"It makes no sense at all" comes to mind, but wouldn't ever work.

"All the stuff is vague enough to fit a lot of people" followed by reference to the Derren Browne astrology vid on youtube seems better, but lacking.

Perhaps I'm coming at this from the wrong angle, and appeals to reason should be replaced by stubborn baldfaced facts.

For example, I imagine that birthdate has an insignificant correlation with: income, suicide rates, drug addiction, and other things.

Is there anything it has a real correlation with? I'm guessing yes, but only things that are easily explainable, and those should be outnumbered by 854 other things. There wouldn't be much profit potential in a book called "854 statistical studies showing astrology can predict exactly nothing", but what would the best chapters contain?

A friend of mine who is very much into parapsychology and astrology and all the other pseudo-spiritual crap was trying to convince me to read one of my horoscopes for the day (it was longer than I expected). It was around ten in the morning and I agreed to humour him. In it mentioned that I would - and this was explicitly stated - nearly have an accident hitting a pedestrian. Sure enough later that day I almost hit a crossing pedestrian, and it was while my friend and I were absorbed in the discussion of astrology. This ironic coincidence left me without a pot to piss in.

It's moments like this that strengthen the false positives of superstitions, all thanks to chance.

How could it be possibly be correlated in any way? Does your life completely change if some extenuating circumstance caused you to be born on one day instead of another? What if your pregnant (with you) mom missed her train, and this meant that you were born in one town as opposed to the one she was probably going to. Does this really have an affect on things like how much money you'll make when you're 47, what sex your second child will be, and how many friends you make in a lifetime?

I don't know, I guess it could somehow. Perhaps your present state would be dramatically different if the chain of events prior to your birth were altered in some dramatic way.

Have the believer read his/her horoscope. If it says they will have a good day, shoot them in the foot.

The problem is that it could — it's about as likely as there being a god who regularly affects events in the real world, and requires the same sort of unknowable, extra-material mechanism to work. And it's about as difficult to prove or disprove.

Given that a majority of humans believe in some or other version of religion that is approximately as badly supported by the real-world evidence, I think OP is likely to be disappointed — no pithy proof (nor any complex one) will convince a believer.

My preferred method for getting people to stop believing the primitive horoscopes is to read them all 12 from yesterday's paper in random order and ask them to rate each one according to how well it described their day. (Of course the believers in horoscopes will all tell you you need an individualized horoscope written for you, and those take a bit more effort.)

On another note:

On the contrary! Being born on New Year's Day dramatically increases your risk of drug addiction, by about a factor of 5. It must be because of those wild parties on new year's eve sending the mothers to the hospitals while intoxicated. (Seriously, about 3% of drug abuse patients write down January 1st on their paperwork when admitted for treatment, compared with between 0.5 and 1% of patients without substance abuse problems. The real explanation, of course, is that some patients are unwilling to reveal their real names/birthdays/SSNs, and among those who provide false birthdays, most of them use january 1st.)

The best thing to do is not to go right at the core of their beliefs, that astrology is real.

Instead I like to lead people on a bit of a round about route so they are completely agreeing with me about the ubiquitous existence of confirmation bias in our society...as well as variance, false positives, placebo effect etc. Usually you can just about anyone to agree that OTHER people fall sway to confirmation bias in things like religion, politics or something else (don't use religion if they are religious!). Once they agree on the phenomenon it is then you can turn the topic to their particular core belief of astrology and the people are usually forced to do such a full circle to get away from the comments about confirmation bias they were completely agreeing with moments before. Of course many peoples faith is strong enough for them to be completely okay with such cognitive dissonance but nonetheless it seems more effective than going directly for "astrology is stupid because of xxxx"

The total lack of forces between the planets and ordinary people pretty much tells you all you need to know. The only forces that can affect things over those sort of distances are electromagnetism and gravity, both of which are overwhelmed by random noise (gravity by the objects around you, magnetic fields from the earth and power lines, etc.)

Not that this will give you much help; believers in astrology are generally immune to reason.

People who believe astrology is real have bigger problems imo. Usually a very small sense of self-worth.

Now that is what I call pithy.

What would a world exactly like ours except for astrology not working at all look like?

Much like a world exactly like ours except for humanity being the only intelligent species.

I used to work at a bar and there was usually a newspaper around so I would often offer to read horoscopes to the waitresses for fun. I'd ask their birthdays but then secretly read them random horoscopes, and only reveal the truth after they expressed amazement about how detailed and accurately the horoscopes described what was going on in their lives.

I want to know what fake date people born on Jan 1 provide.

ive been able to explain to some people that the constellations were just invented by mankind and are totally arbitrary and there are no forces are at work between the stars and the people born on earth...

Yes, astrology is absurd on its face, and horoscopes are certainly made-up garbage. But that doesn't necessarily mean that astrological signs do not correlate to particular traits. Most of these correlations are probably just confirmation bias, but I can't rule it all out without some study. It could be reasonable to believe that people born at different times of the year have different traits on average.

It's jan 1st. a baby is born in new york. another in Melbourne.
The reason contained in reasonable is ........

christmas cheer

Why do you say that? It's genetics and the environment that factor into a persons' traits, not the alignment of stars millions of light years away.

you think the alignment of stars is the only thing that changes throughout the year?

The fact that schizophrenia cycles with flu season doesn't validate astrology.

Perhaps that's why I started my post with:

Precession of the equinoxes FTW. Whatever sign people think they were born under, they were actually born under the one before it. So they've been reading the wrong horoscope all their lives.

What about if I am born in August and you are born in January I have more sunlight in my life than you if we both live for 1 year. And this effects our dopamine? levels and thus might make us act differently...

I don't know about that first sentence.

But this would be reversed in the Southern Hemisphere, no?

Malcolm Gladwell in "Success" points out a correlation between birth month an success in various sports. (The larger kids tend to get picked for teams, leading to a selection bias at a young age.)