setInterval - backgroundPosition

setInterval - backgroundPosition

Hi, can anybody please tell me why my background is not shifting on a loop?
It only seems to shift 200px and then stop.

#image_box_03{
position:relative;
width: 200px;
height:200px;
margin: 0 auto;
background-repeat:repeat-y;
border: thin solid #000;
background-image: url(images/number%20sprites.png);
background-position: left top;
}

<script type="text/javascript">
setInterval(function(){
 $('#image_box_03').animate(
{backgroundPosition:"(-200px 0px)"},{});
}, 1500);
</script>

Thanks for any help.