astrology does not have the fivefold irregular division of each sign into "bounds" (called "terms" in Medieval Latin), which is a central concept for Hellenistic astrology. Indian astrology places far more importance on the lunar nodes than does Hellenistic, almost giving them the status of planets. Hellenistic astrology has an elaborately developed system of lots (often but mistakenly called "Arabic Parts"), which are lacking in Indian astrology proper, although they were introduced later as a result of Persian influence. Conversely, Indian astrology has an extensive development of divisional (or "harmonic" charts), while Hellenistic basically employed only a version of the twelfth (and possibly thirteenth) harmonic. Then there is the nagging problem of the choice of the beginning of the zodiac, where Indian astrology has various alternative starting points for a sidereal zodiac; by contrast, there is reason to believe that many Hellenistic astrologers -- certainly those influenced by Ptolemy -- favored a tropical zodiac, although it is still unresolved whether the earliest Hellenistic astrologers used a sidereal or a tropical zodiac. These two astrologies also differ on quite a number of points of detail.
How are we to account for these likenesses and differences? There is an opinion current among many Indian astrologers that horoscopic astrology originated in India at some distant time in the past as the formulation of insights obtained by certain enlightened "seers" in meditative states. From India, it spread throughout the Middle East and was introduced to the Hellenistic world by Indian colonists, whereupon it underwent its own characteristic development.
Compare this with the opinion of the Western scholar David Pingree, who argues that the Yavanajataka, a Sanskrit translation made circa 150 C.E. of a Greek astrological text, whose title he translates as The Horoscopy of the Greeks, is one of the two primary source texts for all later Indian astrology, along with a lost translation from a second Greek text. In support of his thesis he cites in detail many parallels between particular Indian doctrines and those of Hellenistic astrology. In particular, he demonstrates that much of the technical Sanskrit astrological and astronomical vocabulary consists of simple transliterations of the corresponding Greek terms, producing words that have no meaning in Sanskrit itself.
To one who approaches the evidence with an open mind, it is hard to avoid the conclusion that sometime in the first few centuries of the Christian era, Hellenistic astrology left an unmistakable imprint on Indian astrology, particularly in the terminology and practices concerned with the astrological houses. However, this still does not rule out the possibility that Indian astrology had undergone substantial indigenous development prior to this Hellenistic influence, especially since many of its concepts such as the naksatras have no clear Greek antecedents.
We do not intend to go further into this divisive historical problem at this time. Even if Hellenistic astrology did ultimately derive from Indian sources, we take the view that it is effectively a fresh starting point in the development of astrology, which has enough internal consistency that it can be understood on its own terms and from its own presuppositions. In fact, we intend to show that Hellenistic astrology possessed a clear and coherent theoretical foundation that motivated the introduction of its concepts and techniques. It is true that the search for theoretical foundations is a uniquely Western obsession. Indian astrologers, with their view that astrological doctrine was communicated to ordinary mortals by enlightened beings, seem to exhibit little interest in the question of theoretical foundations, although I have several times heard them say that astrology makes no sense without the the concept of karma and the doctrine of reincarnation. To us, the question is whether such a hypothesis gives a sufficient rationale for the astrological techniques they use, and whether it is capable of giving coherence to the dizzying array of concepts; for I believe that this can be said of the theoretical assumptions underlying Hellenistic astrology.