var scrollSpeed = 20;
var step = 1;
var current = 0;
var imageHeight = 310;
var headerHeight = 310;

var restartPosition = -(imageHeight - headerHeight);

function scrollBg(){
	current -= step;
	if (current == restartPosition){
		current = 0;
	}
	$('#sky').css("background-position",+current+"px 30px");
}
var init = setInterval("scrollBg()", scrollSpeed);