window.addEvent('domready',function() {

	//settings
	var duration = 6000;
	var length = 400;
	var count = 0;
	var element = $('gravity');
	var tweener = element.set('tween',{ duration: duration, transition: 'linear' });
	//showtime!
	var run = function() {
		tweener.tween('background-position','-' + (++count * length) + 'px 0px');
	};
	run();
	run.periodical(duration);
});