You may already know the difference between these two words, but I think that they can be easily confused, so it’s useful to make a distinction. To put it most simply: astronomy is real, and astrology isn’t.
Astronomy is the branch of science that studies space, celestial bodies, and related phenomena.
Astrology is the belief that the position of certain celestial bodies in the sky visible from Earth dictates our lives to a greater or lesser extent.
Obviously, even though both are completely different, the words look and sound similar, so confusion is understandable. Their shared astro- comes from the Ancient Greek astron, meaning star. Obviously both astronomy and astrology are interested in more than just stars, but it’s only been relatively recently in our history that we’ve learned that not all those lights in the night sky are stars. It’s amazing to think how far we’ve come actually. From looking up and not know what all those little lights were, or why they only appeared at night, to now knowing what so many of them are and how they fit into the cosmos.
If you try to differentiate between astronomy and astrology by looking at their different suffixes, you probably won’t have much luck. There is a little logic to the difference though. Astronomy comes from the Greek astronomia, meaning regulation of the stars. Which isn’t exactly what the science about, though there is a logic to it in the sense that any science can be seen to attempt to place some order or logic on some aspect of the universe by understanding it.
The -logy of astrology comes from the Greek -logia, which originally meant to speak. And that’s what’s an astrologist does: tells your future or describes your character based on when you were born. Or at least they claim to. And of course they actually don’t, because what they claim to do isn’t possible.
The link between -logy and speaking isn’t so obvious though, is it? The ending quickly became synonymous with learning and science in general, based on the idea that experts would speak about their areas of study, as in a lecture. You can probably think of a handful of -ologies off the top of your head: biology, etymology, psychology, pharmacology et al. Because we associate this suffix with science in general, it’s easy to forget which one of the two is an actual science, and which one is a pseudoscience at best. It also helps believers in astrology as the word makes their belief system seem more like a credible, rational, science.
Of course it’s easy for me to say all that now, but for hundreds of years the words were used interchangeably. To be more precise, for a long time astronomy and astrology were the same thing, because our knowledge of space was so limited that we believed that the movements of the lights in the sky were responsible for changes in the world around us, like the seasons. It’s understandable that people believed that hundreds of years ago, but I have to admit, I find it hard to understand how someone could believe it now.