astrology

Don't expect a comprehensive account of astrology here, I wouldn't want to get bogged down that much. Astrology is ancient and no doubt many branched and possibly internally riven. As I sit down to write this, I confess that I know little about it beyond the claim that stars and their movements and relations somehow affect the character of humans [and presumably also koalas and stick insects, but we don't so much care about them] on this planet, and that these influences are related to signs of the zodiac, cusps, risings and ascendants. I'm also pretty sure that there's no real testable, verifiable mechanism to explain these influences and effects, though there's always been a lot of talk, of a more or less serious nature, about so-and-so being a typical gemini, leo or whatever. I've always considered astrology to be pure bullshit just on a prima facie basis, without feeling the need to investigate further. After all, if it were true that our stars largely form our character, we would be able to use such knowledge to slot people into their rightful or most fruitful places in society, a la Plato's Republic. But of course I also know that astrological ascriptions of character are so notoriously vague as to be unfalsifiable. I myself am a 'cancer', described as sensitive, artistic and home-loving. I can barely think of anyone who couldn't be tweaked to fit that description, from Jesus to Hitler. There is actually a useful term for this - the Forer effect.
So, is there more to astrology than these vagueries, and why is it so god-damned popular?

Wikipedia has a painfully lengthy article on the subject, and early on they make this interesting remark:

Few astrologers believe that the movements and positions of celestial bodies either directly influence life on Earth or correspond to events experienced on a human scale. More common is the idea that astrology is a symbolic language, an art form, or a form of divination.

They're talking no doubt about modern, western astrologers. The idea is that modern astrologers know better than to make large, testable claims they won't get away with, better to retreat to the byways of 'art' or 'divination'. Unfortunately this retreat from a mechanism by which the celestial influences the terrestrial just won't work. Like theology, there just has to be something at the heart of it, though maybe not, for astrology has affinities with theology in that labyrinthine arguments can be developed out of the skein of the human mind, on the flimsiest of pretexts, when some kind of perceived self-interest is involved - though to me the self-interest in astrology is less clear than the self-interest in theology. That might seem silly in that, in one sense, it's clear that astrology is less about stars and zodiacs than it is about us, but astrology almost seems like mystification for its own fun sake, whereas theology is more clearly about a god who makes us specially and therefore makes us special. 
Anyway, I'm not sure how astrology can be classified as an art, as it has no clear aesthetic dimension. And I don't know how you can determine that someone is good at the art. If you think of the art of violin-making, for example, the product will be a really fine violin, supposing the maker to be a true artist. Fine violins are measurable in their fineness. I presume that with astrologers, the measure of their art is the success of their divinations. What else could it be? Surely not their beauty or richness. The richness of their divinations would only be significant if they came true - everything hangs on that.
So when the success of an activity depends on results  - think of medicine, or science in general [the question of whether or not psychology is a science is really a question of whether it produces results in the form of cumulative knowledge, as biology and physics have] - it's slightly ridiculous, it seems to me, to retreat to the claim that it's really an art.
Astrology is of course more ancient than astronomy. In fact, last year marked what some described as the 400th anniversary of astronomy's birth, when Galileo trained a telescope upon the heavens. Of course this is an insult, not only to Copernicus and the many other pioneers immediately preceding Galileo, but to Ptolemy and the many other ancients who used the heavens to produce real knowledge. But it's largely only in the modern era that the stars were examined for their own sakes rather than for ours. Astrology is a typical product of those pre-scientific times it seems to me.
I'm not going to provide an account of the historical development of the various branches and cultural traditions in astrology here - you'll find much of that in the above-mentioned Wiki article and at many other sites, including this sceptical one. Suffice to say that they were valuable traditions in that they represent the fruits of our first obsessions with the behavior of the stars. I'm sure that the first genuine theorists about what was out there gleaned much from the painstaking records of the early astrologers. Interesting to note, though, that astrology was to some degree driven underground by the spread of Christianity - the 'intervention' of the stars no doubt being seen as devilishly competing with divine intervention. This quote from Johannes Kepler at the very beginning of the scientific renaissance provides an interesting example of the half-Christian, half-scientific negativity towards astrology:

"It should not be considered unbelievable that one can retrieve useful knowledge and sacred relics from astrological folly and godlessness. From this filthy mud one can glean even an occasional escargot, oysters or an eel for one's nutrition; in this enormous heap of worm-castings there are silk-worms to be found; and, finally, out of this foul-smelling dung-heap a diligent hen can scratch up an occasional grain-seed -- indeed, even a pearl or a gold nugget."

Sometimes astrologers make a half-hearted attempt to associate a mechanism for astrological influence with the well-known tidal effects of the moon. Of course this is absurd for many reasons. Gravitational effects diminish very precisely with the square of the distance between two bodies, and with stars other than the sun, the distances are ginormous, though the objects may be very massive. I don't think mass and distance are much taken into account anyway in astrology. Also, tidal effects occur only with oceans, bodies of water which cover significant regions of the surface of the earth. They don't occur with small bodies of water, nor do they occur with people [and we're only talking about tidal effects, never mind the personality effects claimed]. Of course there's the age-old claim that the moon, especially the full moon, has a definite effect on people's behaviour, including the inducing of madness [lunacy]. However, the vast majority of carefully conducted studies indicate that this belief is a myth - but I'm going off topic.
To finish off, the onus is and always has been upon astrologers that their field is about something. In this it's in a similar position to theology. Astrologers, in defending their turf against the analyses of empiricists, argue that the field is incredibly complex, so that all known tests inevitably miss something - usually something incredibly vital. Basically, they're arguing that the field is untestable, usually the first sign of a pseudoscience. It seems to me, though, that all the talk of ascendants, descendants, cusps and houses only obscures the central issue - how can these celestial bodies influence human behaviour? That simple, central question is never answered, and is usually never even addressed. The same problem applies to theology. The constant elaboration of the properties or essence of the deity obscures rather than clarifies the central question - what evidence is there for a supernatural mechanism causing natural events? What lies at the centre of astrology, and theology, is not a mechanism but a massive, untestable assumption.