Astrology is complete and utter bullshit. Horoscopes are written so vaguely as to apply to anyone. Constellations themselves are just the result of abstract games connect the dots. Not to mention that the idea that the gravity of certain planets tens or even hundreds of million kilometers away has any noticeable effect on us is laughable.
Now that I think about it, wouldn't it make more sense for zodiac signs to be assigned at conception, instead of birth?