The mythos and culture surrounding the practice or astrology itself is undoubtedly a fabrication. This has been demonstrated before. Researchers have taken astrological readings, changed the names of the signs, and tested them against the original settings to see how accurate they would be. They had just as many "accurate" predictions as they had before the sign names were changed. The readings behind the signs are so general that they will always be applicable to large demographic of the population no matter which sign you are. In all cases, the subjects said that their readings have the same rate of accuracy, no matter which sign they were reading.
The experiment was to discover if the zodiac was actually a description of certain personality types based on time of birth, or just a series of statements so general that they could be applied to the majority of the population, no matter which sign it fell under.
Here's one report. It is not the one I read, but it is similar.
Gravity, illumination and assorted forms of electromagnetic radiation on the local scale of earth, moon, and sun are certainly responsible for some evolutionary patterns. Just how vulnerable we might be to more subtle inputs of light, gravity, and radiation of more distant objects is likely not going to be resolved for a while. But in the meantime, if you believe it, it works, and therefore it is true.
If you think it works then it works. The study proves my point. The subjects had no reason to doubt the validity of their horoscopes and took them on subjectively making such adjustments of logic, rationality, timing, etc, as were necessary to conform to their belief. It's a rationalization process found in most belief systems.
It's like moon phases effecting people too. Go to a mental asylum on a full moon and see if there's a difference. Science is slowly catching up but there are always exceptions as well due to circumstance or just random chance. Personally I find the Chinese calendar more accurate but they can only ever be used in general terms I personally believe.
A study in the U.S. concluded that 30 years of crime records of a dozen states showed a spike in crime rates around the full moon. The legend of the werewolf signifies this effect of the moon on the human mind. Similarly, the movements of the rest of the universe also have an effect. It's all atoms.
Descriptions of certain zodiac signs seem accurate because they all list general human traits that just about anyone experiences at one point or another. So when someone who believes in astrology sees a person exhibit a trait in line with their zodiac sign, it’s more notable than someone exhibiting a trait opposed to their sign.
Whatever match with what a persons sign, will sticks in the observers memory because it confirms a previously held belief, whatever actions or characteristics that go against what a sign predicts are rationalized and forgotten. Zodiac signs become a self fulfilling prophecy when a person analyzes him self because they're convinced that these completely normal emotions that everyone experiences at one point or the other are part of their core identity. Once bias is "confirmed" this person sees their sign as a reason behind everything they do and they begin to act along the lines of their sign because they've made it a all encompassing justification and explanation for why they act, feel, or think a certain way.
The problem with zodiac relating to people is the complexity of people. If you make a broad enough statement about characteristics of people, then people will match them. Because people are complex enough to have contrasting traits at different times, then most broad statements will match everyone at least some of the time. For example, the statement "You like to think of yourself as an intelligent person, but sometimes things make you doubt yourself. You are also kind, but in your heart you know you could do more." Sounds profound and personal. But it will fit 90% of people.
In broad terms there are personality traits and some interesting correlations shown between zodiac signs, but we are a complex product in a variety of situation, so generalizations will be difficult to make. It is a bit like saying it is February therefore the weather will be … Even ten kilos from me they have a foot of hard rain, yet I've had none and I’m sitting in dry. Astrology works like the weather, it depends of what else is going on around you in the cosmos and where yourself are at.
There is some evidence that seasonal changes and being born at certain times in the year can affect personality changes as you grow older, but there is not enough data available to make a definite determination. As always, it's a gray area.
It makes sense that in a certain area with a certain culture, kids born around the same time of year would tend to have similar experiences. Therefore they would have a high likelihood to share some similar traits. However, extending the provenance of a very conditional rule of thumb to all people everywhere to a level in par with science is stupid. A kid born on October 16th in Jakarta would have little in common with a kid from Alaska born on the same day. You are = genetic predisposition + personal experiences + environmental influences. That's it.