Bunraku, if you are interested in the different "non-conforming" astrologies practiced in the past, it seems clear that our horoscopic astrology was only one version. The others, perhaps because they were more magical or secretive, did not show up in our principal sources on how to read a horoscope.
Speaking of Valens, as a professional astrologer who read horoscopes for clients, he was critical of the alternative mystical traditions of his day, probably because, whatever spiritual value they might have had, he found them worthless for actual horoscope interpretation.
Among these alternative pre-Enlightenment astrologies:
1. Kabbalah (Qabbalah, various spellings)
2. alchemy
3. Egyptian magic (some of it was pretty gruesome black magic)
4. Hermeticism (cf. Hermes Trismegistus, with Hermes the Greek name for Mercury.)
5. Mithraism
This was a secret religion or cult dedicated to a god named Mithras, practiced extensively in the Roman empire during the early centuries CE. Mithraism left no written records but there is tons of archaeological evidence. The initiates were classified according to the planets, with Mercury being the first level of initiation, and Saturn being the highest-- the level of "the father".
The Mercury level was symbolized by the raven, the beaker, and the god Mercury's serpent staff.
https://en.wikipedia.org/wiki/Mithraism
"Mithras is the guide of souls which he leads from the earthly life into which they had fallen back up to the light from which they issued ... It was not only from the religions and the wisdom of Orientals and Egyptians, even less from Christianity, that the notion that life on earth was merely a transition to a higher life was derived by the Romans. Their own anguish and the awareness of senescence made it plain enough that earthly existence was all hardship and bitterness. Mithras-worship became one, and perhaps the most significant, of the religions of redemption in declining paganism.[150]"
5.
theurgy
This one pretty much cuts across the board, including professional horoscopic astrologers. The belief was that through a series of rituals and spiritual exercises, the astrologer could literally become the god. Versus praying for divine guidance in chart-reading, the astrologer supposedly became the god, or merged with him. A mortal astrologer could make mistakes in chart-reading, but when unified with a god, he was far-seeing. Invoking Hermes (Mercury, Thoth) made sense to the more magical astrologers.)
This about the process from Iamblichus (b. 245 CE,) a proponent of astrology and a student of Porphyry:
http://www.esotericarchives.com/oracle/iambl_th.htm
In sec. 9, he seems to be saying that the astrologer should look for "the lord of the house" in question, and then merge with that god or spirit. This might or might not be Mercury. In general, though,
"Hermes, the patron of literature, was rightly considered of old to be a god common to all the priests and the one presiding over the genuine learning relating to the gods, one and the same among all. Hence our predecessors were wont to ascribe to him their discoveries in wisdom and to name all their respective works Books of Hermes."
The translator notes of Hermes/Thoth, "He was also the revealer of the divine will to men." Pretty potent stuff for an astrologer predicting someone's future.
Of course, societies of the past who practiced horoscopic astrology also had other sorts of star-lore and beliefs about the heavens that I wouldn't call "astrology" per se, but rather,
cultural astronomy.
But I want to make the point that the planetary god Mercury, notably as the Greek Hermes, Egyptian Thoth or a composite god Hermanubis, was far more important in traditional esoteric astrology than simply calling him "the ruler of traditional astrology" lets on.
Mercury was kind of the keeper of the keys.
