Haskell, 143 bytes

This uses the gloss library:

import Graphics.Gloss main=animate(InWindow "" (100,100) (0,0))white$ \t->color(makeColor 0.5 0.0 0.5 1.0).rotate(t*6).translate 75 0$circle 1 

The prettier version is

import Graphics.Gloss purple :: Color purple = makeColor 0.5 0.0 0.5 1.0 main :: IO () main = animate (InWindow "" (100,100) (0,0)) white $ \t -> color purple . rotate (t * 6) . translate 75 0 $ circle 1