function scroll_sky(){
  var sky = null;
  var sky_pos = 0;
  var speed = 70;
  var step = 1;
  sky = document.getElementById( 'sky-wrap' );
  setInterval( function() {
    sky_pos++;
    sky.style.backgroundPosition =  ( sky_pos / speed ) + 'em 0px';
  }, step);

}

$( document ).ready( function () {
  scroll_sky();
});