Liz, Bagpuss and Kevin have got it bang on, I think. It seems quite logical that day length, temperature and nutrition during pregnancy and the early months will have an effect on a person.
But as for all the mumbo-jumbo about stars, planets and the like...well, it's a nice little earner for a lot of people and a crutch for the gullible, I suppose.
On two newspapers I've worked on we had a panic over the horoscopes, with the relevant astrologers failing to file their copy (one was a prat called Justin Toper and the other a ghastly old paedophile called Patric Walker, who made so much money from his baloney that he was able to buy a Greek island).
I suggested to the features sub that an easy way out was simply to take some horoscopes at random from past issues, mix 'em up and bung 'em in. Thus Virgos got last week's Capricorn reading, the Geminis got a Pisces from a month or so back and so on.
It worked a treat - the gullible loons who follow such things continued to write in with their testimonials as to the amazing accuracy of the predictions, while the columns themselves continued to be padded out with crap.
Lord Rothermere, the strange but charming old fascist who owned the paper, was a a passionate believer in the accuracy of horoscopes (as was Hitler), and would insist on knowing what Walker's predictions were daily. Naturally they were always terribly auspicious - so much so that Walker's salary just kept going up and up. Shrewd guy!
Personally I'm a great believer in reading entrails. A nice plate of calves liver tells me I'm in for a treat at supper time.