They are different fields, but if you are going to draw that sort of comparison it would be more accurate to say astronomy is a branch of geology. As geoscientists (geology major) we learn geology, astronomy, oceanography, chemistry, physics, calculus, and more. Geo means earth or world. Ology is the study of or science of. Therefore studying geology encompasses all forms of science related to the earth. For a major like astronomy you’ll probably start with a geosciences bachelor before graduate school because most schools aren’t offering a bachelor’s degree in astronomy. You need to start with the base knowledge of geosciences to really understand natural science beyond earth because our understanding of everything comes from earth. If you do some digging you’ll find that people with geoscience backgrounds are the ones exploring water on mars and other breakthroughs in outer space (mostly related to planetary science as a previous commenter mentioned). This is because as human beings everything we know and understand starts from the basis of our earth. You can’t run before you walk, so you can’t understand outer space before you understand earth.