What do you have to major in to be an astronomer?

I'm always surprised to hear that geography is a major.  Physics is the one you really need, so it's good that's one of your majors.  You don't need to know much astronomy to get into grad school in physics, astrophysics, or astronomy - the physics is the important part, and knowing some astronomy helps, so if you can take some courses in that as well, that's good, but not as crucial as physics.  Spend your summers doing research, either at your school, or through an REU or NASA program - get a feel for what research is like, and get a jump start on grad school that way.

As eri said physics is the most important if you want to go into astronomy. Also chemistry and geology can be helpful. Even biology if you want to get into astrobiology. Though I'm not sure how much call there is for that at the moment.

At the undergraduate level, concentrate on math, physics, and computer science.

As an undergrad, it's important to take a whole lot of physics as it will be used in astronomy a whole lot. Mathematics may also be a good idea.

I did it with two undergraduate degrees in math (one in maths & stats).  

However, I would gladly change one of them for science (physics and chemistry).

I also did a lot of specialized geography at the college level and it helps.

But physics is more important (and math is a lot of help if you go into cosmology)