Here are some of my thoughts on the rationale for astrology, adapted from a recent Livejournal posting. I'm reposting them here because they'll be of interest to the Lebwog audience too; see the link for discussion.
First of all, I consider astrology to be primarily a form of art. It's a way for us to express our creative powers. From that point of view, asking whether celestial forces influence the tenor of our lives becomes somewhat like asking whether The Starry Night is an accurate portrayal of how some real starry night physically appeared, such as might be produced by a skilled but not particularly artistic photographer. It's just not an interesting question. Side note - I once read an entertaining article in Science News about researchers who had studied the question, "Did Van Gogh use more yellow in his later paintings because the aging of his eyes caused him to see the world as yellow-tinged?" and arrived at the answer "No."
One of the several theories I have for how divination works, and the one I usually use when talking to rationalists, is that it's all about being able to activate our unconscious minds in a useful way. I have a little story I sometimes tell Tarot querants about how inside our heads we each have a monkey and a dinosaur who work as a team - the dinosaur is big and powerful but not good at making plans and decisions, and the monkey sits on its head giving directions: "Go this way! Go that way! Ooh, ooh, go over there!" The partnership has a lot of advantages for both parties, but it also has a cost. The dinosaur never gets to go anywhere that isn't interesting to the monkey, and the monkey never gets to go anywhere that the dinosaur can't fit. If you try to talk to them, you're only going to hear from the monkey. The dinosaur isn't going to get a word in edgewise as long as the monkey is doing its monkey thing and demanding attention. So if you want to hear the dinosaur's opinion, you have to distract the monkey by giving it a complicated toy that will keep it busy for a while. Divination systems work like that. They're complicated symbolic systems that present endless chains of questions for the conscious mind, often involving arithmetic and foreign languages and so on, and if you use suspension of disbelief to get your conscious mind working on "Okay, the cards or the heavenly bodies have a secret message for me which I have to decode" then you have the opportunity to get answers to your questions from your unconscious mind.
Here's another thought. If I tell you "The ancients knew that persons whose natal Sun is in Aries have blah, blah, blah personality characteristics," that sounds like a superstition. But if I tell you "Experience has shown that persons born in early Spring, defined as a one-month period starting from the equinox, have blah, blah, blah personality characteristics," that sounds like it could be plausible; you might not be prepared to believe it just because I said it, but you probably wouldn't immediately throw it out. It would at least be an hypothesis that someone could respectably do research on. One could imagine mechanisms for such a statement to be true - for instance, babies born immediately after the Spring equinox will see longer days before they ever see shorter days, and their families and the other human beings around will be doing the activities they do in Spring and Summer in the babies' first few months of life, which are significantly different from the activities they do in other seasons, and there could be differences in the mother's diet that would affect her milk in the first few months of life, and so on. But "natal Sun in Aries" is just a precise way of saying "born in early Spring"; those statements should have the same credibility.
Three words: "Seasonal Affective Disorder". If you can believe that that could be real, Sun signs shouldn't be any less plausible in principle.
And it certainly should come as no surprise that the nature of birth-date seasonal effects on personality, if there are any, could be very weird and non-obvious. You know, I recently read a claim that heterosexual men with introverted personalities have a strong tendancy to prefer women with small to medium-sized breasts as potential sex partners, and not to consider breast size a high priority anyway, while extroverts tend to prefer large-breasted women, and to care much more strongly about breast size. The claim was that that's a well-known studied fact in psychology. I haven't read the references or asked a psychologist, and I'd like to before making any firm statements on it, but: A. that's very weird and non-obvious - I was aware that different men had different preferences and strengths of preference on this point, and the ones that differ significantly from mine are as baffling to me as the preferences of homosexual men, but I don't think it would ever have occurred to me to ask if it correlated with introversion/extroversion, and what the fuck kind of bizarre evolutionary adaptation or social pressure or chemical imbalance or whatever would cause us to behave this way? - and B. damned if the prediction doesn't agree very well with my own personal experience and my observations of numerous other men and their preferences.
My father taught kindergarten for a long time and he would often complain about the specific challenges he faced teaching students with December birthdays. The thing was that most parents would send their children to school as soon as they were legally allowed to do so, and (at least where he worked, in BC) that was based on the child's age as of January 1, which meant that the children with December birthdays would be the youngest in their first class, about 11 months younger than the eldest in the class (who were the ones with January birthdays). At that age, 11 months is a lot of age difference, and it affected them socially and academically and because the age-cohort classification from when they started school would stay with them for the rest of their school careers (the schools in BC basically aren't allowed to make kids repeat grades no matter how educationally advisable it might be), there were plenty of opportunities for permanent effects on their personalities and interactions with others. So at least among his students, it'd be pretty surprising if there weren't some personality characteristics that correlated with "natal Sun in Sagittarius", which is close to the same thing as "birthday in December".
Because astrology - even when mediated by a computer according to well-defined rules, like my compatibility test - always involves humans in some way, there are opportunities for the humans to slip in correct information independent of whatever is objectively going on in the calculations. You put in your birth data and that of a girl you're interested in, and the test tells you "Compatible!" or "Not compatible!" and you have an immediate reaction of "That's right!" or "That's wrong!" and for at least half a second, you know the right answer to the question even if the answer produced by the test is not that answer. Look at the comments made by people who're taking the test - they're not really using it to answer questions for which they don't already know the answer, they're asking questions for which they do know the answer, and then they say "There, you see, I was right and the test supports me!" or "Ha, the test was wrong!" If they think they know the answer, but have some doubts, then the test answer will make them pay attention to those doubts. With this kind of question, and most of the questions people take to astrology, even when they think they don't know or have preferences about the answer on some level they at least have hopes, hunches, and other unconscious promptings - and that's where the monkey and dinosaur effect comes in. Part of the point is to give you an excuse for, or a reason to admit to, something you already knew but weren't ready to say before.
There are reasons for astrology to be worthwhile that have nothing to do with whether it works. I have had the experience of sitting at a table wearing my pointy hat while a seemingly endless procession of cute sorority girls (both small-breasted and large-breasted) lined up of their own accord to wait as much as half an hour to talk to me. That was with Tarot, but it could just as easily have been astrology - and a skill that can do that for me is worth having. Open question: does the fact that even this didn't get me laid have anything to do with the fact that in the time and place I was born, a piece of rock named after a Goddess whose cult were best known for their fanatical celibacy, was at its highest point in the local sky, and if that is or even could be so, isn't it useful to have a way of describing it as concise as "Vesta is on my MC"?
Now, here's a short shameful confession: I really don't like classical ballet. I like some of the music that goes with it, but I find the performances terribly boring. They're not something I'd do as a form of entertainment; although I've attended many and will probably do so in the future, it's always been and probably always will be for other reasons than to be entertained. I think I'm by no means alone in that, and my impression is that there would be a whole lot less classical ballet in the world today if the people doing it didn't get government funding for their activities. I doubt that very many ballet companies could survive while both sticking to the traditions and having to do it on ticket sales. Nonetheless: I'm glad that this art form still exists. I think we're better off for keeping it alive, and I don't begrudge the portion of my tax dollars that go to that effort. I think that astrology has similar value as a living cultural tradition that has existed for thousands of years, and I think it's a good thing for people to keep the tradition alive instead of letting it slip away. Yes, of course it's easy to list other cultural traditions that aren't good, don't make our lives richer, and shouldn't be kept alive, but that's not the point. Think of it as comparable to a language. We'd all communicate a lot better if there were just one language that everyone learned as their native language, but it still seems like a shame when the last native speaker of some obscure no-longer-popular language dies. Someone's got to speak those minority languages even if they don't provide much practical advantage in the speaker's daily life.
On the language thing: astrology provides precisely defined technical terms for describing a lot of concepts that cannot easily be described in colloquial English. If I say, "I know this guy - he has a classic Virgo personality" then in many of the circles where I move, I've conveyed a lot of information about my impression of him quickly and easily. A crowd of trained psychologists might have their own technical terms that would be similarly good for communicating such concepts, but the general population doesn't, and the terms of astrology happen to be well optimized for some of the kinds of discussions I participate in, and already known by many of the people I talk to.