For astrology to work, one of two assumptions must be established: either the heavenly bodies somehow affect us directly by means of some form of transmitted energy or force emanating from those bodies which alters our behavior, or, our bodies and minds have somehow evolved the means to alter themselves by using heavenly bodies as cues or signals. Let's look at both of these hypotheses critically.

It is not unreasonable to assume that we are somehow affected by solar system objects. After all, we are affected by the sun, it's light and heat influence us during the daily cycle in an obvious way, as well as by the yearly cycle of the seasons. Modern man is affected by the light and dark, cold and heat, and it is clear that primitive man was influenced by these rhythms even more so. The mechanisms by which these changes manifest themselves are also self-evident, we are vulnerable to predators at night when they can see well and we can't, for example. Travel and other actiivities such as food gathering are also easier for primates during daylight hours and when the weather is comfortable. How the other celestial bodies, such as the planets, might affect us, is never explained by astrologers. There is a lot of hand-waving and smoke-and-mirrors about mystical forces and emanations, influences, and other action-at-a-distance phenomena, but none has ever been suggested so the rest of us can verify it. Gravity is the only force "emitted" by these bodies (their light is just reflected sunlight) and as Carl Sagan pointed out, the gravitational field of a doctor who delivers a baby is much stronger than that of any celestial object. The same can be said for the magnetic fields some of these bodies possess.
Until some testable mechanism by which the planets can affect us is proposed, we can't even begin to verify it.

The moon's gravity does cause the tides, but this is because the weak gravity field of the moon affects the entire planet and it's effects are visible globally. Why and how this happens is explained beautifully by the theory of gravity and the mechanism is well understood by science; published tide tables are marvelously accurate, as any sailor can tell you. On a local scale, these effects are swamped by the gravity of nearby masses, you don't see the water in your swimming pool rising and falling (although it does); that effect is completely overwhelmed by nearby wind, vibration, etc. Besides, if the moon has an effect on us, everyone should feel that effect at the same time. It doesn't happen; as I pointed out in an earlier post: bartenders, cops, emergency personell are just wrong. The moon does not cause accidents, crime, or drunken brawls as has been repeatedly demonstrated by many statistical studies, including my own.

Granted, the moon does affect marine life, and the feeding and reproductive behavior of many marine organisms is keyed in to lunar cycles because of the tidal connection. Horseshoe crabs, for example, synchronize their spawning for August tides, ensuring ideal conditions for their larvae. These creatures have been doing this for so long that the lunar clock is already part of their body cycles, and works even when these creatures are in aquariums out of sight of the moon. Other life forms need to actually see the celestial body in order to synchronize.

It has been argued that a woman's 28 day menstrual cycle is regulated by the moon's monthly orbit, and this cannot be ruled out a priori. After all, we know women living together tend to synchronize their menses (probably by hormonal cues) and if there is any advantage to simultaneous menstruation, it is not unrealistic to assume that an external cycle in nature could have been seized on by evolution as a means to put all the females in the band on the same page. But this mechanism (if it's not just a coincidence that it's on a 28 day cycle) obviously is only a relic of the past and plays no role in our survival today. Most people today don't go outdoors often enough to know what the phase of the moon is, and women menstruate throughout the lunar month.

The lunar example is a plausible astrological mechanism because we can see how nature MIGHT use astronomical phenomena to regulate physiological or even psychological cycles, just as horseshoe crabs do. This is a perfectly reasonable assumption, but that doesn't necessarily mean it's true. Neither does it begin to explain how the planets could provide this function, after all, they just look like stars and their motions are extremely complex and predictable only after long observation and record keeping. And unlike the moon, the planets are hard to observe even if you're trying to do so. Clear, cloudless nights may have been common on the savanna but not in the forests of ice-age Eurasia. If there is a connection, it is up to the champions of astrology to explain just how it operates.

So if the planets don't emit some form of influencing agent which affects our behavior, and if our bodies can't use them to synchronize physiological activities, just how does astrology work? How do we even know that it works. The answer is that it doesn't and we don't. There has never been any statistical study showing an astrological effect that has convinced anyone except other astrologers. No mechanism can be suggested as to how it might work. All we have is anecdotal evidence (hearsay) which never seems be to documented well enough so you can repeat the experiment and see for yourself.

Just because we can't detect a natural mechanism does not mean one does not exist. After all, no one knew about magnetism for thousands of years and it certainly exists, and it is very important. We can't rule out that perhaps there is some unknown agent responsible for astrological phenomena, but astrologers have been studying their craft for thousands of years and not only have they not come up with the mechanism, they haven't even bothered to look for it. They simply don't care. They have no interest in how astrology works, all they care about is how to cheat their fate. That alone is enough to tell me they don't care a fig about science and nothing they say can about it can be trusted.