Is it harmless?
To raise this question these days is largely to invite catcalls. Especially when we raise it as Catholics. Our postmodern neighbor retorts, "What's the difference between your religion with its prophets and miracles and this stuff about horoscopes and magic? If it works for some people, what's the harm?"
Conversely, some of our Evangelical Protestant friends will take a position opposite from this, yet just as hostile to the Catholic Church. "Horoscopes and seances are forbidden by Scripture," they will say, "and that's why Catholic prayer to the saints is evil, too. It's all satanic."
And finally, of course, uneducated Catholics can sometimes get fuddled by apologists for the occult who tell them horoscopes and consultation with the dead are "in the Bible," so it must be okay. How do we navigate such turbulent waters and retain a healthy Catholic Faith?

A good place to start.
A good place to start is the Catechism of the Catholic Church (CCC 2115-2116):
"God can reveal the future to his prophets or to other saints. Still, a sound Christian attitude consists in putting oneself confidently into the hands of Providence for whatever concerns the future, and giving up all unhealthy curiosity about it.

"All forms of divination are to be rejected: recourse to Satan or demons, conjuring up the dead or other practices falsely supposed to 'unveil' the future. Consulting horoscopes, astrology, palm reading, interpretation of omens and lots, the phenomena of clairvoyance, and recourse to mediums all conceal a desire for power over time, history, and, in the last analysis, other human beings, as well as a wish to conciliate hidden powers. They contradict the honor, respect, and loving fear that we owe to God alone."

Let's unpack this. If you ask most modern Catholics which of the Ten Commandments astrology or divination violates, they'll likely tell you, "Thou shalt not steal." That's because, being more modern than Catholic, most tend to think of sins against the wallet before thinking of sins against God. They figure the thing is quackery, so the main sin is in bubbling somebody out of their cash. Interestingly, though, the Catechism teaches us to regard divination, horoscopes and the rest as sins against the First, not the Seventh, Commandment.

The First Commandment says, "You shall have no other gods before me." The reason the Church regards divination, consulting horoscopes, astrology, palm reading, summoning the dead and all the rest of it as sinful is because they are all attempts to treat creatures like the Creator. They are all attempts to wring from (and give to) creatures (whether crystals, tea leaves, stars, dead people or spirits) that which is proper only to God. And they are all, without exception, motivated by a desire for power and control.