function docWidth(){
  var docWidth = 0;
  if(document.width){
    docWidth = document.width;
  }
  else if(self.innerWidth) // all except Explorer
  {
    docWidth = self.innerWidth;
  }else if (document.documentElement && document.documentElement.clientWidth)
  // Explorer 6 Strict Mode
  {
    docWidth = document.documentElement.clientWidth;
  }else if (document.body) // other Explorers
  {
    docWidth = document.body.clientWidth;
    
  }
  return docWidth;
}

function docCenter(){
  return docWidth()/2;
}