Constellations are instantly recognizable and have historically had a profound impact on culture, but astronomers - people who study the stars themselves, independent of society - don't talk about them.
Different cultures tend to group stars into different constellations, but that doesn't mean these constellations aren't "real", or that they don't have meaning, in that context. When you say "Orion's belt", there's a specific, real set of stars that you're referring to, and you can make claims like "stars in this constellation are brighter, or closer together" and have them be true. You can draw elaborate maps of the night sky that are accurate and predict where constellations will occur night by night.
But from an astronomical perspective, all this is useless. Constellations are social constructs.
Now imagine you're in a society governed by astrology, and you're trying to fight against that. One might find it helpful to explain all this, and clear up what exactly constellations are, and how little scientific weight they hold. You could talk about how stars that appear close together to us can actually be really far apart, and how the patterns can look different from different parts of the world. You could even bring up some of the rather poor decisions that have been made in history when people tried to look for messages in the stars.
But no matter what, those opposed to you can always say, "I don't need astronomers to tell me that there isn't a big spoon in the sky. I can see it, and so can you - why are you denying the obvious truth?"
By this point you've probably figured out what I'm getting at, but suffice it to say this is kind of a follow-up to my post from last month, and comes from similar origins. Analogies are never a good persuasive tool, because anyone who cares enough can just nitpick them, but sometimes I'm so naively charitable towards my interlocutors that I mistake their willful ignorance and prejudice for simply a lack of proper understanding - which I think a good analogy can help with. I'll leave it at that.