Jquery, offset(), and IE

Jquery, offset(), and IE

I am needing to center a fixed position header in a website and decided to go with jquery for it. I finished it and it of course looks great in FF and Safari, but not in ie7 or 8. Any ideas?

http://testing.stoobs.com/firefly/index.html
http://testing.stoobs.com/firefly/js/global.js