Raisonner avec un astrologue, c'est boxer avec un oreiller à plumes: on l'enforce en un point, il se regonfle ailleurs (Michel Gauquelin, 1955) (my translation: To reason with an astrologer is like boxing with a feather bed: you make an impression at one point and it/he inflates at another)

I'm glad that the discussion turns from belief to empirical research for that's where it belongs. There are numerous studies about astrology. By far most of them find no effect at all, some find an effect which may or may not be consistent with astrological predictions. If I compare studies that come out in support of astrology with those that don't I cannot find a difference in the general approach to method of study. Mostly, specific astrological predictions are singled out and looked for in a large sample. Since many astrological predictions are specific about the direction of the effect but vague about effect size ("there is a tendency...", "may be counteracted by...") the reasoning of all those studies is that even if there is but a "tendency" it has to show in a large unbiased sample because all potential counteracting or supporting tendencies should cancel out. The astrologers cite all the supporting findings and deal with the contradictory evidence in the way Bearheart has demonstrated above: The researchers should have looked at...instead and don't know a thing about astrology. Since there are so many conflicting predictions and so many conflicting astrological systems, this reasoning will never run out of arguments. What Gauquelin has thought about that I have cited above.
Two examples and I start with Gauquelin. The 1955 book referenced by Alice has the bulk of his data about the so called Mars effect, and his findings about Jupiter and Saturn. Gauqulin explicitly states that his findings contradict traditional astrology and he does so in no kind terms (see above or: "[astrological arguments are full of] l'ignorance, l'incohérance, la déformation des faits"; no need to translate that, I guess). He has done many more experiments with only negative results and I am at a loss of understanding why only a tiny fraction of the work of the Gauquelins which (a) does not fit well within the astrological theory, (b) is seen by Gauquelin himself as a refutation of traditional astrology, and (c) is known for potentially serious artifacts of data selection, is cited by astrologers.
Peter Niehenke was once (and perhaps still is) chairman of one of the German astrological societies. He has made his doctoral dissertation about astrology and I haven't checked the details but I trust a professional astrologer to get the astrological predictions correct. In his two million items of data he looked for several thousand correlations and found nothing at all beyond chance (if you look for several thousand correlations a sizable amount of them must be correct by chance alone) and stated that clearly. He was of course severely criticised in his own astrological journal for singling out effects ('you should have also taken ... into account') and defended his research bravely against his fellow astrologers. When he found that his results did not support his belief he wrote: "I experience every day that the interpretation of the horoscope allows deep insight into the human nature"; he then spoke about "two conflicting realities" and said that astrology is in the danger of becoming a kind of religion, a faith and went on: "But why not? One surely need not worry about astrology and its survival even with more negative results upcoming. A world in which astrology is true is once and for all a more beautiful world than one in which astrology does not exist...The want for astrology to be true, therefore, is much stronger than all rational counterproofs." That's a demonstration how to explain away results one doesn't like.

Wolfgang