<script type="text/javascript">
       var directX=1;
       var directY=1;
       var sunX=0;
       var sunY=0;

     function move(){
         
             
             sunX+=directX;
             sunY+=directY;
             
             var sunDiv=$("sun");
             sunDiv.style.left=sunX+"px";
             sunDiv.style.top=sunY+"px";
             //判断小球的x坐标
             //offsetWitdth用于获取当前对象的宽度
            if(sunX+sunDiv.offsetWidth>=document.body.clientWidth || sunX<=0){
                directX=-directX;   
             }
             
             if(sunY+sunDiv.offsetHeight>=document.body.clientHeight || sunY<=0){
                directY=-directY;
                 
             }
           
       }
       window.setInterval("move()",50);
       
       function $(id){
         return document.getElementById(id);
       }
    
    </script>