window.addEvent("domready", function () 
{
	var left_top = $("wrapper_left").getPosition(document).y;
	var left_bottom = left_top + $("wrapper_left").offsetHeight;

	window.addEvent("scroll",function () 
	{
		// get height of window
			var window_height = window.getHeight();

		var scroll_top = this.getScroll().y;

		if (scroll_top > left_bottom || (scroll_top > left_top && window_height > $("wrapper_left").offsetHeight))
		{
			$("back_to_top").setStyles({
				"top"			: (scroll_top + window_height - 60) + "px",
				"visibility"	: "visible"
			});
		}
		else
			$("back_to_top").setStyle("visibility", "hidden");
		
	});
});