Heh heh heh...
I'd LOVE to discuss the Decans... at any degree of specificity you'd like. Till you cry uncle, in fact. But I'm going to have to do some fierce oversimplification to do it in under about 50,000 words.
The Chaldean order is a straight lift from the
Picatrix, by way of Agrippa which the GD pulls out of Barrett's plagiarized
Magus. Historically, the likeliest explanation for the Chaldean order is Harranian star worship. (cf Green's
City of the Moon God and Swerdlow's
Babylonian Theory of the Planets). I'd bet a lot that it's a straight pull from astral religion and the temple geography and architecture of the Sabaeans.
Now, having gone out on a theoretical limb there I'll add another boulder to the mill. When the Decans are described as being one of the Seven Wanders "in" a division of a sign, it doesn't literally mean that that Decan is identical to that planet in that sign in a chart. Aries I is not equivalent to having Mars in Aries in a chart, it is characterized by a force that resembbles Mars as he expresses himself in Aries. In fact the oldest references to them say that the Decan is always a place of dignity for the given planet, even when the planet is otherwise ill-dignified. This goes back to my argument in the seven planets thread (to which I'll return as soon as I've got the proper time on my hands) about the
Geography of the zodiac vs. the
agency of the Planets.
The Decans are directly and explicitly linked with the belief in spirits and "lords of time" from their creation as an Egyptian calendrical system. This fundamental association continues through their absorption into the Yahwist mythologies and later on, Hellenistic astrology and European magick. HUGE topic there that particular evolution. And one that really will take a book to look at properly.
The important thing (far as I can tell) is that the Decans (unlike the fundamentally passive zodiac terrain) seem to have been regarded as ACTIVE spirits holding sway over capital-T Time... what the Gnostics would have called Aeon. The Decans are often referred to as controlling the planets (!) because they existed in the sphere "above" them. To get even straner, they are explicitly referenced as dæmons in early Solomonic magickal material (cf Peterson, Ameisenowa, Rankine & Skinner)
Here's the weird thing: the tradition of the Decans seems to fall out of favor largely because of Ptolemy. He literally ignores them in the
Tetrabiblos without explanation or exegesis. Most scholars agree on the reason; Ptolemy (who many scholars do not even believe was a practicing astrologer) was determined to codify astrology, emphasizing the science over the art. And the more pagan, magickal elements were usurped by a mechanistic post-Stoic view of the cosmos as interlocking crystalline spheres... but those damn spirits just wouldn't go away. Even as the original potency of the Decans (which are virtually orphaned in western astrology after the
Tetrabiblos) is forgotten, those strange images persist and mutate over thousands of miles and years, while retaining versions their original Egyptian names. (!!!) The Deacns literally refused to die and continue to pop up in Western magick and astrology for 2000 years, maybe because of oral tradition or because the efficacy made them something of a trade secret that didn't quite fit the much-vaunted pristine Ptolemaic model. Over and over the belief in the Decans and the Planets as conscious entities reemerges in the magickal tradition. At root, western magick is concerned with traffic with spirits/dæmons (cf Kieckhefer, Fanger, Walker, et al) . The tug of war in astrology between scientific "de-spirited" astrology" and magickal "dæmonic" astrology reaches a fever pitch in the Enlightenment as materialist science becomes the new state religion and magic is kicked to the curb. We all know how wonderfully that turned out.
But still still still... there is this lingering belief that the universe is not a machine, that spirits are at work around us, that events are not random and empty, that the world is pregnant with meaning and consciousness. In many senses, traditional astrology is the loam from which this belief springs.
It literally comes down to the old Fate vs. Free Will debate that plagues Western civilization: are our lives meaningful and what controls them? The empty, facile, psychologized New Age horsehit that has passed for astrology for 100 years is predicated on a nonsensical view: that the universe is a mechanism BUT that the mechanism can be bent to our will if we just wish hard enough. We might call this Free Will with a lower case "f" (as in
F.U. mom and dad! You're not the boss of me! 
) in which there is infinite entitlement and no responsibility. The Aquarian Age rejection of "Fate" as ineluctable necessity is a kneejerk reaction based on people liking to have their own way. A me-generation wankfest where everything is a reason to pat yourself on the back and feel proud. Getting your way and Free Will are not the same thing. The trouble with a universe possessing deities and spirits and meaning (and Fate) is that some things MEAN more than others, some events WILL happen, everyone doesn't get to be the protagonist in the universal drama, we cannot be FREE agents. Only in a universe empty of spirits (and therefore magick) could people believe they can act blindly and randomly without consequence or cosmic impact. Only in a universe devoid of consciousness could someone believe that EVERY horoscope is positive, that EVERY person is special, that EVERY fate is happy.
Now, this is only a position, I'm sure people will take exception to the above and have lots to say. I'm sure we'll have fun fighting over that one.
The thing I can't figure out is if people want to reject the spirits/magick how they wrap their heads around astrology in the first place. How do they justify astrology as some kind of psychological bait-n-switch for people who couldn't be arsed to get their PhD or MD and practice as a licensed mental health profesional? And why bother mutilating something so robust and venerable for such a questionable gain? As much as it may pain the editors at Llewellyn, premodern humanity did not see the world in the way we do. The world was pregnant with spirits being born and taking action at every moment. The idea of a generic "goddess" or a person being described as a "Scorpio," or magick as a mood would have seemed ridiculous.
In a sense, this philosophizing takes us right back to your original question: why the Chaldean order for the Decans? I'd argue long and hard that the answer is because the astrological world, that is to say, the worldview which permits astrology to exist in a coherent meaningful way, is swollen with consciousness, populated with spirits that follow a (super)natural Order. The power in Astrology, the force that drives events is the Planets. They are literally described as conscious entities. Only modern astrologers treat them like softserve dispensers squirting out "anger" or "love" or "invention" as they pass overhead and under our feet. For the folks living before the 17th century, for people who lived under monarchs, who understood the hierarchies of their world and cosmos, who believed that the universe was meaningful and that lives could spool out like Fate-spun thread, the Planets were the rulers obeyed even by rulers. Situating that power in the PLanets is (in a sense) the "spring" at the center of the astrological clock. No... that's too materialist an image. How about: the Planets are the sap in the astrological tree, and the light in its leaves and the tickle of nutrients in its roots, the fruit sparkiling in its branches.
There was nothing more powerful in the ancient consciousness than the 7 wanderers. Lots of folks make a good case for them being the source of the gods in the ancient pantheons of a whole BUNCH of cultures including pretty much any one of which you've ever heard. Every culture in the ancient near east ordered the planets according to the "Chaldean" order. (n.b. The name itself is a kind of historical joke). As rulers over these awesome forces, the Decans would reflect the Planets as much as the Planets reflect the Decans. As below, so above. In turn, since the Decans were both spirits AND (as such) rulers over regions of the sky, the only way for mere mortals to make sense of the Empyrean would have been to judge the sway of the 7 as they moved through the 36, and in turn how the 36 drove the 7. The Planets' comparative dignity would have been of the utmost importance in making sense of them (and making use of them) effectively. But that's another larger topic...
Blather blather... is that an okay start?
