Drawing a perfect circle that rotates

Hi

I tried to draw a circle while holding down the Shift-key. Then centered it inside a MovieClip named 'circle_mc'

Every frame I'm rotating it like this:

circle_mc.rotation += 5;

That seems pretty simple but the circle is not perfectly round. Why?

. 

You can see the result here:

http://boostmode.com/dev/circle/circle.html

Any ideas?

Thanks

Rolf