Come, astrology purports to predict more than one's mood. It claims a supernatural influence, or at least predictive power, over the course of future events. Nostradamus himself employed astrology for his predictions. (Of course, in today's litigious environment, horoscopes are usually more circumspect in making predictions than formerly, and have become little more than mood rings.)
Moreover, even if the stars and planets need only influence man's thoughts to produce astrological efficacy, there still must be some sort of anomalous physical interaction.
We can rule out gravity, as it doesn't distinguish its influence on us by our birth date, and its effect is too small even to measure over a human length scale.
That leaves light as the only conventional mechanism: perhaps the way the stars look influences our mood in complex ways, and perhaps those susceptibilities are exquisitely age-dependent. But I never have seen horoscopes that correct for latitude or eyesight, or tell you not to bother if it's cloudy.