AS3 horizontal rotation- how to?

Hello:

I have this movie clip on stage (dsp), which I would like to keep rotating horizontally. With the following script it rotates in circular mode from left to right, but not horizontally (left to right or vice versa). How do I rotate it horizontally?

dsp.addEventListener(Event.ENTER_FRAME, rotateHorizontal);
function rotateHorizontal(event:Event): void
{
dsp.rotation += 0.5;

}

Thanks in advance

German