I badly need post bachelor's education advice. Not sure where to go after this.

Here's my situation: Graduated about a year ago with a degree in Economics. I never really liked the subject, but I wanted to graduate on time, so I just kind of grinded through it during college.

Now, I'm sitting at a job with decent pay but where I find the subject matter mind numbing and meaningless. I find myself spending hours reading about paleontology and astronomy. In many ways, having gotten a little bit older, I think I've matured enough to know where my passions really lie.

My plan is this: go back to school and study either Geology or Astronomy. The only problem is, do I go for a second bachelors degree? Or is it possible to apply to a graduate program somewhere and then switch subjects to what I want to study (apply for MBA and switch to physics) and if so can I take undergraduate prerequisite courses while a graduate student? Or can I apply directly to the graduate subject that I want to take? Or do I take the undergraduate prerequisites at a community college and then try to transfer to a state school? I could really use some help figuring this out.

tl;dr: I strongly dislike the subject that I have a degree in. Want to go back to school but unsure how.