Position.GetWindowSize = function(w) {
	var width, height;
  w = w ? w : window;
  this.width = w.innerWidth || (w.document.documentElement.clientWidth || w.document.body.clientWidth);
  this.height = w.innerHeight || (w.document.documentElement.clientHeight || w.document.body.clientHeight);
  return this;
};

alignMan = function() {
  var dim = Position.GetWindowSize();
  var width = dim.width / 2 - 450;
  $('wrapper').style.backgroundPosition = width + 'px 0';
  $('header').style.backgroundPosition = width +  'px 0';
};

alignMan();
Event.observe(window, 'resize', alignMan);