When you say astronomy, do you mean you want to actually be an astronomy major or a physics major with an astronomy focus/minor? Do you know what school you want to go to? If so, make sure they actually have astronomy as a major. Many schools do not.
I've never taken an astronomy class, so I can't be sure about what math you'll need. But I do share an office with the guy who teaches the astronomy classes and they hardly use any math. But if your school has an astronomy major (ours doesn't), then may be you'll be expected to know more math than the students in our astronomy class.
What instruments do you play? It's so weird how so many physics/science people play an instrument or sing. At my REUs, I always brought my piano (and my guitar to my last one) and we had plenty of physicists/chemists jam sessions since most of us played an instrument and/or sung.
Edit: My reading comprehension sucks today. Apparently you already know which schools have the astronomy major and which doesn't. Look at the schools you're thinking of going to and look at their astronomy program. It should have some sort of sample program to show what classes a typical astronomy major takes. This should give you at least some sort of idea of what math you'll be expected to know. For example, if you were a physics major at my school (since we don't have astro), you'd find this webpage
http://www.uakron.edu/physics/academ...uate/index.dot
(Sample curriculum at the bottom)
And you'd see that a physics student is expected to take calculus and calc-based physics the first year (at the same time, apparently. So they obviously assume the person has taken AP physics in high school). However, it's not unusual at all for someone to make it through the physics or engineering program here without having taken calculus in high school (and if it can be done for physicists and engineers, it can be done for astronomers). You could try checking the sample curriculum at some of the schools you're looking at to see what they expect you to take.
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