Why doesn't Astronomy end in Ology?
Astro definition diagram and a spiral galaxy
 
The word "ASTRONOMY" comes from the Greek term meaning "knowledge of the stars."

But today, astronomy includes planets, black holes.

Astronomy is the study of the entire universe and everything in it. It's full of huge distances, gigantic sizes, and long periods of time. It's a BIG subject.

Luckily, just a few BIG ideas can help you understand the whole thing: